The respiratory system consists of organs that deliver oxygen to the circulatory system for transport to the 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 the particles are subsequently 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. A pair of 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 are 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 5 cm (2 inches) 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 such other functions 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 12 to 15 cm (5 to 6 inches) long. Fifteen to twenty 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 brain stem 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 such irritants as cigarette smoke, overuse of the voice, or a viral infection. A person with laryngitis may become hoarse, or they may be able only to whisper until the inflammation is reduced.
Lung cancer—Occurs in those individuals who are exposed to such cancer-causing agents 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-threatening 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 mulitple 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).
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.
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, block 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 such conditions 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, lightheadedness, and numbing of the hands and feet. Treatments include breathing into a paper bag or a mask that induces rebreathing of carbon dioxide.
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 than0.5mm (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.
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: W.B. 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>.
