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Respiratory System

Respiratory system

Respiration is the process by which living organisms take in oxygen and release carbon dioxide. The human respiratory system, working in conjunction with the circulatory system, supplies oxygen to the body's cells, removing carbon dioxide in the process. The exchange of these gases occurs across cell membranes both in the lungs (external respiration) and in the body tissues (internal respiration). Breathing, or pulmonary ventilation, describes the process of inhaling and exhaling air. The human respiratory system consists of the respiratory tract and the lungs.

Respiratory tract

The respiratory tract cleans, warms, and moistens air during its trip to the lungs. The tract can be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity, pharynx (throat), and larynx (voice box). The lower part consists of the trachea (windpipe), bronchi, and bronchial tree.

The nose has openings to the outside that allow air to enter. Hairs inside the nose trap dirt and keep it out of the respiratory tract. The external nose leads to a large cavity within the skull, the nasal cavity. This cavity is lined with mucous membrane and fine hairs called cilia. Mucus moistens the incoming air and traps dust. The cilia move pieces of the mucus with its trapped particles to the throat, where it is spit out or swallowed. Stomach acids destroy bacteria in swallowed mucus. Blood vessels in the nose and nasal cavity release heat and warm the entering air.

Air leaves the nasal cavity and enters the pharynx. From there it passes into the larynx, which is supported by a framework of cartilage (tough, white connective tissue). The larynx is covered by the epiglottis, a flap of elastic cartilage that moves up and down like a trap door. The epiglottis stays open during breathing, but closes during swallowing. This valve mechanism keeps solid particles (food) and liquids out of the trachea. If something other than air enters the trachea, it is expelled through automatic coughing.

Words to Know

Alveoli: Tiny air-filled sacs in the lungs where the exchange of oxygen and carbon dioxide occurs between the lungs and the bloodstream.

Bronchi: Two main branches of the trachea leading into the lungs.

Bronchial tree: Branching, air-conducting subdivisions of the bronchi in the lungs.

Diaphragm: Dome-shaped sheet of muscle located below the lungs separating the thoracic and abdominal cavities that contracts and expands to force air in and out of the lungs.

Epiglottis: Flap of elastic cartilage covering the larynx that allows air to pass through the trachea while keeping solid particles and liquids out.

Pleura: Membranous sac that envelops each lung and lines the thoracic cavity.

Air enters the trachea in the neck. Mucous membrane lines the trachea and C-shaped cartilage rings reinforce its walls. Elastic fibers in the trachea walls allow the airways to expand and contract during breathing, while the cartilage rings prevent them from collapsing. The trachea divides behind the sternum (breastbone) to form a left and right branch, called bronchi (pronounced BRONG-key), each entering a lung.

The lungs

The lungs are two cone-shaped organs located in the chest or thoracic cavity. The heart separates them. The right lung is somewhat larger than the left. A sac, called the pleura, surrounds and protects the lungs. One layer of the pleura attaches to the wall of the thoracic cavity and the other layer encloses the lungs. A fluid between the two membrane layers reduces friction and allows smooth movement of the lungs during breathing.

The lungs are divided into lobes, each one of which receives its own bronchial branch. Inside the lungs, the bronchi subdivide repeatedly into smaller airways. Eventually they form tiny branches called terminal

bronchioles. Terminal bronchioles have a diameter of about 0.02 inch (0.5 millimeter). This branching network within the lungs is called the bronchial tree.

The terminal bronchioles enter cup-shaped air sacs called alveoli (pronounced al-VEE-o-leye). The average person has a total of about 700 million gas-filled alveoli in the lungs. These provide an enormous surface area for gas exchange. A network of capillaries (tiny blood vessels) surrounds each alveoli. As blood passes through these vessels and air fills the alveoli, the exchange of gases takes place: oxygen passes from the alveoli into the capillaries while carbon dioxide passes from the capillaries into the alveoli.

This processexternal respirationcauses the blood to leave the lungs laden with oxygen and cleared of carbon dioxide. When this blood reaches the cells of the body, internal respiration takes place. The oxygen diffuses or passes into the tissue fluid, and then into the cells. At the same time, carbon dioxide in the cells diffuses into the tissue fluid and then into the capillaries. The carbon dioxide-filled blood then returns to the lungs for another cycle.

Breathing

Breathing exchanges gases between the outside air and the alveoli of the lungs. Lung expansion is brought about by two important muscles, the diaphragm (pronounced DIE-a-fram) and the intercostal muscles. The diaphragm is a dome-shaped sheet of muscle located below the lungs that separates the thoracic and abdominal cavities. The intercostal muscles are located between the ribs.

Nerves from the brain send impulses to the diaphragm and intercostal muscles, stimulating them to contract or relax. When the diaphragm contracts, it moves down. The dome is flattened, and the size of the chest cavity is increased. When the intercostal muscles contract, the ribs move up and outward, which also increases the size of the chest cavity. By contracting, the diaphragm and intercostal muscles reduce the pressure inside the lungs relative to the pressure of the outside air. As a consequence, air rushes into the lungs during inhalation. During exhalation, the reverse occurs. The diaphragm relaxes and its dome curves up into the chest cavity, while the intercostal muscles relax and bring the ribs down and inward. The diminished size of the chest cavity increases the pressure in the lungs, thereby forcing air out.

A healthy adult breathes in and out about 12 times per minute, but this rate changes with exercise and other factors. Total lung capacity is about 12.5 pints (6 liters). Under normal circumstances, humans inhale and exhale about one pint (475 milliliters) of air in each cycle. Only about three-quarters of this air reaches the alveoli. The rest of the air remains in the respiratory tract. Regardless of the volume of air breathed in and out, the lungs always retain about 2.5 pints (1200 milliliters) of air. This residual air keeps the alveoli and bronchioles partially filled at all times.

Respiratory disorders

The respiratory system is open to airborne microorganisms and outside pollution. Some respiratory disorders are relatively mild and, unfortunately, very familiar. Excess mucus, coughing, and sneezing are all symptoms of the common cold, which is an inflammation of the mucous membrane lining the nose and nasal cavity. Viruses, bacteria, and allergens are among the causes of the common cold.

Since the respiratory lining is continuous, nasal cavity infections often spread. Laryngitis, an inflammation of the vocal cords, results in hoarseness and loss of voice. Viruses, irritating chemicals in the air, and overuse of the voice are causes of laryngitis.

Pneumonia, inflammation of the alveoli, is most commonly caused by bacteria and viruses. During a bout of pneumonia, the inflamed alveoli fill up with fluid and dead bacteria (pus). Breathing becomes difficult. Patients come down with fever, chills, and pain, coughing up phlegm and sometimes blood.

Sufferers of bronchitis, an inflammation of the bronchi, also cough up thick phlegm. There are two types of bronchitis, acute and chronic. Acute bronchitis can be a complication of a cold or flu. Bacteria, smoking, and air pollution can also cause acute bronchitis. This type of bronchitis clears up in a short time. Chronic bronchitis is a long-term illness that is mainly caused by air pollution and tobacco smoke. There is a persistent cough and congestion of the airways.

In emphysema, also caused by smoking, the walls of the alveoli disintegrate and the alveoli blend together. They form large air pockets from which air cannot escape. This cuts down the surface area for gas exchange. It becomes difficult for the patient to exhale. The extra work of exhaling over several years can cause the chest to enlarge and become barrel-shaped. The body is unable to repair the damage to the lungs, and the disease can lead to respiratory failure.

Asthma is a disorder of the nervous system. While the cause for the condition is unknown, it is known that allergies can trigger an asthma attack. Nerve messages cause extreme muscle spasms in the lungs that either narrow or close the bronchioles. A tightness is felt in the chest and breathing becomes difficult. Asthma attacks come and go in irregular patterns, and they vary in degree of severity.

Lung cancer is the leading cause of cancer death in men. It is the second leading cause of cancer death (after breast cancer) in women. Cigarette smoking is the main cause of lung cancer. Air pollution, radioactive minerals, and asbestos also cause lung cancer. The symptoms of the disease include a chronic cough from bronchitis, coughing up blood, shortness

of breath, and chest pain. Lung cancer can spread in the lung area. Unchecked, it can spread to other parts of the body.

[See also Blood ]

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Respiratory System

Respiratory System

All animals require oxygen. Oxygen enables living things to metabolize (or burn) nutrients, which releases the energy they need to grow, reproduce, and maintain life processes. Some animals can exist for months on fats or other foods stored in their bodies, and many can live a shorter time without water. Yet few can survive for long without oxygen because little can be stored in the body. Most animals obtain oxygen from their environment. It is generally believed that life originated in the oceans, where many animals still live, obtaining their oxygen in a dissolved form from the water. In the course of evolution, various animals have become earth dwellers and have developed structures that allow them to breathe air.

Along with supplying oxygen, the respiratory system assists in the removal of carbon dioxide, preventing a dangerous and potentially lethal buildup of this waste product. The respiratory system also helps regulate the balance of acid and bases in tissues, which is a crucial process that enables cells to function. Without the prompt of conscious thought, the respiratory system carries out this life-sustaining activity. If any of the functions of the system are interrupted for more than a few minutes, serious and irreversible damage to body tissues would occur, and possibly result in death.

Respiratory Systems of Various Species

Depending on the animal, the organs and structures of the respiratory system vary in composition and complexity. The respiratory system is composed of the organs that deliver oxygen to the circulatory system for movement or transport to all of the cells in the body. Organs or systems such as body covering, gills, lungs, or trachea allow the movement of gases between the animal and its environment. These structures vary in appearance but function in a similar way by allowing gases to be exchanged.

Animals obtain oxygen in a number of ways: (1) from water or air through a moist surface directly into the body (protozoan); (2) from air or water through the skin to blood vessels (earthworms); (3) from air through gills to a system of air ducts or trachae (insects); (4) from water through moist gill surfaces to blood vessels (fishes, amphibians); (5) from air through moist lung surfaces to blood vessels (reptiles, mammals, humans).

In one-celled aquatic animals, such as protozoans, and in sponges, jellyfish, and other aquatic organisms that are a few cell layers thick, oxygen and carbon dioxide diffuse directly between the water and the cell. This process of diffusion works because all cells of the animal are within a few millimeters of an oxygen source.

Insects, centipedes, millipedes, and some arachnids have fine tubes or trachea connecting all parts of the body to small openings on the surface of the animal. Movements of the thoracic and abdominal parts and the animal's small size enable oxygen and carbon dioxide to be transported from the trachea to the blood by way of diffusion.

In more complex animals, respiration requires a blood circulatory system and gills, in combination with blood, blood vessels, and a heart. Many aquatic animals have gills, thin-walled filaments that increase the surface area and increase the amount of available oxygen. The oxygen and carbon dioxide exchange occurs between the surrounding water and the blood within the gills. The gills of some larvae and worms are simply exposed to the water, while some aquatic crustaceans, such as crayfish, have special adaptations to force water over their gills. The gills of fishes and tadpoles are located in chambers at the sides of the throat, with water taken into the mouth and forced out over gills.

All land vertebrates, including most amphibians, all reptiles, birds and mammals have lungs that enable these animals to get oxygen from air. A heart and a closed circulatory system work with the lungs to deliver oxygen and to remove carbon dioxide from the cells. A lung is a chamber lined with moist cells that have an abundance of blood capillaries. These membranes take different forms. In amphibians and reptiles they can form a single balloon-like sac. In animals that require large amounts of oxygen, the lungs are a spongy mass composed of millions of tiny air sacs called alveoli that supply an enormous surface area for the transfer of gases.

In birds a special adaptation allows for the high-energy demands of flight. The lungs have two openings, one for taking in oxygen-filled air, the other for expelling the carbon dioxide. Air flows through them rather than in and out as in the other lunged vertebrates.

Respiratory System in Humans

The human respiratory system consists of the nasal cavity, throat (pharynx), vocal area (larynx), windpipe (trachea), bronchi, and lungs. Air is taken in through the mouth and or nose. The nasal passages are covered with mucous membranes that have tiny hairlike projections called cilia. They keep dust and foreign particles from reaching the lungs.

Approximately halfway down the chest the trachea or windpipe branches into two bronchi, one to each lung. Each branch enters a lung, where it divides into increasingly smaller branches known as bronchioles. Each bronchiole joins a cluster of tiny airsacs called alveoli. The pair of human lungs contain nearly 300 million of these clusters and together can hold nearly four quarts of air. After oxygen has crossed the alveolar membrane, oxygen is delivered to the cells by the pigment hemoglobin , found in blood.

The lungs in humans are cone-shaped and are located inside the thorax or chest, in the cavity framed by the rib cage. One lung is on either side of the heart. The right lung has three lobes; the left has two lobes. A thin membrane known as pleura covers the lungs, which are porous and spongy. The base of each lung rests on the diaphragm, a strong sheet of muscle that separates the chest and abdominal cavities.

The respiratory center at the base of the brain is a cluster of nerve cells that control breathing by sending impulses to the nerves in the spinal cord. These signals stimulate the diaphragm and muscles between the ribs for automatic inhalation. During inhalation the rib muscles elevate the ribs and the diaphragm moves downward, increasing the chest cavity. Air pressure in the lungs is reduced, and air flows into them. During the exhalation, the rib muscles and diaphragm relax and the chest cavity contracts. The average adult takes about sixteen breaths per minute while awake and about six to eight per minute while sleeping. If breathing stops for any reason, death soon follows, unless breathing movements are artificially restored by mouth to mouth breathing.

see also Circulatory System; Transport.

Leslie Hutchinson

Bibliography

Hickman, Cleveland, Larry Roberts, and Frances Hickman. Integrated Principles of Zoology, 8th ed. St. Louis, MO: Times Mirror/Mosby College Publishing, 1990.

Johnson, George B. Biology: Visualizing Life. New York: Holt, Rinehart and Winston, Inc., 1998.

Randall, David, Warren Burggren, and Kathleen French. Eckert Animal Physiology: Mechanisms and Adaptations, 4th ed. New York: W. H. Freeman, 1997.

Internet Resources

"Respiratory System." Britannica Online. 1994-2000. Encyclopedia Britannica. <http://www.eb.com/>.

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respiratory system

respiratory system System in air-breathing animals concerned with gas exchange. The respiratory tract begins with the nose and mouth, through which air enters the body. Air then passes through the larynx and into the trachea. At its lower end, the trachea branches into two bronchi, each bronchus leads to a lung. The bronchi divide into many bronchioles, which lead in turn to bunches of tiny air sacs (alveoli), where the exchange of gases between air and blood takes place. Exhaled air leaves along the same pathway. See also alveolus

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respiratory system

respiratory system n. the combination of organs and tissues associated with breathing. It includes the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.

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respiratory system

respiratory system: see respiration.

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respiratory system

respiratory system

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Respiratory System

Respiratory System

Definition

The respiratory system consists of organs that deliver oxygen to the circulatory system for transport to all the cells of the body. The respiratory system also assists in the removal of carbon dioxide (CO2), thus preventing a deadly buildup of this waste product in the body.

Description

The respiratory system consists of the upper and lower respiratory tracts, extending from the nose to the lungs.

The upper respiratory tract encompasses the:

  • nose
  • pharynx, more commonly called the throat

The lower respiratory tract includes the:

  • larynx, also called the voice box
  • the trachea or windpipe, which splits into two main branches called bronchi
  • tiny branches of the bronchi called bronchioles
  • the lungs

These organs all work together to provide air to and from the lungs. The lungs then operate in conjunction with the circulatory system to deliver oxygen and remove carbon dioxide.

Nasal passages

The flow of air begins in the nose, which is divided into the left and right nasal passages and ends in the lungs. The nasal passages are lined with epithelial cells, a mucous membrane composed mostly of a layer of flat, closely packed cells. Each epithelial cell is fringed with thousands of tiny fingerlike extensions of the cells called cilia. Goblet cells are specialized cells that produce mucus, and are among the epithelial cells. Mucus is a thick, moist fluid that coats epithelial cells and cilia. Beneath the mucous membrane, near the surface of the nasal passages, are many tiny blood vessels called capillaries. The nasal passages play two critical roles in transporting air to the pharynx. First, the nasal passages filter air to remove potentially disease-causing particles. Secondly, they moisten and warm the air to protect the respiratory system.

Filtering air through the nasal passage prevents airborne bacteria, viruses, smog, dust particles, and other potentially disease-causing substances from entering the lungs or the bronchioles. Just inside the nostrils are coarse hairs that assist in trapping airborne particles as they are inhaled. The particles then drop down onto the mucous membranes in the lining of the nasal passages. The particles are then propelled out of the nose or downward to the pharynx by the wave of mucus created by the cilia in the mucous membranes. From the pharynx, mucus is swallowed and travels to the stomach where subsequently the particles are destroyed by stomach acid. If there are more particles in the nasal passages than the cilia can cope with, a reflex will be triggered producing a sneeze. The sneeze, designed to flush out the polluted air, is due to particles building up on the mucus and irritating the membrane below it.

Pharynx

As air leaves the nasal passages, it flows to the pharynx, which is a short, funnel-shaped tube about 13 cm (5 inches) long. The pharynx is also lined with a mucous membrane and ciliated cells that filter air from the nasal passages. The pharynx also includes the tonsils, which are lymphatic tissues that contain white blood cells. If any impurities escape the hairs, cilia, and mucus of the nasal passages and pharynx, the white blood cells attack the disease-causing organisms. To prevent these organisms from moving further into the body, the tonsils are strategically located. One pair of growths of lymphoid tissue, referred to as the adenoids, is located high in the rear wall of the pharynx. Two tonsils called the palatine tonsils are positioned on either side of the tongue at the back of the pharynx. Another pair called the lingual tonsils is found deep in the pharynx at the base of the tongue. The tonsils may become swollen with infection during their fight against disease-causing organisms.

Larynx

Air passes from the pharynx to the larynx, which is approximately 2 inches (5 cm) long and situated near the middle of the neck. The larynx is comprised of several layers of cartilage, a tough and flexible tissue.

In addition to transporting air to the trachea, the larynx serves other functions such as:

  • It prevents food and fluid from entering the air passage which would cause choking.
  • Its mucous membranes and cilia-bearing cells help filter air.
  • It plays a primary role in producing sound.
  • The cilia in the larynx move airborne particles up toward the pharynx to be swallowed.

A thin, leaflike flap of tissue called the epiglottis prevents food and fluids from entering the larynx from the pharnyx. The epiglottis is held in a vertical position, like an open trap door when a person is breathing. When swallowing, a reflex forces the larynx and the epiglottis to move toward each other. This reflex diverts food and fluids to the esophagus. The swallowing reflex may not work if one eats or drinks too rapidly, or laughs while swallowing. Food or fluid enters the larynx and a coughing reflex is initiated to clear the obstruction. This situation may cause life-threatening choking if coughing does not clear the larynx of the obstruction.

Trachea, bronchi, and bronchioles

Air is passed from the larynx into the trachea, the largest airway in the respiratory system. The trachea is a tube located just below the larynx, approximately 5 to 6 inches (12 to 15 cm) long. Fifteen to 20 C-shaped rings of cartilage form the trachea. Air passes freely at all times because the trachea is held open by the rings of sturdy cartilage. The open part of the C-shaped cartilage rings is situated at the back of the trachea with the ends connected by muscle tissue. The trachea branches into two tubes at its base, located just below where the neck meets the trunk of the body. These two tubes are called the left and right bronchi and they deliver air to the left and right lungs, respectively. The bronchi branch into smaller tubes called bronchioles within the lungs. The trachea, bronchi, and the first few bronchioles are lined with mucous membranes and ciliated cells; thus they contribute to the cleansing action of the respiratory system by moving mucus upward to the pharynx.

Alveoli and lungs

The bronchioles divide many more times in the lungs into an upside-down tree-like structure with progressively smaller branches. Tiny air sacs called alveoli are at the end of the branches. Some of the bronchioles are no larger than 0.5 mm (0.02 inches) in diameter. The alveoli comprise most of the lung tissue, with about 150 million alveoli per lung, and resemble bunches of grapes. The alveoli send oxygen to the circulatory system while removing carbon dioxide. Alveoli have thin elastic walls, thus allowing air to flow into them when they expand; they collapse when the air is exhaled. Alveoli are arranged in clusters, and each cluster is surrounded by a dense network of capillaries. The walls of the capillaries are very thin; thus the air in the wall of the alveoli is very near to the blood in the capillaries (only about 0.1 to 0.2 microns). Carbon dioxide is a waste product that is dumped into the bloodstream from the cells. It flows throughout the body in the bloodstream to the heart, and then to the alveolar capillaries. The oxygen diffuses from the alveoli to the capillaries since the concentration of oxygen is much higher in the alveoli than in the capillaries. From the capillaries, the oxygen flows into larger vessels and is then carried to the heart where it is pumped to the rest of the body. The forces of exhalation cause the carbon dioxide to go back up through the respiratory passages and out of the body. Numerous macrophages are interspersed among the alveoli. Macrophages are large white blood cells that remove foreign substances from the alveoli that have not been previously filtered out. The presence of the macrophages ensures that the alveoli are protected from infection; they are the last line of defense of the respiratory system.

The lungs are the largest organ in the respiratory system and resemble large pink sponges. The left lung is slightly smaller than the right lung since it shares space with the heart, which is also located in the left side of the chest. Each lung is divided into lobes, with two in the left lung and three in the right. A slippery membrane called the pleura covers the lungs and lines the inside of the chest wall. It helps the lungs move smoothly during each breath. Normally, the two lubricated layers of the pleura have very little space between them. They glide smoothly over each other when the lungs expand and contract.

The diaphragm is the most important muscle involved in respiration. It lies just under the lungs and is a muscle shaped like a large dome. The sternum (or breastbone), ribs, and spine protect the lungs and the other organs in the chest. Twelve pairs of ribs curve around the chest and are joined to the vertebrae of the spine. The intercostal muscles are also important for respiration. They lie between the ribs and assist in breathing by helping to move the rib cage.

Function

The main function of the respiratory system is the delivery of oxygen and removal of carbon dioxide. To achieve this purpose, the nervous system controls the flow of air in and out of the lungs while maintaining a regular rate and pattern of breathing. Regulation is controlled by the respiratory center, a cluster of nerve cells in the brain stem. These cells simultaneously send signals to the muscles involved in inhalation: the diaphragm and rib muscles. The diaphragm flattens out when stimulated by a nervous impulse. The thoracic or chest cavity contains the lungs. The volume of the cavity expands with the downward movement of the diaphragm, thus expanding the lungs. The rib muscles also contract when stimulated, which pulls the rib cage up and out, at the same time expanding the thoracic cavity. This movement reduces pressure in the chest. When the volume is increased in the thoracic cavity, air rushes into the lungs to equalize the pressure. This nervous stimulation is quick, and when it is over, the diaphragm and rib muscles relax and a person exhales.

Working in conjunction with the circulatory system, the oxygen-rich blood travels from the lungs through the pulmonary veins into the left side of the heart. From there, blood is pumped to the rest of the body. Blood that is oxygen-depleted, but carbon dioxide-rich, returns to the right side of the heart through two large veins called the superior and inferior venae cavae. This blood is then pumped through the pulmonary artery to the lungs, where oxygen is picked up and carbon dioxide is released. This process is repeated continually under normal circumstances.

Other functions the respiratory system assist in just by normal respiration are the regulation of acid-base balance in the body, a critical process for normal cellular function. It also protects the body against toxic substances inhaled as well as against disease-causing organisms in the air. The respiratory system also assists in detecting smell using the olfactory receptors located in the nasal passages. Furthermore, it aids in producing sounds for speech.

Role in human health

Breathing is an unconscious process carried out on a constant basis and is necessary for survival. Under normal conditions, a person takes 12-20 breaths per minute, although newborns breathe at a faster rate, at approximately 30-50 breaths per minute. The breathing rate set by the respiratory center can be altered by conscious control, for example, by holding the breath. This alteration occurs when the part of the brain involved in thinking, the cerebral cortex, sends signals to the diaphragm and rib muscles to momentarily ignore the signals from the respiratory center. If a person holds his or her breath too long, carbon dioxide accumulates in the blood, which then causes the blood to become more acidic. The increased acidity interferes with the action of enzymes, which are specialized proteins that coordinate all biochemical reactions in the body. To prevent too much acid from building up in the blood, special receptors located in the brainstem and in the blood vessels of the neck called chemoreceptors monitor the acid level in the blood. These chemoreceptors send nervous signals to the respiratory center when acid levels are too high, which overrides the signals from the cerebral cortex, forcing a person to exhale and then resume breathing. The blood acid level is brought back to normal levels by exhalation, which expels the carbon dioxide. Irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death if the respiratory system's tasks are interrupted for more than a few minutes.

Common diseases and disorders

The diseases and disorders of the respiratory system can affect any part of the respiratory tract and may range from mild to life-threatening conditions such as:

  • Colds—A virus that targets the nasal passages and pharynx. Symptoms include a stuffy and runny nose.
  • Hay fever and asthma—Allergic reactions that may occur when the immune system is stimulated by pollen, dust, or other irritants. A runny nose, watery eyes, and sneezing characterizes hay fever. In asthma, because the bronchi and bronchioles are temporarily constricted and inflamed, a person has difficulty breathing.
  • Bronchitis—Characterized by inflamed bronchi or bronchiole membranes, resulting from viral or bacterial infection or from chemical irritants.
  • Emphysema—A non-contagious disease that results from multiple factors including: smog, cigarette smoke, infection, and a genetic predisposition to the condition. Emphysema partially destroys the alveolar tissue and leaves the remaining alveoli weakened and enlarged. When a person exhales, the bronchioles collapse, trapping air in the alveoli. This process eventually impedes the ability to exchange oxygen and carbon dioxide, leading to breathing difficulties.
  • Pneumonia—Infections caused by bacteria or viruses can lead to this potentially serious condition. The alveoli become inflamed and fill with fluid, impairing the flow of oxygen and carbon dioxide between the capillaries and the alveoli.
  • Tuberculosis—A condition caused by a bacterium that attacks the lungs and occasionally other body tissues. Left untreated, the disease destroys lung tissue.
  • Laryngitis—An inflammation of the larynx caused by irritants such as cigarette smoke, overuse of the voice, or a viral infection. People with laryngitis may become hoarse, or they may only be able to whisper until the inflammation is reduced.
  • Lung cancer—Occurs in those individuals who are exposed to cancer-causing agents, such as tobacco smoke, asbestos, or uranium; or who have a genetic predisposition to the disease. Treatments are very effective if the cancer is detected before the cancer has spread to other parts of the body. About 85% of cases are diagnosed after the cancer has spread; thus the prognosis is very poor.
  • Respiratory distress syndrome (RDS)—Refers to a group of symptoms that indicate severe malfunctioning of the lungs affecting adults and infants. Adult respiratory distress syndrome (ARDS) is a life-threat-ening condition that results when the lungs are severely injured, for example, by poisonous gases, in an automobile accident, or as a response to inflammation in the lungs.
  • Wheezing—A high-pitched whistling sound produced due to air flowing through narrowed breathing tubes. It may have many causes such as asthma, emphysema, pneumonia, bronchitis, etc.
  • Shortness of breath or dyspnea—This condition may have multiple causes such as asthma, emphysema, hyperventilation, obesity, cigarette smoking, lung disease, excessive exercise, etc.
  • Chronic respiratory insufficiency (or chronic obstructive pulmonary disease; COPD)—A prolonged or persistent condition characterized by breathing or respiratory dysfunction resulting in reduced rates of oxygenation or the ability to eliminate carbon dioxide. These rates are insufficient to meet the requirements of the body and may be severe enough to impair or threaten the function of vital organs (respiratory failure ).

KEY TERMS

Acidosis— A dangerous condition in which the blood and body tissues are less alkaline (or more acidic) than normal.

Alkalosis— Excessive alkalinity of the blood and body tissue.

Bronchi— The trachea branches into two tubes at the base of the trachea called the left and right bronchi, which extend from the trachea to deliver air to the left and right lungs, respectively. The bronchi branch into smaller tubes called bronchioles within the lungs.

Bronchioles— The bronchioles are no larger than 0.5mm (0.02 inches) in diameter and divide many times in the lungs to form a tree-like structure; they have progressively smaller branches and tiny air sacs called alveoli at the end.

Capillaries— Tiny blood vessels that lie beneath the mucous membrane, near the surface of the nasal passages.

Carbon dioxide (CO2)— A gaseous waste product that is dumped into the bloodstream from the cells; a byproduct of respiration, it is released upon exhalation of air from the body.

Cilia— Each epithelial cell is fringed with thousands of these tiny fingerlike extensions of the cells.

Diaphragm— The diaphragm is involved in inhalation. It lies just under the lungs and is a muscle shaped like a large dome.

Epiglottis— A thin, leaflike flap of tissue that prevents food and fluids from entering the larynx from the pharynx.

Mucus— A thick, moist fluid that coats epithelial cells and cilia.

pH— the negative logarithm of H+ (hydrogen) concentration. Acid-base balance can be defined as homeostatis (equilibrium) of the body fluids at a normal arterial blood pH ranging between 7.37 and 7.43.

Thoracic cavity— Also called the chest cavity, it is the portion of the ventral body cavity located between the neck and the diaphragm. It is enclosed by the ribs, the vertebral column, and the sternum. It is separated from the abdominal cavity by the diaphragm.

Some of the most common symptoms of respiratory disorders are a cough, shortness of breath, chest pain, wheezing, cyanosis (bluish discoloration), finger clubbing, stridor (a crowing sound when breathing), hemoptysis (coughing up of blood), and respiratory failure. These symptoms do not necessarily signify a respiratory problem, but can be a sign of another problem. For example, chest pain may be due to a heart or a gastrointestinal problem.

Cystic fibrosis is a genetic disease that causes excessive mucus production and clogs the airways.

Acidosis is a condition resulting from higher than normal acid levels in the body fluids. It is not a disease but may be an indicator of disease. Respiratory acidosis is due to a failure by the lungs to remove carbon dioxide, therefore reducing the pH in the body. Several conditions such as chest injury, blockage of the upper air passages, and severe lung disease may result in respiratory acidosis. Blockage of the air passages may be due to bronchitis, asthma, or airway obstruction resulting in mild or severe acidosis. Regular, consistent retention of carbon dioxide in the lungs is referred to as chronic respiratory acidosis. This disorder results in only mild acidosis because of an increased bicarbonate (alkali) production by the kidneys.

Alkalosis is a condition resulting from a higher than normal level of base or alkali in the body fluids. Respiratory alkalosis results from decreased carbon dioxide levels caused by conditions such as hyperventilation (a faster breathing rate), anxiety, and fever. The pH becomes elevated in the body. Hyperventilation causes the body to lose excess carbon dioxide in expired air and can be triggered by altitude or a disease that reduces the amount of oxygen in the blood. Symptoms of respiratory alkalosis may include dizziness, light-headedness, and numbing of the hands and feet. Treatments include breathing into a paper bag or a mask that induces rebreathing of carbon dioxide.

Resources

BOOKS

Ganong, William F. Review of Medical Physiology, 20th ed. New York: McGraw-Hill Professional Publishing, 2001.

Hlastala, Michael P., and Albert J. Berger. Physiology of Respiration, 2nd ed. Oxford, UK: Oxford University Press, 2001.

Murray, John F., and Jay A. Nadel. Textbook of Respiratory Medicine (Two-Volume Set), 3rd ed. Philadelphia: WB Saunders Co, 2000.

West, John B. Respiratory Physiology: The Essentials, 6th ed. Philadelphia: Lippincott, Williams and Wilkins, 2000.

PERIODICALS

Baker, Frank, et al. "Health risks associated with cigar smoking." Journal of the American Medical Association 284, no. 6 (2000):735-740.

Beckett, W. S. "Current concepts: occupational respiratory diseases." New England Journal of Medicine 342 (2000):406-413.

Napoli, Maryann. "Alleviating cold symptoms: what works, what doesn't." Healthfacts (January 2001). 〈http://www.findarticles.com/cf_0/m0815/2001_Jan/68277444/p1/article.jhtml〉.

ORGANIZATIONS

The American Lung Association. 1740 Broadway, NY, NY, 10019. (212) 315-8700. 〈http://www.lungusa.org〉.

National Center for Complementary and Alternative Medicine (NCCAM). 31 Center Dr., Room #5B-58, Bethesda, MD 20892-2182. (800) NIH-NCAM Fax: (301) 495-4957. 〈http://nccam.nih.gov〉.

National Heart, Lung and Blood Institute. Building 31, Room 4A21, Bethesda, MD 20892. (301) 496-4236. 〈http://www.nhlbi.nih.gov〉.

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Respiratory System

Respiratory System

Respiration in the earthworm

Respiration in insects

Respiratory system of fish

Respiration in terrestrial vertebrates

Human respiratory system

The respiratory tract

The lungs

Pulmonary ventilation

Respiratory disorders

Resources

Aerobic organisms take in oxygen from the external environment and release carbon dioxide in a process known as respiration. At the most basic level, this exchange of gases takes place in cells and involves the release of energy from food materials by oxidation. Carbon dioxide is produced as a waste product of these oxidation reactions. The gas exchange in cells is called cellular respiration. In single-celled organisms, the oxygen and carbon dioxide simply diffuse through the cellmembrane. Respiration in multicellular organisms, however, is a much more complex process involving a specialized respiratory system that plays an intermediary role between the cells and the external environment. While the respiratory organs of some complex organisms such as insects communicate directly with internal tissues, respiration in vertebrates also involves the circulatory system, which carries gases between cells and respiratory organs.

The respiratory system must meet two important criteria. First, the respiratory surface must be large enough to take in oxygen in sufficient quantities to meet the organisms needs and release all waste gas quickly. Some animals, such as the earthworm, use the entire body surface as a respiratory organ. The internal respiratory organs of vertebrates generally have many lobes to enlarge the surface area. Second, the respiratory membrane must be moist, since gases require water to diffuse across membranes. The watery environment keeps the respiratory surface moist for aquatic animals. A problem exists for land animals, whose respiratory surfaces can dry out in open air. As a result, animals such as the earthworm must live in damp places. Internal respiratory organs provide an environment that is easier to keep moist.

Respiration in the earthworm

The earthworm uses its moist outer skin as a respiratory organ. Oxygen diffuses across the body surface and enters blood in the dense capillary mesh that lies just below the skin. Blood carries the oxygen to the body cells. There, it picks up carbon dioxide and transports it to the skin capillaries where it diffuses out of the body. The skin is effective as an organ of respiration in small worm-like animals where there is a high ratio of surface to volume.

Respiration in insects

Tiny air tubes called tracheae branch throughout the insects body. Air enters the tracheae through holes in the body wall called spiracles, which are opened and closed by valves. In larger insects, air moves through the tracheae when the body muscles contract. The tracheae are invaginatedthat is, folded into the bodyand thereby kept moist. Thickened rings in the walls of the tracheae help support them. These vessels branch into smaller vessels called tracheoles, which lack the supportive rings. The tracheoles carry air directly to the surface of individual cells, where they branch further to deliver oxygen and pick up carbon dioxide. A fluid in the endings of tracheoles regulates how much air contacts the cells. If a cell needs oxygen, the fluid pulls back and exposes the cell membrane to the air.

Respiratory system of fish

Gills mediate the gas exchange in fish. These organs, located on the sides of the head, are composed of gill filaments, feathery structures that provide a large surface for gas exchange. The filaments are arranged in rows in the gill arches, and each filament has lamellae, discs that contain capillaries. Blood enters and leaves the gills through these small blood vessels. Although gills are restricted to a small section of the body, the immense respiratory surface created by the gill filaments provides the whole animal with an efficient gas exchange. The surrounding water keeps the gills wet.

A flap, the operculum, covers and protects the gills of bony fish. Water containing dissolved oxygen enters the fishs mouth, and the animal moves its jaws and operculum in such a way as to pump the incoming water through the gills. As water passes over the gill filaments, blood inside the capillaries picks up the dissolved oxygen. Since the blood in the capillaries flows in a direction opposite to the flow of water around the gill filaments, there is a good opportunity for absorption. The circulatory system then transports the oxygen to all body tissues and picks up carbon dioxide, which is removed from the body through the gills. After the water flows through the gills, it exits the body behind the fishs operculum.

Respiration in terrestrial vertebrates

Lungs are the internal respiratory organs of amphibians, reptiles, birds, and mammals. The lungs, paired invaginations located in one area of the body, provide a large, thin, moist surface for gas exchange. Lungs work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body. The circulatory system also transports carbon dioxide from body cells to the lungs to be exhaled. The process of inhaling and exhaling is called pulmonary ventilation.

Besides these similarities, there is a great variety in the respiratory systems of terrestrial vertebrates. Frogs, for instance, have balloon like lungs that do not have a very large surface area. Diffusion across the frogs moist skin supplements the gas exchange through the lungs. Birds have about eight thin-walled air sacs attached to their lungs. The air sacs take up space in the entire body cavity and in some of the bones. When birds inhale, air passes through a tube called the bronchus and enters the air sacs located in the posterior (rear) of the animal. At the same time, air in the lungs moves forward to air sacs located in the anterior (front). When birds exhale, the air from the anterior air sacs moves to the outside, while air from the posterior sacs moves into the lungs. This efficient system moves air forward through the lungs both when the bird inhales and exhales. Blood in the capillaries of the lungs flows against the air current that, again, increases respiratory efficiency. Birds are capable of flying at high altitudes, where the air has a low oxygen content, because of these adaptations of the respiratory system.

Human respiratory system

The human respiratory system, working in conjunction with the circulatory system, supplies oxygen and removes carbon dioxide. The respiratory system conducts air to the respiratory surfaces of lung units. There, the blood in the lung capillaries readily absorbs oxygen, and gives off carbon dioxide gathered from the body cells. The circulatory system transports oxygen-laden blood to the body cells and picks up carbon dioxide. The term respiration describes the exchange of gases across cell membranes both in the lungs (external respiration) and in the body tissues (internal respiration). Pulmonary ventilation, or breathing, exchanges volumes of air with the external environment.

The respiratory tract

The human respiratory system consists of the respiratory tract and the lungs. The respiratory tract can, again, be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity, pharynx (throat) and larynx (voicebox). The lower part consists of the trachea (windpipe), bronchi, and bronchial tree. The respiratory tract cleans, warms, and moistens air during its trip to the lungs. The nose has openings to the outside that allow air to enter. Hairs inside the nose trap dirt and keep it out of the respiratory tract. The external nose leads to a large cavity within the skull. This cavity and the space inside the nose make up the nasal cavity. A nasal septum, supported by cartilage and bone, divides the nasal cavity into a right and left side. Epithelium, a layer of cells that secrete mucus and cells equipped with cilia, lines the nasal passage. Mucus moistens the incoming air and traps dust. The cilia move pieces of the mucus with its trapped particles to the throat, where it is spit out or swallowed. Stomach acids destroy bacteria in swallowed mucus. Sinuses, epithelium-lined cavities in bone, surround the nasal cavity. Blood vessels in the nose and nasal cavity release heat and warm the entering air.

Air leaves the nasal cavity and enters the throat or pharynx. From there it passes into the larynx, which is located between the pharynx and the trachea or wind-pipe. A framework of cartilage pieces supports the larynx, which is covered by the epiglottis, a flap of elastic cartilage that moves up and down like a trap door. When humans breathe, the epiglottis stays open, but when one swallows, it closes. This valve mechanism keeps solid particles and liquids out of the trachea. If humans breathe in something other than air, the person automatically coughs and expels it. Should these protective mechanisms fail, allowing solid food to lodge in and block the trachea, the victim is in imminent danger of asphyxiation.

Air enters the trachea in the neck. Epithelium lines the trachea as well as all the other parts of the respiratory tract. C-shaped cartilage rings reinforce the wall of the trachea and all the passageways in the lower respiratory tract. Elastic fibers in the trachea walls allow the airways to expand and contract when humans inhale and exhale, while the cartilage rings prevent them from collapsing. The trachea divides behind the sternum to form a left and right bronchus, each entering a lung. Inside the lungs, the bronchi subdivide repeatedly into smaller airways. Eventually they form tiny branches called terminal bronchioles. Terminal bronchioles have a diameter of about 0.02 in (0.5 mm). The branching air-conducting network within the lungs is called the bronchial tree.

The respiratory tract is not dedicated to respiration alone but plays a major role in many other bodily functions as well. The pharynx in particular is a multi-purpose organ. It is a passageway for food as well as air, since the mouth cavity also leads to it. The back of the pharynx leads into the esophagus (food tube) of the digestive system. The front leads into the larynx and the rest of the respiratory system. Small amounts of air pass between holes in the pharynx and the Eustachian tubes of the ear to equalize the gas pressure inside the ears, nose, and throat. The pharynx also contains lymph glands called tonsils and adenoids, which play a role in the immune system. Finally, the pharynx, which doubles as a resonating chamber, also plays a role in the production of sound, to which many other parts of the respiratory tract also contribute.

The vocal cords, a pair of horizontal folds inside the larynx, vibrate to produce sound from exhaled air. When humans speak, muscles change the size of the vocal cords and the space between them, known as the glottis. The shape and size of the vocal cords determine the pitch of the sound produced. The glottis widens for deep tones and narrows for high-pitched ones. Longer, thicker vocal cords, which vibrate more slowly, produce a deeper sound. The force with which air is expelled through the larynx determines the volume of the sound produced. Voice quality also depends on several other factors, including the shape of the nasal cavities, sinuses, pharynx, and mouth, which all function as resonating chambers.

The lungs

The lungs are two cone-shaped organs located in the thoracic cavity, or chest, and are separated by the heart. The right lung is somewhat larger than the left. The pleural membrane surrounds and protects the lungs. One layer of the pleural membrane attaches to the wall of the thoracic cavity, and the other layer encloses the lungs. A fluid between the two membrane layers reduces friction and allows smooth movement of the lungs during breathing. The lungs are divided into lobes, each one of which receives its own bronchial branch. The bronchial branch subdivides and eventually leads to the terminal bronchi. These tiny airways lead into structures called respiratory bronchioles.

The respiratory bronchioles branch into alveolar ducts that lead into outpocketings called alveolar sacs. Alveoli, tiny expansions of the wall of the sacs, form clusters that resemble bunches of grapes. The average person has about 300 million gas-filled alveoli in the lungs. These provide an enormous surface area for gas exchange. Spread flat, the average adult males respiratory surface would be about 750 sq ft (70 m2), approximately the size of a handball court. Arterioles and venules make up a capillary network that surrounds the alveoli. Gas diffusion occurs rapidly across the walls of the alveoli and nearby capillaries. The alveolar-capillary membrane together is extremely thin, about 0.5 in (1.3 cm) thick.

The rate of external respiration in the lungs depends on several factors. One is the difference in concentration (partial pressure) of the respiratory gases in the alveolus and in the blood. Oxygen diffuses out of the alveolus into the blood because its partial pressure is greater in the alveolus than in the capillary. In the capillary, oxygen binds reversibly to hemoglobin in red blood cells and is transported to body tissues. Carbon dioxide diffuses out of the capillary and into the alveolus because its partial pressure is greater in the capillary than in the alveolus. In addition, the rate of gas exchange is higher as the surface area is larger and the membrane thinner. Finally, the diffusion rate depends on airflow. Rapid breathing brings in more air and speeds up the gas exchange.

The result of external respiration is that blood leaves the lungs laden with oxygen and cleared of carbon dioxide. When this blood reaches the cells of the body, internal respiration takes place. Under a higher partial pressure in the capillaries, oxygen breaks away from hemoglobin, diffuses into the tissue fluid, and then into the cells. Conversely, concentrated carbon dioxide under higher partial pressure in the cells diffuses into the tissue fluid and then into the capillaries. The deoxygenated blood carrying carbon dioxide then returns to the lungs for another cycle.

Pulmonary ventilation

Pulmonary ventilation, or breathing, exchanges gases between the outside air and the alveoli of the lungs. Ventilation, which is mechanical in nature, depends on a difference between the atmospheric air pressure and the pressure in the alveoli. When humans expand the lungs to inhale, the lungs increase internal volume and reduce internal pressure. Lung expansion is brought about by two important muscles, the diaphragm and the intercostal muscles. The diaphragm is a dome-shaped sheet of muscle located below the lungs that separates the thoracic and abdominal cavities. When the diaphragm contracts, it moves down. The dome is flattened and the size of the chest cavity is increased, lowering pressure on the lungs. When the intercostal muscles, which are located between the ribs, contract, the ribs move up and outward. Their action also increases the size of the chest cavity and lowers the pressure on the lungs. By contracting, the diaphragm and intercostal muscles reduce the internal pressure relative to the atmospheric pressure. Consequently, air rushes into the lungs. When humans exhale, the reverse occurs. The diaphragm relaxes, and its dome curves up into the chest cavity, while the intercostal muscles relax and bring the ribs down and inward. The diminished size of the chest cavity increases the pressure in the lungs, thereby forcing out the air.

Physicians use an instrument called a spirometer to measure the tidal volume, that is, the amount of air that lungs exchange during a ventilation cycle. Under normal circumstances, humans inhale and exhale about 500 ml, or about a pint, of air in each cycle. Only about 350 ml of the tidal volume reaches the alveoli. The rest of the air remains in the respiratory tract. With a deep breath, humans can take in an additional 3,000 ml (3 liters or a little more than 6 pints) of air. The total lung capacity is about 6 liters on average. The largest volume of air that can be ventilated is referred to as the vital capacity. Trained athletes have a high vital capacity. Regardless of the volume of air ventilated, the lung always retains about 1,200 ml (3 pints) of air. This residual volume of air keeps the alveoli and bronchioles partially filled at all times.

A healthy adult ventilates about 12 times per minute, but this rate changes with exercise and other factors. The basic breathing rate is controlled by breathing centers in the medulla and the pons in the brain. Nerves from the breathing centers conduct impulses to the diaphragm and intercostal muscles, stimulating them to contract or relax. There is an inspiratory center for inhaling and an expiratory center for exhaling in the medulla. Before we inhale, the inspiratory center becomes activated. It sends impulses to the breathing muscles. The muscles contract and we inhale. Impulses from a breathing center in the pons turn off the inspiratory center before the lungs get too full. A second breathing center in the pons stimulates the inspiratory center to prolong inhaling when needed. During normal quiet breathing, humans exhale passively as the lungs recoil and the muscles relax. For rapid and deep breathing, however, the expiratory center becomes active and sends impulses to the muscles to bring on forced exhalations.

The normal breathing rate changes to match the bodys needs. Humans can consciously control how fast and deeply the body breathes. Humans can even stop breathing for a short while. This occurs because the cerebral cortex has connections to the breathing centers and can override their control. Voluntary control of breathing allows humans to avoid breathing in water or harmful chemicals for brief periods of time. People cannot, however, consciously stop breathing for a prolonged period. A buildup of carbon dioxide and hydrogen ions in the bloodstream stimulates the breathing centers to become active no matter what humans want to do.

Humans are not in conscious control of all the factors that affect the breathing rate. For example, tension on the vessels of the bronchial tree affects the breathing rate. Specialized stretch receptors in the bronchi and bronchioles detect excessive stretching caused by too much air in the lungs. They transmit the information on nerves to the breathing centers, which in turn inhibit breathing. Certain chemical substances in the blood also help control the rate of breathing. Hydrogen ions, carbon dioxide, and oxygen are detected by specialized chemoreceptors. Inside cells, carbon dioxide (CO2) combines with water (H2O) to form carbonic acid (H2CO3). The carbonic acid breaks down rapidly into hydrogen ions and bicarbonate ions. Therefore, an increase in carbon dioxide results in an increase in hydrogen ions, while a decrease in carbon dioxide brings about a decrease in hydrogen ions. These substances diffuse into the blood. When people exercise, cells use up oxygen and produce carbon dioxide at a higher than average rate. As a result, chemoreceptors in the medulla and in parts of the peripheral nervous system detect a raised level of carbon dioxide and hydrogen ions. They signal the inspiratory center, which in turn sends impulses to the breathing muscles to breathe faster and deeper. A lack of oxygen also stimulates increased breathing, but it is not as strong a stimulus as the carbon dioxide and hydrogen ion surpluses. A large decrease in oxygen stimulates the peripheral chemoreceptors to signal the inspiratory center to increase breathing rate.

In addition to chemoreceptors, there are receptors in the body that detect changes in movement and pressure. Receptors in joints detect movement and signal the inspiratory center to increase breathing rate. When receptors in the circulatory system detect a rise in blood pressure, they stimulate slower breathing. Lowered blood pressure stimulates more rapid breathing. Increased body temperature and prolonged pain also elevate the rate of pulmonary ventilation.

Respiratory disorders

The respiratory system is open to airborne microbes and to outside pollution. It is not surprising that respiratory diseases occur, in spite of the bodys defenses. Some respiratory disorders are relatively mild and, unfortunately, very familiar. People all experience the excess mucus, coughing, and sneezing of the common cold from time to time. The common cold is an example of rhinitis, an inflammation of the epithelium lining the nose and nasal cavity. Viruses, bacteria, and allergens are among the causes of rhinitis.

Since the respiratory lining is continuous, nasal cavity infections often spread. Laryngitis, an inflammation of the vocal cords, results in hoarseness and loss of voice. Swelling of the inflamed vocal cords interferes with or prevents normal vibration. Pathogens, irritating chemicals in the air, and overuse of the voice are causes of laryngitis.

Pneumonia, inflammation of the alveoli, is most commonly caused by bacteria and viruses. During a bout of pneumonia, the inflamed alveoli fill up with fluid and dead bacteria, and the external respiration rate drops. Patients come down with fever, chills, and pain, coughing up phlegm and sometimes blood. Sufferers of bronchitis, an inflammation of the bronchi, also cough up thick phlegm. There are two types of bronchitis, acute and chronic. Acute bronchitis can be a complication of a cold or flu. Bacteria, smoking, and air pollution can also cause acute bronchitis. This type of bronchitis clears up in a short time.

Chronic bronchitis and emphysema are termed chronic obstructive pulmonary disease (COPD), in which the airways are obstructed and the respiratory surface is diminished. COPD patients do not improve without treatment. Air pollution and cigarette smoking are the main causes of COPD. Nonsmokers who inhale the smoke of otherspassive smokersare also at risk. Smoking stimulates the lining cells in the bronchi to produce mucus. This causes the epithelium lining the bronchi and its branches to thicken and thereby narrow. Patients cough up phlegm and experience breathlessness as well as strain on the heart. In emphysema, also caused by smoking, the alveolar walls disintegrate and the alveoli blend together. They form large air pockets from which the air does not escape. This cuts down the surface area for gas exchange. It becomes difficult for the patient to exhale. The extra work of exhaling over several years can cause the chest to enlarge and become barrel-shaped. The body is unable to repair the damage to the lungs brought on by COPD, and the disease can lead to respiratory failure. During respiratory failure, the respiratory system does not supply sufficient oxygen to sustain the organism.

In addition to COPD, lung cancer also destroys lung tissue. The most common type of cancer in the United States, lung cancer is the leading cause of cancer death in men. According to the Cancer Journal for Clinicians, lung cancer surpassed breast cancer (in 1987) as the largest cause of cancer death in women. As of 2005, lung cancer accounted for 27% of all female cancer deaths. Cigarette smoking is the main cause of lung cancer. Passive smokers are also at risk. Air pollution, radioactive minerals, and asbestos also cause lung cancer. The symptoms of the disease include a chronic cough from bronchitis, coughing up blood,

KEY TERMS

Alveolus (plural, alveoli) An air sac of the lung, in which oxygen and carbon dioxide exchange occurs.

Breathing centers Specialized areas in the medulla and pons that regulate the basic rate of breathing.

Bronchial tree Branching, air-conducting subdivisions of the bronchi in the lungs.

COPD Chronic obstructive pulmonary disease, in which the air passages of the lungs become narrower and obstructed. Includes chronic bronchitis and emphysema.

Gill filaments Finely divided surface of a gill of a fish or other aquatic animal where gas exchange takes place.

Tracheae Tubes in land arthropods that conduct air from opening in body walls to body tissues.

shortness of breath, and chest pain. Lung cancer can spread in the lung area. Unchecked, it can metastasize (spread) to other parts of the body. Physicians use surgery, anticancer drugs, and radiation therapy to destroy the cancer cells and contain the disease.

See also Cigarette smoke.

Resources

BOOKS

LaRue, Charles J. Biology. Circle Pines, MN: AGS Publishing, 2004.

Miller, Kenneth R., and Joseph S. Levine. Prentice Hall Biology. Upper Saddle River, NJ: Pearson Prentice Hall, 2006.

Petechuk, David. The Respiratory System. Westport, CT: Greenwood Press, 2004.

Thibodeau, Gary A. Anatomy & Physiology. St. Louis, MP: Elsevier Mosby, 2007.

PERIODICALS

Martinez, F.D. The Coming-of-age of the Hygiene Hypothesis. Respiratory Research no. 2 (March 2001): 129-132.

OTHER

The Canadian Lung Association. Inside the Human Body: The Respiratory System. <http://www.lung.ca/children/index_kids.html> (accessed October 25, 2006).

Bernice Essenfeld

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Respiratory System

Respiratory system

Aerobic organisms take in oxygen from the external environment and release carbon dioxide in a process known as respiration . At the most basic level, this exchange of gases takes place in cells and involves the release of energy from food materials by oxidation. Carbon dioxide is produced as a waste product of these oxidation reactions. The gas exchange in cells is called cellular respiration. In single-celled organisms, the oxygen and carbon dioxide simply diffuse through the cell membrane . Respiration in multicellular organisms, however, is a much more complex process involving a specialized respiratory system that plays an intermediary role between the cells and the external environment. While the respiratory organs of some complex organisms such as insects communicate directly with internal tissues, respiration in vertebrates also involves the circulatory system , which carries gases between cells and respiratory organs.

The respiratory system must meet two important criteria. First, the respiratory surface must be large enough to take in oxygen in sufficient quantities to meet the organism's needs and release all waste gas quickly. Some animals, such as the earthworm, use the entire body surface as a respiratory organ . The internal respiratory organs of vertebrates generally have many lobes to enlarge the surface area. Second, the respiratory membrane must be moist, since gases require water to diffuse across membranes. The watery environment keeps the respiratory surface moist for aquatic animals. A problem exists for land animals, whose respiratory surfaces can dry out in open air. As a result, animals such as the earthworm must live in damp places. Internal respiratory organs provide an environment that is easier to keep moist.


Respiration in the earthworm

The earthworm uses its moist outer skin as a respiratory organ. Oxygen diffuses across the body surface and enters blood in the dense capillary mesh that lies just below the skin. Blood carries the oxygen to the body cells. There, it picks up carbon dioxide and transports it to the skin capillaries where it diffuses out of the body. The skin is effective as an organ of respiration in small wormlike animals where there is a high ratio of surface to volume .


Respiration in insects

Tiny air tubes called tracheae branch throughout the insect's body. Air enters the tracheae through holes in the body wall called spiracles, which are opened and closed by valves. In larger insects, air moves through the tracheae when the body muscles contract. The tracheae are invaginated—that is, folded into the body—and thereby kept moist. Thickened rings in the walls of the tracheae help support them. These vessels branch into smaller vessels called tracheoles, which lack the supportive rings. The tracheoles carry air directly to the surface of individual cells, where they branch further to deliver oxygen and pick up carbon dioxide. A fluid in the endings of tracheoles regulates how much air contacts the cells. If a cell needs oxygen, the fluid pulls back and exposes the cell membrane to the air.

Respiratory system of fish

Gills mediate the gas exchange in fish . These organs, located on the sides of the head, are made up of gill filaments, feathery structures that provide a large surface for gas exchange. The filaments are arranged in rows in the gill arches, and each filament has lamellae, discs that contain capillaries. Blood enters and leaves the gills through these small blood vessels. Although gills are restricted to a small section of the body, the immense respiratory surface created by the gill filaments provides the whole animal with an efficient gas exchange. The surrounding water keeps the gills wet.

A flap, the operculum, covers and protects the gills of bony fish . Water containing dissolved oxygen enters the fish's mouth, and the animal moves its jaws and operculum in such a way as to pump the incoming water through the gills. As water passes over the gill filaments, blood inside the capillaries picks up the dissolved oxygen. Since the blood in the capillaries flows in a direction opposite to the flow of water around the gill filaments, there is a good opportunity for absorption. The circulatory system then transports the oxygen to all body tissues and picks up carbon dioxide, which is removed from the body through the gills. After the water flows through the gills, it exits the body behind the fish's operculum.


Respiration in terrestrial vertebrates

Lungs are the internal respiratory organs of amphibians , reptiles , birds , and mammals . The lungs, paired invaginations located in one area of the body, provide a large, thin, moist surface for gas exchange. Lungs work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body. The circulatory system also transports carbon dioxide from body cells to the lungs to be exhaled. The process of inhaling and exhaling is called pulmonary ventilation.

Besides these similarities, there is a great variety in the respiratory systems of terrestrial vertebrates. Frogs , for instance, have balloon-like lungs that do not have a very large surface area. Diffusion across the frog's moist skin supplements the gas exchange through the lungs. Birds have about eight thin-walled air sacs attached to their lungs. The air sacs take up space in the entire body
cavity and in some of the bones. When birds inhale, air passes through a tube called the bronchus and enters the air sacs located in the posterior (rear) of the animal. At the same time, air in the lungs moves forward to air sacs located in the anterior (front). When birds exhale, the air from the anterior air sacs moves to the outside, while air from the posterior sacs moves into the lungs. This efficient system moves air forward through the lungs both when the bird inhales and exhales. Blood in the capillaries of the lungs flows against the air current, which again increases respiratory efficiency. Birds are capable of flying at high altitudes, where the air has a low oxygen content, because of these adaptations of the respiratory system.

Human respiratory system


The human respiratory system, working in conjunction with the circulatory system, supplies oxygen and removes carbon dioxide. The respiratory system conducts air to the respiratory surfaces of lung units. There, the blood in the lung capillaries readily absorbs oxygen, and gives off carbon dioxide gathered from the body cells. The circulatory system transports oxygen-laden blood to the body cells and picks up carbon dioxide. The term respiration describes the exchange of gases across cell membranes both in the lungs (external respiration) and in the body tissues (internal respiration). Pulmonary ventilation, or breathing, exchanges volumes of air with the external environment.


The respiratory tract

The human respiratory system consists of the respiratory tract and the lungs. The respiratory tract can again be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity, pharynx (throat) and larynx (voicebox). The lower part consists of the trachea (windpipe), bronchi, and bronchial tree . The respiratory tract cleans, warms, and moistens air during its trip to the lungs. The nose has openings to the outside that allow air to enter. Hairs inside the nose trap dirt and keep it out of the respiratory tract. The external nose leads to a large cavity within the skull. This cavity and the space inside the nose make up the nasal cavity. A nasal septum, supported by cartilage and bone, divides the nasal cavity into a right and left side. Epithelium, a layer of cells that secrete mucus and cells equipped with cilia, lines the nasal passage. Mucus moistens the incoming air and traps dust. The cilia move pieces of the mucus with its trapped particles to the throat, where it is spit out or swallowed. Stomach acids destroy bacteria in swallowed mucus. Sinuses, epithelium-lined cavities in bone, surround the nasal cavity. Blood vessels in the nose and nasal cavity release heat and warm the entering air.

Air leaves the nasal cavity and enters the throat or pharynx. From there it passes into the larynx, which is located between the pharynx and the trachea or windpipe. A framework of cartilage pieces supports the larynx, which is covered by the epiglottis, a flap of elastic cartilage that moves up and down like a trap door. When we breathe, the epiglottis stays open, but when we swallow, it closes. This valve mechanism keeps solid particles and liquids out of the trachea. If we breathe in something other than air, we automatically cough and expel it. Should these protective mechanisms fail, allowing solid food to lodge in and block the trachea, the victim is in imminent danger of asphyxiation.

Air enters the trachea in the neck. Epithelium lines the trachea as well as all the other parts of the respiratory tract. C-shaped cartilage rings reinforce the wall of the trachea and all the passageways in the lower respiratory tract. Elastic fibers in the trachea walls allow the airways to expand and contract when we inhale and exhale, while the cartilage rings prevent them from collapsing. The trachea divides behind the sternum to form a left and right bronchus, each entering a lung. Inside the lungs, the bronchi subdivide repeatedly into smaller airways. Eventually they form tiny branches called terminal bronchioles. Terminal bronchioles have a diameter of about 0.02 in (0.5 mm). The branching air-conducting network within the lungs is called the bronchial tree.

The respiratory tract is not dedicated to respiration alone but plays a major role in many other bodily functions as well. The pharynx in particular is a multipurpose organ. It is a passageway for food as well as air, since the mouth cavity also leads to it. The back of the pharynx leads into the esophagus (food tube) of the digestive system . The front leads into the larynx and the rest of the respiratory system. Small amounts of air pass between holes in the pharynx and the Eustachian tubes of the ear to equalize the gas pressure inside the ears, nose, and throat. The pharynx also contains lymph glands called tonsils and adenoids, which play a role in the immune system . Finally, the pharynx, which doubles as a resonating chamber, also plays a role in the production of sound, to which many other parts of the respiratory tract also contribute.

The vocal cords, a pair of horizontal folds inside the larynx, vibrate to produce sound from exhaled air. When we speak, muscles change the size of the vocal cords and the space between them, known as the glottis. The shape and size of the vocal cords determine the pitch of the sound produced. The glottis widens for deep tones and narrows for high-pitched ones. Longer, thicker vocal cords, which vibrate more slowly, produce a deeper sound. The force with which air is expelled through the larynx determines the volume of the sound produced. Voice quality also depends on several other factors, including the shape of the nasal cavities, sinuses, pharynx, and mouth, which all function as resonating chambers.


The lungs

The lungs are two cone-shaped organs located in the thoracic cavity, or chest, and are separated by the heart . The right lung is somewhat larger than the left. The pleural membrane surrounds and protects the lungs. One layer of the pleural membrane attaches to the wall of the thoracic cavity, and the other layer encloses the lungs. A fluid between the two membrane layers reduces friction and allows smooth movement of the lungs during breathing. The lungs are divided into lobes, each one of which receives its own bronchial branch. The bronchial branch subdivides and eventually leads to the terminal bronchi. These tiny airways lead into structures called respiratory bronchioles.

The respiratory bronchioles branch into alveolar ducts that lead into outpocketings called alveolar sacs. Alveoli, tiny expansions of the wall of the sacs, form clusters that resemble bunches of grapes . The average person has a total of about 300 million gas-filled alveoli in the lungs. These provide an enormous surface area for gas exchange. Spread flat, the average adult male's respiratory surface would be about 750 sq ft (70 m2), approximately the size of a handball court. Arterioles and venules make up a capillary network that surrounds the alveoli. Gas diffusion occurs rapidly across the walls of the alveoli and nearby capillaries. The alveolar-capillary membrane together is extremely thin, about 0.5 in (1.3 cm) thick.

The rate of external respiration in the lungs depends on several factors. One is the difference in concentration (partial pressure) of the respiratory gases in the alveolus and in the blood. Oxygen diffuses out of the alveolus into the blood because its partial pressure is greater in the alveolus than in the capillary. In the capillary, oxygen binds reversibly to hemoglobin in red blood cells and is transported to body tissues. Carbon dioxide diffuses out of the capillary and into the alveolus because its partial pressure is greater in the capillary than in the alveolus. In addition, the rate of gas exchange is higher as the surface area is larger and the membrane thinner. Finally, the diffusion rate depends on airflow. Rapid breathing brings in more air and speeds up the gas exchange.

The result of external respiration is that blood leaves the lungs laden with oxygen and cleared of carbon dioxide. When this blood reaches the cells of the body, internal respiration takes place. Under a higher partial pressure in the capillaries, oxygen breaks away from hemoglobin, diffuses into the tissue fluid, and then into the cells. Conversely, concentrated carbon dioxide under higher partial pressure in the cells diffuses into the tissue fluid and then into the capillaries. The deoxygenated blood carrying carbon dioxide then returns to the lungs for another cycle.


Pulmonary ventilation

Pulmonary ventilation, or breathing, exchanges gases between the outside air and the alveoli of the lungs. Ventilation, which is mechanical in nature, depends on a difference between the atmospheric air pressure and the pressure in the alveoli. When we expand the lungs to inhale, we increase internal volume and reduce internal pressure. Lung expansion is brought about by two important muscles, the diaphragm and the intercostal muscles. The diaphragm is a dome-shaped sheet of muscle located below the lungs that separates the thoracic and abdominal cavities. When the diaphragm contracts, it moves down. The dome is flattened, and the size of the chest cavity is increased, lowering pressure on the lungs. When the intercostal muscles, which are located between the ribs, contract, the ribs move up and outward. Their action also increases the size of the chest cavity and lowers the pressure on the lungs. By contracting, the diaphragm and intercostal muscles reduce the internal pressure relative to the atmospheric pressure . As a consequence, air rushes into the lungs. When we exhale, the reverse occurs. The diaphragm relaxes, and its dome curves up into the chest cavity, while the intercostal muscles relax and bring the ribs down and inward. The diminished size of the chest cavity increases the pressure in the lungs, thereby forcing out the air.

Physicians use an instrument called a spirometer to measure the tidal volume, that is, the amount of air we exchange during a ventilation cycle. Under normal circumstances, we inhale and exhale about 500 ml, or about a pint, of air in each cycle. Only about 350 ml of the tidal volume reaches the alveoli. The rest of the air remains in the respiratory tract. With a deep breath, we can take in an additional 3,000 ml (3 liters or a little more than 6 pints) of air. The total lung capacity is about 6 liters on average. The largest volume of air that can be ventilated is referred to as the vital capacity. Trained athletes have a high vital capacity. Regardless of the volume of air ventilated, the lung always retains about 1,200 ml (3 pints) of air. This residual volume of air keeps the alveoli and bronchioles partially filled at all times.

A healthy adult ventilates about 12 times per minute, but this rate changes with exercise and other factors. The basic breathing rate is controlled by breathing centers in the medulla and the pons in the brain . Nerves from the breathing centers conduct impulses to the diaphragm and intercostal muscles, stimulating them to contract or relax. There is an inspiratory center for inhaling and an expiratory center for exhaling in the medulla. Before we inhale, the inspiratory center becomes activated. It sends impulses to the breathing muscles. The muscles contract and we inhale. Impulses from a breathing center in the pons turn off the inspiratory center before the lungs get too full. A second breathing center in the pons stimulates the inspiratory center to prolong inhaling when needed. During normal quiet breathing, we exhale passively as the lungs recoil and the muscles relax. For rapid and deep breathing, however, the expiratory center becomes active and sends impulses to the muscles to bring on forced exhalations.

The normal breathing rate changes to match the body's needs. We can consciously control how fast and deeply we breathe. We can even stop breathing for a short while. This occurs because the cerebral cortex has connections to the breathing centers and can override their control. Voluntary control of breathing allows us to avoid breathing in water or harmful chemicals for brief periods of time. We cannot, however, consciously stop breathing for a prolonged period. A buildup of carbon dioxide and hydrogen ions in the bloodstream stimulates the breathing centers to become active no matter what we want to do.

We are not in conscious control of all the factors that affect our breathing rate. For example, tension on the vessels of the bronchial tree affects the breathing rate. Specialized stretch receptors in the bronchi and bronchioles detect excessive stretching caused by too much air in the lungs. They transmit the information on nerves to the breathing centers, which in turn inhibit breathing. Certain chemical substances in the blood also help control the rate of breathing. Hydrogen ions, carbon dioxide, and oxygen are detected by specialized chemoreceptors. Inside cells, carbon dioxide (CO2) combines with water (H2O) to form carbonic acid (H2CO3). The carbonic acid breaks down rapidly into hydrogen ions and bicarbonate ions. Therefore, an increase in carbon dioxide results in an increase in hydrogen ions, while a decrease in carbon dioxide brings about a decrease in hydrogen ions. These substances diffuse into the blood. When we exercise, our cells use up oxygen and produce carbon dioxide at a higher than average rate. As a result, chemoreceptors in the medulla and in parts of the peripheral nervous system detect a raised level of carbon dioxide and hydrogen ions. They signal the inspiratory center, which in turn sends impulses to the breathing muscles to breathe faster and deeper. A lack of oxygen also stimulates increased breathing, but it is not as strong a stimulus as the carbon dioxide and hydrogen ion surpluses. A large decrease in oxygen stimulates the peripheral chemoreceptors to signal the inspiratory center to increase breathing rate.

In addition to chemoreceptors, there are receptors in the body that detect changes in movement and pressure. Receptors in joints detect movement and signal the inspiratory center to increase breathing rate. When receptors in the circulatory system detect a rise in blood pressure, they stimulate slower breathing. Lowered blood pressure stimulates more rapid breathing. Increased body temperature and prolonged pain also elevate the rate of pulmonary ventilation.


Respiratory disorders

The respiratory system is open to airborne microbes and to outside pollution . It is not surprising that respiratory diseases occur, in spite of the body's defenses. Some respiratory disorders are relatively mild and, unfortunately, very familiar. We all experience the excess mucus, coughing, and sneezing of the common cold from time to time. The common cold is an example of rhinitis, an inflammation of the epithelium lining the nose and nasal cavity. Viruses, bacteria, and allergens are among the causes of rhinitis.

Since the respiratory lining is continuous, nasal cavity infections often spread. Laryngitis , an inflammation of the vocal cords, results in hoarseness and loss of voice. Swelling of the inflamed vocal cords interferes with or prevents normal vibration. Pathogens , irritating chemicals in the air, and overuse of the voice are causes of laryngitis.

Pneumonia , inflammation of the alveoli, is most commonly caused by bacteria and viruses. During a bout of pneumonia, the inflamed alveoli fill up with fluid and dead bacteria, and the external respiration rate drops. Patients come down with fever, chills, and pain, coughing up phlegm and sometimes blood. Sufferers of bronchitis , an inflammation of the bronchi, also cough up thick phlegm. There are two types of bronchitis, acute and chronic. Acute bronchitis can be a complication of a cold or flu. Bacteria, smoking, and air pollution can also cause acute bronchitis. This type of bronchitis clears up in a short time.

Chronic bronchitis and emphysema are termed chronic obstructive pulmonary disease (COPD), in which the airways are obstructed and the respiratory surface is diminished. COPD patients do not improve without treatment. Air pollution and cigarette smoking are the main causes of COPD. Nonsmokers who inhale the smoke of others—passive smokers—are also at risk. Smoking stimulates the lining cells in the bronchi to produce mucus. This causes the epithelium lining the bronchi and its branches to thicken and thereby narrow. Patients cough up phlegm and experience breathlessness as well as strain on the heart. In emphysema, also caused by smoking, the alveolar walls disintegrate and the alveoli blend together. They form large air pockets from which the air does not escape. This cuts down the surface area for gas exchange. It becomes difficult for the patient to exhale. The extra work of exhaling over several years can cause the chest to enlarge and become barrel-shaped. The body is unable to repair the damage to the lungs brought on by COPD, and the disease can lead to respiratory failure. During respiratory failure, the respiratory system does not supply sufficient oxygen to sustain the organism .

In addition to COPD, lung cancer also destroys lung tissue. The most common type of cancer in the United States, lung cancer is the leading cause of cancer death in men. It is the second leading cause of cancer death, after breast cancer, in women. Cigarette smoking is the main cause of lung cancer. Passive smokers are also at risk. Air pollution, radioactive minerals , and asbestos also cause lung cancer. The symptoms of the disease include a chronic cough from bronchitis, coughing up blood, shortness of breath, and chest pain. Lung cancer can spread in the lung area. Unchecked, it can metastasize (spread) to other parts of the body. Physicians use surgery , anticancer drugs, and radiation therapy to destroy the cancer cells and contain the disease.

See also Cigarette smoke.

Resources

books

Blaustein, Daniel. Biology: The Dynamics of Life. Westerville, OH: McGraw-Hill Companies, 1998.

Essenfeld, Bernice, Carol R. Gontang, and Randy Moore. Biology. Menlo Park, CA: Addison-Wesley Publishing Co., 1996.

Marieb, Elaine Nicpon. Human Anatomy & Physiology. 5th ed. San Francisco: Benjamin/Cummings, 2000.


periodicals

Crapo, Robert O. "Pulmonary Function Testing." New EnglandJournal of Medicine (July 7, 1994).

Martinez, F.D. "The Coming-of-age of the Hygiene Hypothesis." Respiratory Research no. 2 (March 2001): 129-132.


other

The Human Voice. VHS. Princeton, NJ: Films for the Humanities and Sciences, 1995.

Respiration. VHS. Princeton, NJ: Films for the Humanities and Sciences, 1995.


Bernice Essenfeld

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alveolus (plural, alveoli)

—An air sac of the lung, in which oxygen and carbon dioxide exchange occurs.

Breathing centers

—Specialized areas in the medulla and pons that regulate the basic rate of breathing.

Bronchial tree

—Branching, air-conducting subdivisions of the bronchi in the lungs.

COPD

—Chronic obstructive pulmonary disease, in which the air passages of the lungs become narrower and obstructed. Includes chronic bronchitis and emphysema.

Gill filaments

—Finely divided surface of a gill of a fish or other aquatic animal where gas exchange takes place.

Tracheae

—Tubes in land arthropods that conduct air from opening in body walls to body tissues.

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Respiratory System

Respiratory System


The respiratory system is composed of those organs and processes that allow an animal to take oxygen into its body and expel carbon dioxide. All living things require oxygen to survive, and although respiratory systems may vary in size and the way they function, they share many basic features and operate on the same principles.

All animals need oxygen to survive, because oxygen is the fuel they use to convert their food into energy. As with any fuel that is consumed, there is always a byproduct given off, and in the case of animals, this waste product is carbon dioxide. Since all animals need to constantly take in oxygen and eliminate carbon dioxide, they need some system to perform this gas exchange, which is called respiration. Although a variety of systems and methods accomplish this, all operate on the same basic principle called diffusion.

DIFFUSION VITAL TO RESPIRATION

Diffusion can be described as the movement or spreading out of a substance from an area in which it is highly concentrated to an area of its lowest concentration. Diffusion takes place at the level of molecules, and since molecules are constantly in motion, they have a natural tendency to mix randomly with one another. Although it is not known exactly why molecules behave this way, they have a natural tendency to move from where they are all together to where there are the fewest of them, in an effort to spread themselves out more evenly. Although any given single molecule may not always behave this way, the net or overall movement of a group of molecules will always do so. This net movement is called diffusion. An everyday example of diffusion is the way in which tobacco smoke or strong perfume will spread throughout the still air of a closed room.

The respiratory system of an animal always carries out diffusion across a respiratory surface. There is always some sort of membrane across which the gases (oxygen and carbon dioxide) pass in and out of a body. When there is more oxygen in the environment outside a membrane than there is inside, oxygen will automatically drift through the membrane to equalize the pressure on both sides. Although this may sound like osmosis (which could be described as diffusion across a membrane), osmosis only involves the passage of a substance dissolved in a liquid solution. In respiration, however, gases are exchanged.

INTEGUMENTARY EXCHANGE

In the simplest animal respiratory system, gases are exchanged directly through the body tissues themselves. This is called integumentary exchange, since an animal's integument is its protective outer covering or skin. The one-celled organisms that practice this form of respiration are too simple to have any skin, and instead have a membrane that allows oxygen and carbon dioxide to diffuse right through it and reach every part of its "body." More complicated animals, like earthworms, also perform this type of "skin breathing," but they also use a simple type of circulatory system to move the gases from the inside of their body to their skin. Studying an earthworm also reveals a major characteristic of all respiratory systems—they must always be wet. The slimy, mucus-covered skin of an earthworm demonstrates that diffusion will only take place if the respiratory surface or membrane is always kept moist. If an earthworm dries out, it will suffocate.

TRACHEAL SYSTEM

Skin breathing may be fine for very small organisms, but when the outer area of an animal cannot provide a large enough surface area or is too thick or hardened to allow a good gas exchange, then the animal needs some type of specialized respiratory organs. One simple form of such specialized organs is the tracheal system found in insects. In this unique system, oxygen and carbon dioxide are exchanged through diffusion by means of a network of small, stiff tubes called tracheae that extend into the insect's body and are small enough to supply individual cells. Insects have no need of lungs or any connection to a circulatory system.

GILLS

The gills that fish use to breathe are the next most sophisticated system, and they are designed specifically to work in water. A gill is a collection of thin flaps (called lamellae) that are able to obtain oxygen dissolved in water. As water moves over the lamellae's thin membrane, dissolved oxygen diffuses into the fish's blood, while carbon dioxide diffuses out through the gills and into the water. Since water contains only five percent of the oxygen that air does, gills must be especially efficient.

They only will work, however, if water keeps moving over their surface, and a fish needs to keep its mouth open as its swims to keep the water moving over the gills. When a fish is not moving, it closes its gills, takes a mouthful of water, and forces it out its gills. Fish cannot breathe out of water because the filaments that contain their lamellae collapse when they are not supported by water. The gills of most fish are located behind their head.

THE HUMAN RESPIRATORY SYSTEM

Lungs are the respiratory organs used by mammals, birds, and reptiles, as well as some amphibians. Lungs exchange gases, since it is the job of the circulatory system to collect and distribute gases throughout the organism. The human lungs are located in the chest cavity, although the human respiratory system actually begins at the nose and mouth where air is inhaled and exhaled. The mouth and nose are connected to a common tube at the back of the throat called a pharynx. Air then passes through the larynx, also called the voice box, into a branching system that resembles an upside-down tree. The larynx flows into the trachea (the tree trunk), which divides into two large limbs called the right and left bronchi. Each of these branch off into multiple smaller bronchi, which continue dwindling down into smaller and smaller tubes that finally end in terminal bronchioles. At the end of these bronchioles, like the leaves of tree, are clusters of moist air sacs called alveoli where the actual exchange of gases takes place. Tiny, thin-walled blood vessels called capillaries create vast networks surrounding the alveoli. It is across these thin walls that oxygen is passed into the blood and carbon dioxide out of the blood by diffusion. Humans have a total of 300,000,000 alveoli in both lungs. Air is actually forced into the lungs by a large muscle beneath the lungs called a diaphragm. When its muscles contract and pull down, the ribs above are lifted upward and outward, and air rushes into the elastic lungs. When air is automatically sucked into the lungs as they expand (inspiration), it is called negative pressure breathing. Expiration occurs when the diaphragm muscles relax, allowing the lungs to go back to their retracted state. Air containing carbon dioxide is then forced out of the lungs. Breathing is controlled by a brain command that sends a signal every few seconds. The average adult human takes between twelve and fifteen breaths a minute.

Because the human respiratory system brings in substances from the outside environment, there are safeguards to fight infectious agents that may enter with the oxygen. The normal human lung is sterile, meaning that there are no bacteria or viruses present. The first line of defense includes hair in the nostrils that filters large particles. The epiglottis between the pharynx and larynx acts as a trap door and prevents food and other swallowed substances from entering the larynx and then the trachea. If it does, we cough involuntarily and say that "something went down the wrong pipe." The acts of sneezing and coughing are usually both started by irritants in the respiratory system, and work to forcibly expel them. Finally, mucus exists throughout the system and serves to trap dust and infectious organisms. Cells called macrophages line the respiratory tract and engulf and kill anything they consider an invader.

Plants can be said to "breathe" since they exchange gases during photosynthesis (which is actually the reverse of respiration since it takes in carbon dioxide and gives off oxygen in the process of making food). However, they do not have any respiratory system approximating that of an animal, since plants exchange gases through simple openings or pores in their leaves called stomata.

[See alsoBlood; Respiration ]

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