Respiratory System is responsible to deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death.
While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the normal functioning of cells. It protects the body against disease-causing organisms and toxic substances inhaled with air. The respiratory system also houses the cells that detect smell, and assists in the production of sounds for speech.
The respiratory and circulatory systems work together to deliver oxygen to cells and remove carbon dioxide in a two-phase process called respiration. The first phase of respiration begins with breathing in, or inhalation. Inhalation brings air from outside the body into the lungs. Oxygen in the air moves from the lungs through blood vessels to the heart, which pumps the oxygen-rich blood to all parts of the body. Oxygen then moves from the bloodstream into cells, which completes the first phase of respiration. In the cells, oxygen is used in a separate energy-producing process called cellular respiration, which produces carbon dioxide as a byproduct. The second phase of respiration begins with the movement of carbon dioxide from the cells to the bloodstream. The bloodstream carries carbon dioxide to the heart, which pumps the carbon dioxide-laden blood to the lungs. In the lungs, breathing out, or exhalation, removes carbon dioxide from the body, thus completing the respiration cycle.
NASAL PASSAGE
The flow of air from outside of the body to the lungs begins with the nose, which is divided into the left and right nasal passages. The nasal passages are lined with a membrane composed primarily of one layer of flat, closely packed cells called epithelial cells. Each epithelial cell is densely fringed with thousands of microscopic cilia, fingerlike extensions of the cells. Interspersed among the epithelial cells are goblet cells, specialized cells that produce mucus, a sticky, thick, moist fluid that coats the epithelial cells and the cilia. Numerous tiny blood vessels called capillaries lie just under the mucous membrane, near the surface of the nasal passages. While transporting air to the pharynx, the nasal passages play two critical roles: they filter the air to remove potentially disease-causing particles; and they moisten and warm the air to protect the structures in the respiratory system.
Filtering prevents airborne bacteria, viruses, other potentially disease-causing substances from entering the lungs, where they may cause infection. Filtering also eliminates smog and dust particles, which may clog the narrow air passages in the smallest bronchioles. Coarse hairs found just inside the nostrils of the nose trap airborne particles as they are inhaled. The particles drop down onto the mucous membrane lining the nasal passages. The cilia embedded in the mucous membrane wave constantly, creating a current of mucus that propels the particles out of the nose or downward to the pharynx. In the pharynx, the mucus is swallowed and passed to the stomach, where the particles are destroyed by stomach acid. If more particles are in the nasal passages than the cilia can handle, the particles build up on the mucus and irritate the membrane beneath it. This irritation triggers a reflex that produces a sneeze to get rid of the polluted air.
The nasal passages also moisten and warm air to prevent it from damaging the delicate membranes of the lung. The mucous membranes of the nasal passages release water vapor, which moistens the air as it passes over the membranes. As air moves over the extensive capillaries in the nasal passages, it is warmed by the blood in the capillaries. If the nose is blocked or “stuffy” due to a cold or allergies, a person is forced to breathe through the mouth. This can be potentially harmful to the respiratory system membranes, since the mouth does not filter, warm, or moisten air.
In addition to their role in the respiratory system, the nasal passages house cells called olfactory receptors, which are involved in the sense of smell. When chemicals enter the nasal passages, they contact the olfactory receptors. This triggers the receptors to send a signal to the brain, which creates the perception of smell.
PHARYNX
Air leaves the nasal passages and flows to the pharynx, a short, funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx. Like the nasal passages, the pharynx is lined with a protective mucous membrane and ciliated cells that remove impurities from the air. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth.
LARYNX
Air moves from the pharynx to the larynx, a structure about 5 cm (2 in) long located approximately in the middle of the neck. Several layers of cartilage, a tough and flexible tissue, comprise most of the larynx. A protrusion in the cartilage called the Adam’s apple sometimes enlarges in males during puberty, creating a prominent bulge visible on the neck.
While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage to cause choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed.
Food and fluids from the pharynx usually are prevented from entering the larynx by the epiglottis, a thin leaf like tissue. The “stem” of the leaf attaches to the front and top of the larynx. When a person is breathing, the epiglottis is held in a vertical position, like an open trap door. When a person swallows, however, a reflex causes the larynx and the epiglottis to move toward each other, forming a protective seal, and food and fluids are routed to the esophagus. If a person is eating or drinking too rapidly, or laughs while swallowing, the swallowing reflex may not work, and food or fluid can enter the larynx. Food, fluid, or other substances in the larynx initiate a cough reflex as the body attempts to clear the larynx of the obstruction. If the cough reflex does not work, a person can choke, a life-threatening situation. The Heimlich maneuver is a technique used to clear a blocked larynx (see First Aid). A surgical procedure called a tracheotomy is used to bypass the larynx and get air to the trachea in extreme cases of choking.
TRECHEA, BRONCHI AND BRONCHIOLES
Air passes from the larynx into the trachea, a tube about 12 to 15 cm (about 5 to 6 in) long located just below the larynx. The trachea is formed of 15 to 20 C-shaped rings of cartilage. The sturdy cartilage rings hold the trachea open, enabling air to pass freely at all times. The open part of the C-shaped cartilage lies at the back of the trachea, and the ends of the “C” are connected by muscle tissue.
The base of the trachea is located a little below where the neck meets the trunk of the body. Here the trachea branches into two tubes, the left and right bronchi, which deliver air to the left and right lungs, respectively. Within the lungs, the bronchi branch into smaller tubes called bronchioles. The trachea, bronchi, and the first few bronchioles contribute to the cleansing function of the respiratory system, for they, too, are lined with mucous membranes and ciliated cells that move mucus upward to the pharynx.
ALVEOLI
The bronchioles divide many more times in the lungs to create an impressive tree with smaller and smaller branches, some no larger than 0.5 mm (0.02 in) in diameter. These branches dead-end into tiny air sacs called alveoli. The alveoli deliver oxygen to the circulatory system and remove carbon dioxide. Interspersed among the alveoli are numerous macrophages, large white blood cells that patrol the alveoli and remove foreign substances that have not been filtered out earlier. The macrophages are the last line of defense of the respiratory system; their presence helps ensure that the alveoli are protected from infection so that they can carry out their vital role.
The alveoli number about 150 million per lung and comprise most of the lung tissue. Alveoli resemble tiny, collapsed balloons with thin elastic walls that expand as air flows into them and collapse when the air is exhaled. Alveoli are arranged in grapelike clusters, and each cluster is surrounded by a dense hairnet of tiny, thin-walled capillaries. The alveoli and capillaries are arranged in such a way that air in the wall of the alveoli is only about 0.1 to 0.2 microns from the blood in the capillary. Since the concentration of oxygen is much higher in the alveoli than in the capillaries, the oxygen diffuses from the alveoli to the capillaries. The oxygen flows through the capillaries to larger vessels, which carry the oxygenated blood to the heart, where it is pumped to the rest of the body.
Carbon dioxide that has been dumped into the bloodstream as a waste product from cells throughout the body flows through the bloodstream to the heart, and then to the alveolar capillaries. The concentration of carbon dioxide in the capillaries is much higher than in the alveoli, causing carbon dioxide to diffuse into the alveoli. Exhalation forces the carbon dioxide back through the respiratory passages and then to the outside of the body.
REGULATION
The flow of air in and out of the lungs is controlled by the nervous system, which ensures that humans breathe in a regular pattern and at a regular rate. Breathing is carried out day and night by an unconscious process. It begins with a cluster of nerve cells in the brain stem called the respiratory center. These cells send simultaneous signals to the diaphragm and rib muscles, the muscles involved in inhalation. The diaphragm is a large, dome-shaped muscle that lies just under the lungs. When the diaphragm is stimulated by a nervous impulse, it flattens. The downward movement of the diaphragm expands the volume of the cavity that contains the lungs, the thoracic cavity. When the rib muscles are stimulated, they also contract, pulling the rib cage up and out like the handle of a pail. This movement also expands the thoracic cavity. The increased volume of the thoracic cavity causes air to rush into the lungs. The nervous stimulation is brief, and when it ceases, the diaphragm and rib muscles relax and exhalation occurs. Under normal conditions, the respiratory center emits signals 12 to 20 times a minute, causing a person to take 12 to 20 breaths a minute. Newborns breathe at a faster rate, about 30 to 50 breaths a minute.
The rhythm set by the respiratory center can be altered by conscious control. The breathing pattern changes when a person sings or whistles, for example. A person also can alter the breathing pattern by holding the breath. The cerebral cortex, the part of the brain involved in thinking, can send signals to the diaphragm and rib muscles that temporarily override the signals from the respiratory center. The ability to hold one’s breathe has survival value. If a person encounters noxious fumes, for example, it is possible to avoid inhaling the fumes.
A person cannot hold the breath indefinitely, however. If exhalation does not occur, carbon dioxide accumulates in the blood, which, in turn, causes the blood to become more acidic. Increased acidity interferes with the action of enzymes, the specialized proteins that participate in virtually all biochemical reaction in the body. To prevent the blood from becoming too acidic, the blood is monitored by special receptors called chemoreceptors, located in the brainstem and in the blood vessels of the neck. If acid builds up in the blood, the chemoreceptors send nervous signals to the respiratory center, which overrides the signals from the cerebral cortex and causes a person to exhale and then resume breathing. These exhalations expel the carbon dioxide and bring the blood acid level back to normal.
A person can exert some degree of control over the amount of air inhaled, with some limitations. To prevent the lungs from bursting from overinflation, specialized cells in the lungs called stretch receptors measure the volume of air in the lungs. When the volume reaches an unsafe threshold, the stretch receptors send signals to the respiratory center, which shuts down the muscles of inhalation and halts the intake of air.
CELLEULAR RESPIRATION
The first stage of glucose catabolism is glycolysis. A glucose molecule breaks to give two molecules of pyruvic acid in a series of ten reactions. It is the common process for aerobic as well as anaerobic respiration. Glycolysis is also called EMP pathway in honour of its discoverers.
The second stage of cellular respiration, the three-stage process by which living cells break down organic fuel molecules in the presence of oxygen to harvest the energy they need to grow and divide. This metabolic process occurs in most plants, animals, fungi, and many bacteria. In all organisms except bacteria the TCA cycle is carried out in the matrix of intracellular structures called mitochondria.
The TCA cycle plays a central role in the breakdown, or catabolism, of organic fuel molecules—i.e., glucose and some other sugars, fatty acids, and some amino acids. Before these rather large molecules can enter the TCA cycle they must be degraded into a two-carbon compound called acetyl coenzyme A (acetyl CoA). Once fed into the TCA cycle, acetyl CoA is converted into carbon dioxide and energy.
The TCA cycle consists of eight steps catalyzed by eight different enzymes. The cycle is initiated (1) when acetyl CoA reacts with the compound oxaloacetate to form citrate and to release coenzyme A (CoA-SH). Then, in a succession of reactions, (2) citrate is rearranged to form isocitrate; (3) isocitrate loses a molecule of carbon dioxide and then undergoes oxidation to form alpha-ketoglutarate; (4) alpha-ketoglutarate loses a molecule of carbon dioxide and is oxidized to form succinyl CoA; (5) succinyl CoA is enzymatically converted to succinate; (6) succinate is oxidized to fumarate; (7) fumarate is hydrated to produce malate; and, to end the cycle, (8) malate is oxidized to oxaloacetate. Each complete turn of the cycle results in the regeneration of oxaloacetate and the formation of two molecules of carbon dioxide.
Energy is produced in a number of steps in this cycle of reactions. In step 5, one molecule of adenosine triphosphate (ATP), the molecule that powers most cellular functions, is produced. Most of the energy obtained from the TCA cycle, however, is captured by the compounds nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) and converted later to ATP. Energy transfers occur through the relay of electrons from one substance to another, a process carried out through the chemical reactions known as oxidation and reduction, or redox reactions. (Oxidation involves the loss of electrons from a substance and reduction the addition of electrons.) For each turn of the TCA cycle, three molecules of NAD+ are reduced to NADH and one molecule of FAD is reduced to FADH2. These molecules then transfer their energy to the electron transport chain, a pathway that is part of the third stage of cellular respiration. The electron transport chain in turn releases energy so that it can be converted to ATP through the process of oxidative phosphorylation.
HAZARDS
The respiratory system can be damaged by a variety of chemicals found in the environment, ranging from automobile fumes and industrial smoke to household cleaning products. Cigarette smoke, however, poses a particularly serious threat to the respiratory system because of the tar and other substances that enter the lungs. After a person smokes just one cigarette, for example, tar temporarily paralyzes the cilia of the upper and lower respiratory tracts. The tar also temporarily immobilizes the macrophages in the alveoli of the lungs. With the filtering and cleansing functions inactivated, the air passages and lungs are exposed not only to the irritating effects of tar but also to airborne bacteria, viruses, and other particles. These, along with the tar, settle in the mucous layers of the lungs. The paralyzed cilia recover after about one hour, but repeated smoking eventually kills them. Repeated smoking also causes mucus to build up in the lungs and block the smaller air passages. The blockage triggers a cough reflex—the familiar “smoker’s cough”—the lung’s effort to clear the airways. In addition, tobacco smoke contains over 40 chemicals known to cause cancer. Smoking is responsible for almost 90 percent of lung cancer cases among men, and more than 70 percent among women.
Workers in occupations that produce impurities released into the air are at high risk for respiratory illnesses. Sandblasters, stone cutters, quarry workers, miners, construction workers, people who install brake lining or insulation, people who work in shipyards or on farms, and people who pick cotton are among those at risk. In the United States, the Occupational Safety and Health Administration (OSHA) issues regulations that protect workers—requiring air masks with filters for certain jobs, for example. The Environmental Protection Agency (EPA) monitors and regulates the amount of pollutants released into the air. Despite these efforts, respiratory illnesses remain higher among workers who have significant exposure to air pollutants (see Environmental and Occupational Diseases).
Normal, everyday exposure to air pollution from cars and industrial emissions in the city also weakens the respiratory system of city-dwellers. Even if a person does not smoke, the city air gradually changes pink, healthy lung tissue to tissue darkened with particles of smog, dust, and other pollutants, making the lungs more vulnerable to infection. While outdoor pollutants pose threats to the respiratory system, a far greater threat is created by indoor air pollution. In homes and offices, a variety of substances, including cleaning compounds, air fresheners, synthetic carpets and furniture, and certain construction materials, can emit hazardous gases, which become highly concentrated in unventilated rooms. Individuals at greatest risk are those who spend most of their time indoors, children, the elderly, and people with a history of respiratory illnesses. Like outdoor air pollutants, indoor air pollutants weaken the lungs and invite infection. The long-term effects of air pollution are difficult to measure, but may include cancer and other serious diseases.
While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the normal functioning of cells. It protects the body against disease-causing organisms and toxic substances inhaled with air. The respiratory system also houses the cells that detect smell, and assists in the production of sounds for speech.
The respiratory and circulatory systems work together to deliver oxygen to cells and remove carbon dioxide in a two-phase process called respiration. The first phase of respiration begins with breathing in, or inhalation. Inhalation brings air from outside the body into the lungs. Oxygen in the air moves from the lungs through blood vessels to the heart, which pumps the oxygen-rich blood to all parts of the body. Oxygen then moves from the bloodstream into cells, which completes the first phase of respiration. In the cells, oxygen is used in a separate energy-producing process called cellular respiration, which produces carbon dioxide as a byproduct. The second phase of respiration begins with the movement of carbon dioxide from the cells to the bloodstream. The bloodstream carries carbon dioxide to the heart, which pumps the carbon dioxide-laden blood to the lungs. In the lungs, breathing out, or exhalation, removes carbon dioxide from the body, thus completing the respiration cycle.
NASAL PASSAGE
The flow of air from outside of the body to the lungs begins with the nose, which is divided into the left and right nasal passages. The nasal passages are lined with a membrane composed primarily of one layer of flat, closely packed cells called epithelial cells. Each epithelial cell is densely fringed with thousands of microscopic cilia, fingerlike extensions of the cells. Interspersed among the epithelial cells are goblet cells, specialized cells that produce mucus, a sticky, thick, moist fluid that coats the epithelial cells and the cilia. Numerous tiny blood vessels called capillaries lie just under the mucous membrane, near the surface of the nasal passages. While transporting air to the pharynx, the nasal passages play two critical roles: they filter the air to remove potentially disease-causing particles; and they moisten and warm the air to protect the structures in the respiratory system.Filtering prevents airborne bacteria, viruses, other potentially disease-causing substances from entering the lungs, where they may cause infection. Filtering also eliminates smog and dust particles, which may clog the narrow air passages in the smallest bronchioles. Coarse hairs found just inside the nostrils of the nose trap airborne particles as they are inhaled. The particles drop down onto the mucous membrane lining the nasal passages. The cilia embedded in the mucous membrane wave constantly, creating a current of mucus that propels the particles out of the nose or downward to the pharynx. In the pharynx, the mucus is swallowed and passed to the stomach, where the particles are destroyed by stomach acid. If more particles are in the nasal passages than the cilia can handle, the particles build up on the mucus and irritate the membrane beneath it. This irritation triggers a reflex that produces a sneeze to get rid of the polluted air.
The nasal passages also moisten and warm air to prevent it from damaging the delicate membranes of the lung. The mucous membranes of the nasal passages release water vapor, which moistens the air as it passes over the membranes. As air moves over the extensive capillaries in the nasal passages, it is warmed by the blood in the capillaries. If the nose is blocked or “stuffy” due to a cold or allergies, a person is forced to breathe through the mouth. This can be potentially harmful to the respiratory system membranes, since the mouth does not filter, warm, or moisten air.
In addition to their role in the respiratory system, the nasal passages house cells called olfactory receptors, which are involved in the sense of smell. When chemicals enter the nasal passages, they contact the olfactory receptors. This triggers the receptors to send a signal to the brain, which creates the perception of smell.
PHARYNX
Air leaves the nasal passages and flows to the pharynx, a short, funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx. Like the nasal passages, the pharynx is lined with a protective mucous membrane and ciliated cells that remove impurities from the air. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth.
LARYNX
Air moves from the pharynx to the larynx, a structure about 5 cm (2 in) long located approximately in the middle of the neck. Several layers of cartilage, a tough and flexible tissue, comprise most of the larynx. A protrusion in the cartilage called the Adam’s apple sometimes enlarges in males during puberty, creating a prominent bulge visible on the neck.
While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage to cause choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed.
Food and fluids from the pharynx usually are prevented from entering the larynx by the epiglottis, a thin leaf like tissue. The “stem” of the leaf attaches to the front and top of the larynx. When a person is breathing, the epiglottis is held in a vertical position, like an open trap door. When a person swallows, however, a reflex causes the larynx and the epiglottis to move toward each other, forming a protective seal, and food and fluids are routed to the esophagus. If a person is eating or drinking too rapidly, or laughs while swallowing, the swallowing reflex may not work, and food or fluid can enter the larynx. Food, fluid, or other substances in the larynx initiate a cough reflex as the body attempts to clear the larynx of the obstruction. If the cough reflex does not work, a person can choke, a life-threatening situation. The Heimlich maneuver is a technique used to clear a blocked larynx (see First Aid). A surgical procedure called a tracheotomy is used to bypass the larynx and get air to the trachea in extreme cases of choking.
TRECHEA, BRONCHI AND BRONCHIOLES
Air passes from the larynx into the trachea, a tube about 12 to 15 cm (about 5 to 6 in) long located just below the larynx. The trachea is formed of 15 to 20 C-shaped rings of cartilage. The sturdy cartilage rings hold the trachea open, enabling air to pass freely at all times. The open part of the C-shaped cartilage lies at the back of the trachea, and the ends of the “C” are connected by muscle tissue.
The base of the trachea is located a little below where the neck meets the trunk of the body. Here the trachea branches into two tubes, the left and right bronchi, which deliver air to the left and right lungs, respectively. Within the lungs, the bronchi branch into smaller tubes called bronchioles. The trachea, bronchi, and the first few bronchioles contribute to the cleansing function of the respiratory system, for they, too, are lined with mucous membranes and ciliated cells that move mucus upward to the pharynx.
ALVEOLI
The bronchioles divide many more times in the lungs to create an impressive tree with smaller and smaller branches, some no larger than 0.5 mm (0.02 in) in diameter. These branches dead-end into tiny air sacs called alveoli. The alveoli deliver oxygen to the circulatory system and remove carbon dioxide. Interspersed among the alveoli are numerous macrophages, large white blood cells that patrol the alveoli and remove foreign substances that have not been filtered out earlier. The macrophages are the last line of defense of the respiratory system; their presence helps ensure that the alveoli are protected from infection so that they can carry out their vital role.The alveoli number about 150 million per lung and comprise most of the lung tissue. Alveoli resemble tiny, collapsed balloons with thin elastic walls that expand as air flows into them and collapse when the air is exhaled. Alveoli are arranged in grapelike clusters, and each cluster is surrounded by a dense hairnet of tiny, thin-walled capillaries. The alveoli and capillaries are arranged in such a way that air in the wall of the alveoli is only about 0.1 to 0.2 microns from the blood in the capillary. Since the concentration of oxygen is much higher in the alveoli than in the capillaries, the oxygen diffuses from the alveoli to the capillaries. The oxygen flows through the capillaries to larger vessels, which carry the oxygenated blood to the heart, where it is pumped to the rest of the body.
Carbon dioxide that has been dumped into the bloodstream as a waste product from cells throughout the body flows through the bloodstream to the heart, and then to the alveolar capillaries. The concentration of carbon dioxide in the capillaries is much higher than in the alveoli, causing carbon dioxide to diffuse into the alveoli. Exhalation forces the carbon dioxide back through the respiratory passages and then to the outside of the body.
REGULATION
The flow of air in and out of the lungs is controlled by the nervous system, which ensures that humans breathe in a regular pattern and at a regular rate. Breathing is carried out day and night by an unconscious process. It begins with a cluster of nerve cells in the brain stem called the respiratory center. These cells send simultaneous signals to the diaphragm and rib muscles, the muscles involved in inhalation. The diaphragm is a large, dome-shaped muscle that lies just under the lungs. When the diaphragm is stimulated by a nervous impulse, it flattens. The downward movement of the diaphragm expands the volume of the cavity that contains the lungs, the thoracic cavity. When the rib muscles are stimulated, they also contract, pulling the rib cage up and out like the handle of a pail. This movement also expands the thoracic cavity. The increased volume of the thoracic cavity causes air to rush into the lungs. The nervous stimulation is brief, and when it ceases, the diaphragm and rib muscles relax and exhalation occurs. Under normal conditions, the respiratory center emits signals 12 to 20 times a minute, causing a person to take 12 to 20 breaths a minute. Newborns breathe at a faster rate, about 30 to 50 breaths a minute.
The rhythm set by the respiratory center can be altered by conscious control. The breathing pattern changes when a person sings or whistles, for example. A person also can alter the breathing pattern by holding the breath. The cerebral cortex, the part of the brain involved in thinking, can send signals to the diaphragm and rib muscles that temporarily override the signals from the respiratory center. The ability to hold one’s breathe has survival value. If a person encounters noxious fumes, for example, it is possible to avoid inhaling the fumes.
A person cannot hold the breath indefinitely, however. If exhalation does not occur, carbon dioxide accumulates in the blood, which, in turn, causes the blood to become more acidic. Increased acidity interferes with the action of enzymes, the specialized proteins that participate in virtually all biochemical reaction in the body. To prevent the blood from becoming too acidic, the blood is monitored by special receptors called chemoreceptors, located in the brainstem and in the blood vessels of the neck. If acid builds up in the blood, the chemoreceptors send nervous signals to the respiratory center, which overrides the signals from the cerebral cortex and causes a person to exhale and then resume breathing. These exhalations expel the carbon dioxide and bring the blood acid level back to normal.
A person can exert some degree of control over the amount of air inhaled, with some limitations. To prevent the lungs from bursting from overinflation, specialized cells in the lungs called stretch receptors measure the volume of air in the lungs. When the volume reaches an unsafe threshold, the stretch receptors send signals to the respiratory center, which shuts down the muscles of inhalation and halts the intake of air.
CELLEULAR RESPIRATION
The first stage of glucose catabolism is glycolysis. A glucose molecule breaks to give two molecules of pyruvic acid in a series of ten reactions. It is the common process for aerobic as well as anaerobic respiration. Glycolysis is also called EMP pathway in honour of its discoverers.The second stage of cellular respiration, the three-stage process by which living cells break down organic fuel molecules in the presence of oxygen to harvest the energy they need to grow and divide. This metabolic process occurs in most plants, animals, fungi, and many bacteria. In all organisms except bacteria the TCA cycle is carried out in the matrix of intracellular structures called mitochondria.
The TCA cycle plays a central role in the breakdown, or catabolism, of organic fuel molecules—i.e., glucose and some other sugars, fatty acids, and some amino acids. Before these rather large molecules can enter the TCA cycle they must be degraded into a two-carbon compound called acetyl coenzyme A (acetyl CoA). Once fed into the TCA cycle, acetyl CoA is converted into carbon dioxide and energy.
The TCA cycle consists of eight steps catalyzed by eight different enzymes. The cycle is initiated (1) when acetyl CoA reacts with the compound oxaloacetate to form citrate and to release coenzyme A (CoA-SH). Then, in a succession of reactions, (2) citrate is rearranged to form isocitrate; (3) isocitrate loses a molecule of carbon dioxide and then undergoes oxidation to form alpha-ketoglutarate; (4) alpha-ketoglutarate loses a molecule of carbon dioxide and is oxidized to form succinyl CoA; (5) succinyl CoA is enzymatically converted to succinate; (6) succinate is oxidized to fumarate; (7) fumarate is hydrated to produce malate; and, to end the cycle, (8) malate is oxidized to oxaloacetate. Each complete turn of the cycle results in the regeneration of oxaloacetate and the formation of two molecules of carbon dioxide.
Energy is produced in a number of steps in this cycle of reactions. In step 5, one molecule of adenosine triphosphate (ATP), the molecule that powers most cellular functions, is produced. Most of the energy obtained from the TCA cycle, however, is captured by the compounds nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) and converted later to ATP. Energy transfers occur through the relay of electrons from one substance to another, a process carried out through the chemical reactions known as oxidation and reduction, or redox reactions. (Oxidation involves the loss of electrons from a substance and reduction the addition of electrons.) For each turn of the TCA cycle, three molecules of NAD+ are reduced to NADH and one molecule of FAD is reduced to FADH2. These molecules then transfer their energy to the electron transport chain, a pathway that is part of the third stage of cellular respiration. The electron transport chain in turn releases energy so that it can be converted to ATP through the process of oxidative phosphorylation.
HAZARDS
The respiratory system can be damaged by a variety of chemicals found in the environment, ranging from automobile fumes and industrial smoke to household cleaning products. Cigarette smoke, however, poses a particularly serious threat to the respiratory system because of the tar and other substances that enter the lungs. After a person smokes just one cigarette, for example, tar temporarily paralyzes the cilia of the upper and lower respiratory tracts. The tar also temporarily immobilizes the macrophages in the alveoli of the lungs. With the filtering and cleansing functions inactivated, the air passages and lungs are exposed not only to the irritating effects of tar but also to airborne bacteria, viruses, and other particles. These, along with the tar, settle in the mucous layers of the lungs. The paralyzed cilia recover after about one hour, but repeated smoking eventually kills them. Repeated smoking also causes mucus to build up in the lungs and block the smaller air passages. The blockage triggers a cough reflex—the familiar “smoker’s cough”—the lung’s effort to clear the airways. In addition, tobacco smoke contains over 40 chemicals known to cause cancer. Smoking is responsible for almost 90 percent of lung cancer cases among men, and more than 70 percent among women.
Workers in occupations that produce impurities released into the air are at high risk for respiratory illnesses. Sandblasters, stone cutters, quarry workers, miners, construction workers, people who install brake lining or insulation, people who work in shipyards or on farms, and people who pick cotton are among those at risk. In the United States, the Occupational Safety and Health Administration (OSHA) issues regulations that protect workers—requiring air masks with filters for certain jobs, for example. The Environmental Protection Agency (EPA) monitors and regulates the amount of pollutants released into the air. Despite these efforts, respiratory illnesses remain higher among workers who have significant exposure to air pollutants (see Environmental and Occupational Diseases).
Normal, everyday exposure to air pollution from cars and industrial emissions in the city also weakens the respiratory system of city-dwellers. Even if a person does not smoke, the city air gradually changes pink, healthy lung tissue to tissue darkened with particles of smog, dust, and other pollutants, making the lungs more vulnerable to infection. While outdoor pollutants pose threats to the respiratory system, a far greater threat is created by indoor air pollution. In homes and offices, a variety of substances, including cleaning compounds, air fresheners, synthetic carpets and furniture, and certain construction materials, can emit hazardous gases, which become highly concentrated in unventilated rooms. Individuals at greatest risk are those who spend most of their time indoors, children, the elderly, and people with a history of respiratory illnesses. Like outdoor air pollutants, indoor air pollutants weaken the lungs and invite infection. The long-term effects of air pollution are difficult to measure, but may include cancer and other serious diseases.