Skip to main content
Select Source:

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 ]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Respiratory System." UXL Encyclopedia of Science. . Encyclopedia.com. 17 Dec. 2017 <http://www.encyclopedia.com>.

"Respiratory System." UXL Encyclopedia of Science. . Encyclopedia.com. (December 17, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

"Respiratory System." UXL Encyclopedia of Science. . Retrieved December 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

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/>.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Respiratory System." Animal Sciences. . Encyclopedia.com. 17 Dec. 2017 <http://www.encyclopedia.com>.

"Respiratory System." Animal Sciences. . Encyclopedia.com. (December 17, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/respiratory-system

"Respiratory System." Animal Sciences. . Retrieved December 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/respiratory-system

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

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"respiratory system." World Encyclopedia. . Encyclopedia.com. 17 Dec. 2017 <http://www.encyclopedia.com>.

"respiratory system." World Encyclopedia. . Encyclopedia.com. (December 17, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

"respiratory system." World Encyclopedia. . Retrieved December 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

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.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"respiratory system." A Dictionary of Nursing. . Encyclopedia.com. 17 Dec. 2017 <http://www.encyclopedia.com>.

"respiratory system." A Dictionary of Nursing. . Encyclopedia.com. (December 17, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/respiratory-system

"respiratory system." A Dictionary of Nursing. . Retrieved December 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/respiratory-system

respiratory system

respiratory system: see respiration.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"respiratory system." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 17 Dec. 2017 <http://www.encyclopedia.com>.

"respiratory system." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (December 17, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

"respiratory system." The Columbia Encyclopedia, 6th ed.. . Retrieved December 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

respiratory system

respiratory system

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"respiratory system." The Oxford Companion to the Body. . Encyclopedia.com. 17 Dec. 2017 <http://www.encyclopedia.com>.

"respiratory system." The Oxford Companion to the Body. . Encyclopedia.com. (December 17, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/respiratory-system

"respiratory system." The Oxford Companion to the Body. . Retrieved December 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/respiratory-system