Respiratory System
Respiratory System
Respiration in terrestrial vertebrates
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 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 worm-like 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 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 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 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 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 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 body’s 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 body’s 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 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. 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