The Respiratory System

Breathing, controlled by the respiratory system, is a continuous process of which a person is normally unaware. If breathing stops, however, a person becomes acutely aware of the fact. An individual can go days without food and water and hours without sleep, but only five or six minutes without air. Anything beyond that would be fatal. The trillions of cells in the body need a constant and generous amount of oxygen to carry out their vital functions. As they use that oxygen, they give off carbon dioxide as a waste product. It is the role of the respiration system, working in conjunction with the cardiovascular system, to supply the oxygen and dispose of the carbon dioxide.

DESIGN: PARTS OF THE RESPIRATORY SYSTEM

Breathing describes the process of inhaling and exhaling air. The exchange of gases (oxygen and carbon dioxide) between living cells and the environment is a process known as respiration. The respiratory system, which controls breathing and respiration, consists of the respiratory tract and the lungs.

The respiratory tract cleans, warms, and moistens air on its way 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), larynx, and upper part of the trachea (windpipe). The lower part consists of the lower part of the trachea, bronchi, and lungs (which contain bronchioles and alveoli).

The nose and nasal cavity

The nose is the only external part of the respiratory system. It is made of bone and cartilage (tough connective tissue) and is covered with skin. The two openings to the outside, called nostrils, allow air to enter or leave the body during breathing. The nostrils are lined with coarse hairs that prevent large particles such as dust, insects, and sand from entering.

The nostrils open into a large cavity, the nasal cavity. This cavity is divided into right and left cavities by a thin plate of bone and cartilage called the nasal septum. The hard portion of the palate forms the floor of the entire nasal cavity, separating it from the mouth or oral cavity below. Three flat, spongy folds or plates project toward the nasal septum from the sides of the nasal cavity. These plates, called nasal conchae, help to slow down the passage of air, causing it to swirl in the nasal cavity.

The Respiratory System: Words to Know

Alveoli (al-VEE-oh-lie):

Air sacs of the lungs.

Breathing (BREETH-ing):

Process of inhaling and exhaling air.

Bronchi (BRONG-kie):

Largest branch of the bronchial tree between the trachea and bronchioles.

Bronchial tree (BRONG-key-uhl TREE):

Entire system of air passageways within the lungs formed by the branching of bronchial tubes.

Bronchioles (BRONG-key-ohls):

Smallest of the air passageways within the lungs.

Epiglottis (ep-i-GLAH-tis):

Flaplike piece of tissue at the top of the larynx that covers its opening when swallowing is occurring.

Esophagus (i-SOF-ah-gus):

Muscular tube connecting the pharynx and stomach.

Exhalation (ex-ha-LAY-shun):

Also known as expiration, the movement of air out of the lungs.

Glottis (GLAH-tis):

Opening of the larynx between the vocal cords.

Hemoglobin (HEE-muh-glow-bin):

Iron-containing protein pigment in red blood cells that can combine with oxygen and carbon dioxide.

Inhalation (in-ha-LAY-shun):

Also known as inspiration, the movement of air into the lungs.

Larynx (LAR-ingks):

Organ between the pharynx and trachea that contains the vocal cords.

Lungs:

Paired breathing organs.

Nasal cavity (NAY-zul KAV-i-tee):

Air cavity in the skull through which air passes from the nostrils to the upper part of the pharynx.

Nasal conchae (NAY-zul KAHN-kee):

Flat, spongy plates that project toward the nasal septum from the sides of the nasal cavity.

Nasal septum (NAY-zul SEP-tum):

Vertical plate made of bone and cartilage that divides the nasal cavity.

Nose:

Part of the human face that contains the nostrils and organs of smell and forms the beginning of the respiratory tract.

Nostril (NOS-tril):

Either of the two external openings of the nose.

Paranasal sinuses (pair-a-NAY-sal SIGH-nus-ez):

Air-filled chambers in the bones of the skull that open into the nasal cavity.

Pharynx (FAR-inks):

Short, muscular tube extending from the mouth and nasal cavities to the trachea and esophagus.

Pleura (PLOOR-ah):

Membrane sac covering and protecting each lung.

Pulmonary surfactant (PULL-mo-nair-ee sir-FAK-tent):

Oily substance secreted by the alveoli to prevent their walls from sticking together.

Respiration (res-pe-RAY-shun):

Exchange of gases (oxygen and carbon dioxide) between living cells and the environment.

Trachea (TRAY-key-ah):

Also known as the windpipe, the respiratory tube extending from the larynx to the bronchi. The nasal cavity is lined by mucous membrane containing microscopic hairlike structures called cilia. The cells of the membrane produce mucus, a

thick, gooey liquid. As the nasal conchae cause air to swirl in the nasal cavity, the mucus moistens the air and traps any bacteria or particles of air pollution. The cilia wave back and forth in rhythmic movement, and pieces of mucus with their trapped particles are swept along to the throat. The mucus is then either spat out or (more often) swallowed. Any bacteria present in the swallowed mucus is destroyed by the hydrochloric acid in the gastric juice of the stomach.

Air is not only moistened in the nasal cavity but warmed, as well. A rich network of thin-walled capillaries permeates the mucus membrane (especially the uppermost concha), and the incoming air is warmed as it passes over the vessels. When air finally reaches the lungs, it is similar to the warm, damp air found in the tropics.

The bones that surround the nasal cavity contain hollow spaces known as paranasal sinuses. The sinuses are also lined with mucous membrane containing cilia. The mucus produced in the sinuses drains into the nasal cavity. The main functions of the sinuses are to lighten the skull and to provide resonance (sound quality) for the voice.

The pharynx

The pharynx or throat is a short, muscular tube extending about 5 inches (12.7 centimeters) from the nasal cavity and mouth to the esophagus and trachea. It serves two separate systems: the digestive system (by allowing the passage of solid food and liquids) and the respiratory system (by allowing the passage of air).

The larynx

The larynx, commonly called the voice box, forms the upper part of the trachea. The larynx is made of nine pieces of cartilage connected by ligaments. The largest of these cartilages is the shield-shaped thyroid cartilage, which may protrude at the front of the neck, forming the so-called Adam's apple. The upper cartilage is the epiglottis, a flaplike piece of tissue. During swallowing, the larynx rises up and the epiglottis folds down to cover the glottis, or the larynx's opening. This prevents food or liquids from passing into the lower respiratory tract.

Mucous membrane lines the larynx. A pair of elastic folds in that lining form the vocal cords. During silent breathing, the vocal cords lie against the walls of the larynx. During speech, the cords are stretched across the opening of the larynx and air that passes through causes them to vibrate, generating sound waves. Various muscles produce tension on the cords, making them tighter (shorter) or looser (longer). The tighter the tension, the higher the pitch of the sound produced. Since men's larynges tend to be larger than women's, their vocal cords tend to be thicker and longer. The male voice thus tends to be lower in pitch.

The trachea

The trachea is a tough, flexible tube about 1 inch (2.5 centimeters) in diameter and 4.5 inches (11.4 centimeters) in length. Located in front of the esophagus, it is the principal tube that carries air to and from the lungs. The walls of the trachea are supported by 16 to 20 C-shaped cartilage rings. Elastic fibers in the tracheal walls allow the trachea to expand and contract during breathing, while the cartilage rings prevent it from collapsing. Mucous membrane containing cilia lines the trachea. The mucus produced by the membrane traps dust particles and other debris. The cilia move continuously in a direction opposite that of the incoming air, helping propel the mucus away from the lungs to the throat where it can be swallowed or spat out.

The bronchi

The trachea divides behind the sternum (breastbone) to form a right and left branch called primary bronchi (singular: bronchus). Each bronchus passes into a lung—the right bronchus into the right lung and the left bronchus into the left lung. The right bronchus is wider, shorter, and straighter than the left. As a result, accidentally inhaled objects (such as pieces of food) most often enter the right primary bronchus.

By the time incoming air reaches the primary bronchi, it is warm, moistened, and cleansed of most particles or other impurities.

The lungs

The lungs are two broad, cone-shaped organs located on either side of the heart in the thoracic or chest cavity. They extend from the collarbones to the diaphragm, a membrane of muscle separating the thoracic cavity from the abdominal cavity. The base of each lung rests directly on the diaphragm. The rib cage forms a wall around the lungs, protecting them.

At birth, the lungs are pale pink in color. As people age, their lungs grow darker. The inhaling of dirt and other particles increases this aging process, even scarring the delicate tissue of the lungs.

Each lung is divided into lobes separated by deep grooves or fissures. The right lung, which is larger, is divided into three lobes. The left lung is divided into only two lobes. Combined, the two soft and spongy lungs weigh about 2.5 pounds (1.1 kilograms).

A membrane sac, called the pleura, surrounds and protects each lung. One layer of the pleura attaches to the wall of the thoracic cavity; the other layer encloses the lung. A fluid (pleural fluid) between the two membrane layers reduces friction and allows smooth movement of a lung during breathing.

After the bronchi enter the lungs, they subdivide repeatedly into smaller and smaller bronchi or branches. Eventually they form thousands of tiny branches called bronchioles, which have a diameter of about 0.02 inch (0.5 millimeter). This branching network of bronchial tubes within the lungs is called the bronchial tree.

The bronchioles branch to form even smaller passageways that open into clusters of cup-shaped air sacs called alveoli (singular: alveolus). The average person has a total of about 700 million alveoli (which resemble clusters of grapes) in his or her lungs. These provide an enormous surface area-roughly the size of a tennis court—for gas exchange. A network of capillaries surrounds each alveolus. 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.

The membranes of the alveoli are extremely delicate and thin to allow the gases to pass easily through them. The inner lining of those membranes is coated with a thin layer of tissue fluid (a gas must be dissolved in a liquid in order to enter or leave a cell). To prevent the walls of the alveoli from sticking together (like the inside walls of a wet plastic bag), cells in the alveoli also produce an oily secretion, called pulmonary surfactant, that mixes with the tissue fluid (pulmonary refers to anything relating to or affecting the lungs).

WORKINGS: HOW THE RESPIRATORY SYSTEM FUNCTIONS

The main function of the respiratory system is to provide oxygen for the body's cells and remove the carbon dioxide they produce. Oxygen is the most important energy source for the cells. They need it for cellular respiration: the process by which the simple sugar glucose is oxidized (combined with oxygen) to form the energy-rich compound adenosine triphosphate (ATP). Glucose is produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and complex sugars such as sucrose (cane or beet sugar) and fructose (fruit sugar). ATP is the compound used by all cells to carry out their ordinary functions: growth, the production of new cell parts and chemicals, and the movement of compounds through cells and the body as a whole.

Breathing

The mechanical process by which the body takes in oxygen and then releases carbon dioxide is called breathing or pulmonary ventilation. Inhalation (or inspiration) occurs when air flows into the lungs. Exhalation (or expiration) occurs when air flows out of the lungs. A single breath, called a respiratory cycle, consists of an inhalation followed by an exhalation. Breathing is brought about by the actions of the nervous system and the respiratory muscles.

BREATHING IN AND OUT: THE COMPOSITION OF AIR

When Earth was new, its atmosphere was probably composed of hydrogen, methane, and ammonia gases—much like the other planets in our solar system. Over billions of years, the composition of the atmosphere has changed considerably. Scientists theorize that a series of events that began when gases were released by early volcanic activity led to the formation of Earth's current atmosphere.

The air humans breathe in is Earth's atmosphere. The air humans breathe out, however, has a different composition. The following list breaks down the major components of those two types of air and their approximate percentages:

Nitrogen: 78% (inhaled air)/ 78% (exhaled air)

Oxygen: 21% (inhaled air)/ 16% (exhaled air)

Carbon dioxide: 0.04% (inhaled air)/ 4.5% (exhaled air)

Although most of Earth's atmosphere is composed of nitrogen, the human body cannot utilize this gas, so it is simply exhaled. Exhaled air has a decreased amount of oxygen and an increased amount of carbon dioxide. These amounts show how much oxygen is retained within the body for use by the cells and how much carbon dioxide is produced as a by-product of cellular metabolism.

The respiratory muscles are the diaphragm and the intercostal muscles. When the diaphragm (the dome-shaped sheet of muscle beneath the lungs that separates the thoracic chest cavity from the abdominal cavity) contracts, it flattens and moves downward. The intercostal muscles are found between the ribs. When the external intercostal muscles contract, they pull the ribs upward and outward. When the internal intercostal muscles contract, they pull the ribs downward and inward. The actions of all these muscles produce changes in the pressure within the alveoli and the bronchial tree.

All forms of matter—solid, liquid, and gas—exert pressure. In the case of a gas (like air), that pressure is caused by the motion of the gas particles. Gas particles have a tendency to fly away rapidly from each other and fill any container in which they are placed. As they do so, they constantly collide against the walls of that container and each other. The collisions of the gas particles causes gas pressure. In a large container, the gas particles in a certain amount of gas will be far apart and less collisions will occur. As a result, the gas pressure will be low. In a smaller container, the gas particles in that same amount of gas will be closer together and more collisions will occur. This will result in high gas pressure.

Inhalation occurs when motor nerves from the medulla oblongata in the brain carry impulses to the diaphragm and intercostal muscles, stimulating them to contract. When the diaphragm is stimulated to contact, it moves downward. Its dome is flattened and the size of the chest cavity is increased. The external intercostal muscles are also stimulated to contract, and they move the ribs up and outward. This also increases the size of the chest cavity. Since the lungs are attached to the chest (thoracic) walls, as the chest expands, so do the lungs. This action reduces the pressure inside the lungs relative to the pressure of the outside atmospheric air. As a consequence, a partial vacuum is created in the lungs and air rushes in from the outside to fill them. The quantity of fresh air taken in during an inhalation is referred to as tidal air.

The reverse occurs in exhalation. In healthy people, exhalation is mostly a passive process that depends more on the elasticity of the lungs than on muscle contraction. During exhalation, motor nerve stimulation from the brain decreases. The diaphragm relaxes and its dome curves up into the chest cavity, while the external intercostal muscles relax and the ribs move back down and inward. As the chest cavity decreases in size, so do the lungs. The air in the lungs is forced more closely together and its pressure increases. When that pressure rises to a point higher than atmospheric pressure, the air is expelled or forced out of the lungs until the two pressures are equal again.

Under normal circumstances, energy is expended during inhalation, but not during exhalation. However, air can be forcefully expelled, such as during talking, singing, or playing a musical wind instrument. Forced exhalation is an active process that requires muscle contraction. In such a case, the internal intercostal muscles are stimulated to contract, pulling the ribs down and in. This forces more air out of the lungs. The abdominal muscles (rectus abdominis) may also be stimulated to contract, compressing the abdominal organs and pushing the diaphragm upward. This action forces even more air out of the lungs.

A healthy adult at rest breathes in and out—one respiratory cycle—about twelve to sixteen times per minute (children breathe more rapidly, about eighteen to twenty times per minute). Exercise and other factors can change this rate. Total lung capacity is about 12.5 pints (6 liters). Under normal circumstances, an individual inhales and exhales about 1 pint (475 milliliters) of air in each respiratory 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 (called the tidal volume), about 2.5 pints (1200 milliliters) remains in the respiratory passageways and alveoli. This amount of air, called the residual volume, keeps the alveoli inflated and allows gas exchange between the lungs and blood vessels to go on continuously.

Respiration

Once air has filled the lungs, the oxygen in that air must be transported to all the cells in the body. In return, all cells in the body release carbon dioxide that must be transported back to the lungs to be exhaled. The exchanges of gases in the body is known as respiration. External respiration is the exchange of gases through the thin membranes of the alveoli and those of the blood capillaries surrounding them. Internal respiration is the exchange of gases between the blood capillaries and the tissue cells of the body. Within the body, all gases are exchanged through the process of diffusion.

Diffusion is the movement of molecules from an area of greater concentration (existing in greater numbers) to an area of lesser concentration (existing in lesser numbers). Diffusion takes place because molecules have free energy, meaning they are always in motion. This is the case especially with molecules in a gas, which move quicker than those in a solid or liquid. Oxygen and carbon dioxide, the gases that pass between the alveoli and their capillaries and between the blood and the interstitial fluid (fluid surrounding cells of the body), move by diffusion.

BREATHING UNDERWATER

In 1943, French oceanographer Jacques-Yves Cousteau (1910–1997) and French engineer Emile Gagnan developed the aqualung or scuba gear. This scuba (an acronym for s elf-c ontained u nderwater b reathing a pparatus) system not only benefitted recreational divers, but scientists as well. It has become an indispensable tool in the study of marine biology.

The aqualung allows a diver to swim freely down to about 180 feet (55 meters). Recordsetting dives of over 300 feet (91 meters) have been made with scuba gear. It consists of a canister or canisters of highly compressed air that the diver wears on his or her back. The unit is connected to a demand regulator that automatically supplies air at the same pressure as that of the surrounding water. A mouthpiece attached to the regulator allows the diver to breathe.

EXTERNAL RESPIRATION. After inhalation, the air in the alveoli contains a high concentration of oxygen and a low concentration of carbon dioxide. Conversely, the blood in the pulmonary capillaries surrounding the alveoli (which has come from the body) has a low concentration of oxygen and a high concentration of carbon dioxide. Following the law of diffusion, oxygen molecules in the air in the alveoli flow into the pulmonary capillaries. Carbon dioxide molecules flow in the opposite direction, from the blood in pulmonary capillaries into the air in the alveoli.

After gas exchange occurs in the lungs, the pulmonary capillaries carry the oxygenated (carrying oxygen) blood toward the heart. They merge to form venules, which merge to form larger and larger veins. Finally, the oxygenated blood reaches the left atrium of the heart through the four pulmonary veins. After flowing into the left ventricle, the blood is pumped out to the rest of the body.

Almost all the oxygen that diffuses into the pulmonary capillaries attaches to red blood cells in the blood. The primary element of red blood cells is a protein pigment called hemoglobin. Hemoglobin molecules account for one-third the weight of each red blood cell. At the center of each hemoglobin molecule is a single atom of iron, which gives red blood cells their color. The oxygen molecules bond to the iron atoms to create compounds called oxyhemoglobins. The main function of red blood cells is to transport this form of oxygen to the cells throughout the body.

INTERNAL RESPIRATION. Internal respiration occurs between the cells in the body and the systemic capillaries (capillaries in the body outside of the lungs). The bond between the oxygen molecules and the iron atoms of hemoglobin is not a very strong or stable one. When red blood cells enter tissues in the body where the concentration of oxygen is low, the bond is readily broken and the oxygen molecules are released.

HOW DO FISH BREATHE?

Fish and most other aquatic animals use gills for respiration. In fish, these external respiratory organs are located in gill chambers at the rear of the mouth. Gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries.

Water taken in through a fish's mouth is forced through openings called gill slits. It then washes over the delicate gills. The exchange of gases—oxygen and carbon dioxide—occurs through diffusion, much like in human lungs. Oxygen that is dissolved in the water diffuses through the thin membranes of the gills and passes into the capillaries. Carbon dioxide, produced as a waste product by the fish's cells, diffuses from the capillaries through the gills into the passing water.

All higher vertebrates or animals that have a backbone or spinal column (including humans) have immature gill slits when they are in an embryo stage or initially developing. However, these gill slits never fully mature and become functional. They disappear as the vertebrate embryo develops.

This occurs when the systemic capillaries pass among the body cells. The blood in the systemic capillaries has a high concentration of oxygen molecules and a low concentration of carbon dioxide molecules. The body cells and the interstitial fluid surrounding them have just the opposite: a low concentration of oxygen molecules and a high concentration of carbon dioxide molecules (because cells use oxygen to create energy, giving off carbon dioxide as the waste product of human metabolism).

Thus, in internal respiration, oxygen diffuses from the capillaries into the interstitial fluid to be taken up by the cells. At the same time, carbon dioxide diffuses from the interstitial fluid into the capillaries. Red blood cells in the now deoxygenated (carrying very little oxygen) blood then transport the carbon dioxide molecules back to the heart through ever larger veins. Finally, the blood returns to the right atrium of the heart via the venae cavae. After flowing into the right ventricle, the deoxygenated blood is pumped through the pulmonary arteries to the lungs, where the cycle of respiration begins once again.

DO PLANTS BREATHE?

Plants do not "breathe" like animals. All animals have some mechanism for removing oxygen from the air and transmitting it into their bloodstreams, while expelling carbon dioxide from their bloodstreams in the process. Plants exchange oxygen and carbon dioxide with Earth's atmosphere, but in a different process.

Plants create energy for their cells through the process known as photosynthesis. Simply put, a plant absorbs sunlight into chlorophyll (green pigment located in plant cells called chloroplasts) and takes in carbon dioxide from the air through stomata (microscopic openings on the underside of its leaves). It also absorbs water from the soil through its roots. Using the energy from sunlight, the plant combines carbon dioxide and water to create the simple sugar glucose (which is later used to form more complex carbohydrates such as starch and cellulose). Oxygen is a by-product of this process.

In the second phase of photosynthesis, called respiration, the plant combines glucose and oxygen with enzymes to create adenosine triphosphate (ATP), a high-energy molecule used by cells of all organisms to store energy. Since plants use less oxygen during respiration than is created during photosynthesis, they expel that oxygen through their stomata. This action occurs mainly at night when photosynthesis cannot take place.

AILMENTS: WHAT CAN GO WRONG WITH THE RESPIRATORY SYSTEM

Since the respiratory system is open to airborne microorganisms and out-side pollution, many ailments or maladies can afflict it. Some respiratory disorders are relatively mild and, unfortunately, very familiar. Excess mucus (runny nose), coughing, sneezing, nasal congestion, headache, sore throat, muscle aches, and tiredness are all symptoms of the common cold. A viral infection of the upper respiratory system, the common cold can be caused by one of over 200 different viruses. An average individual usually suffers from between 50 and 100 colds during his or her life. Almost all colds clear up in less than two weeks without complications.

When a cold lingers beyond that time period, the cause may be an inflammation of the sinuses, a condition called sinusitis. Caused by a bacterial infection, sinusitis is often mistaken for a common cold because the symptoms are somewhat similar. With sinusitis, congestion may be the same or even worse. While drainage from the nose during a common cold is often clear, drainage due to sinusitis is often thick and yellowish-green in color. Sinus pain and pressure is frequent. A sore throat and bad breath, resulting from drainage dripping down the back of the throat, may also occur. Antibiotic medications are often necessary to treat sinusitis.

Allergies, abnormal immune reactions to otherwise harmless substances, are among the most common of medical disorders. Symptoms depend on the specific type of allergic reaction. In the most common type of reaction, symptoms can mimic those of a nasal cavity infection: pressure, pain, a runny nose, congestion, and a scratchy or irritated throat.

The following are some of the more serious—and often fatal—disorders and diseases that can impair the functioning of the respiratory system or its parts.

Asthma

Asthma is a chronic (long-term) inflammatory disease of the airways. Although the cause for the condition is unknown, it is known that allergies can trigger an asthma attack. Continuing inflammation makes the airways hypersensitive to stimuli such as cold air, exercise, dust mites, air pollutants, and even stress and anxiety. All can then give rise to an asthma attack.

Asthma usually begins in childhood or adolescence, but it also may first appear during adult years. An estimated 15 million people in the United States suffer from asthma. More than 5,600 people die of severe asthma attacks each year.

RESPIRATORY SYSTEM DISORDERS

Asthma (AZ-ma): Respiratory disease often caused by an allergy that is marked by tightness in the chest and difficulty in breathing.

Bronchitis (bron-KIE-tis): Inflammation of the mucous membrane of the bronchial tubes.

Cystic fibrosis (SIS-tik fie-BRO-sis): Genetic disease in which, among other things, the mucous membranes of the respiratory tract produce a thick, sticky mucus that clogs airways.

Emphysema (em-feh-ZEE-mah): Respiratory disease marked by breathlessness that is brought on by the enlargement of the alveoli in the lungs.

Pneumonia (noo-MOE-nya): Disease of the lungs marked by inflammation and caused by bacteria or viruses.

Tuberculosis (too-burr-cue-LOW-sis): Infectious, inflammatory disease of the lungs caused by a bacteria that results in tissue damage.

In an asthma attack, the muscle tissue in the walls of the bronchi and bronchioles go into spasm. As a result, the cells lining the airways swell and secrete mucus into the air spaces. Both these actions cause the bronchi and bronchioles to become narrowed. This, in turn, produces a tightness in the chest, wheezing, and breathlessness, sometimes to the point where an individual gasps for air. Asthma attacks come and go in irregular patterns, and they vary in degree of severity. Some may last only a few minutes; others may go on for much longer.

Treatment for asthma usually includes identifying the specific substance causing the allergic reaction and subsequently avoiding contact with it. Drugs are often given to relax the muscles of the bronchial tubes and allow increased air flow. They may be taken by mouth or inhaled through a nebulizer, a device that delivers a regulated flow of medication into the airways.

Bronchitis

Bronchitis is an inflammation of the mucous membranes of the lower respiratory passages, especially the trachea and bronchi. Bronchitis can either be acute (short-term) or chronic (long-term).

Acute bronchitis usually follows a viral infection such as a cold or the flu. Anyone can be afflicted with the disorder, but infants, young children, and the elderly are more susceptible because their body immunity (ability to fight disease) is generally weaker. Acute bronchitis usually begins with the symptoms of a cold: runny nose, sneezing, and a dry cough. However, the cough soon becomes deep and painful, and it will bring up greenish-yellow phlegm. High fever and wheezing are also common.

If no additional infection is present, acute bronchitis is treated in the same way as the common cold: drinking plenty of fluids, resting, not smoking, and taking acetaminophen for fever and pain. If an additional infection exists, the infection is treated with an antibiotic. When treated, acute bronchitis usually resolves in one to two weeks without complications.

Chronic bronchitis is a major cause of disability and death in the United States, affecting an estimated 14 million people. The disorder is caused by inhaling respiratory irritants, especially cigarette smoke. The American Lung Association estimates that 80 to 90 percent of all cases are caused by smoking. Other irritants include chemical fumes, air pollution, mold, and dust.

Chronic bronchitis develops slowly over time. When smoke or other irritants are inhaled, the cilia projecting from the mucous membrane lining the respiratory tract become paralyzed or snap off. Airways then become inflamed, narrowed, and clogged with mucus, making breathing difficult. A mild cough, sometimes called smokers' cough, is usually the first symptom. Wheezing and shortness of breath may accompany the cough. As the disease advances, breathing becomes even more difficult and activity decreases.

There is no cure for chronic bronchitis and treatment to help reduce symptoms is complex. As in asthma attacks, drugs may be given to relax the muscles of the bronchial tubes and allow increased air flow. The drugs may be taken by mouth or inhaled through a nebulizer, a device that delivers a regulated flow of medication into the airways. To further reduce the swelling of airway tissue, anti-inflammatory drugs may also be prescribed. As the disease progresses, an individual may be required to breathe supplemental oxygen.

The best way to prevent either type of bronchitis is to stop smoking or not even to begin.

Cystic fibrosis

Cystic fibrosis is an inherited or genetic disease, meaning it is caused by a defect in a person's genes. It affects the lungs, digestive system, sweat glands, and male fertility (ability to produce offspring or children). The disease affects about 30,000 children and young adults in the United States. Approximately 3,000 babes are born each year with cystic fibrosis.

Cystic fibrosis affects the body's ability to move salt and water in and out of cells. This defect causes the lungs and pancreas to secrete thick mucus, blocking passageways and preventing proper functioning. The disease derives its name from the fibrous scar tissue that develops as a result in the pancreas.

In the lungs, the thickened mucus increases irritation and inflammation of lung tissue. This inflammation swells the passageways, partially closing them down. At the same time, infection from bacteria or viruses becomes more likely since the mucus is a rich source of nutrients. Bronchitis and pneumonia frequently develop in individuals with cystic fibrosis.

The body's response to the infection is to increase mucus production. White blood cells fighting the infection thicken the mucus even further as they break down and release their cell contents. These white blood cells also provoke more inflammation. The process is a downward spiral as a person suffering from the disease experiences ever-increasing shortness of breath and tiredness. Untreated, cystic fibrosis leads to severe lung infection, which is the primary cause of death.

There is no cure for cystic fibrosis. Regular monitoring and early treatment are key to maintaining respiratory health. Good general health, especially good nutrition and exercise, can keep the body's immune response working properly. This, in turn, can help decrease the number of infections started by the bacteria always present in the lungs of infected individuals.

Clearing mucus from the lungs also helps to prevent infection, and devices and techniques have been developed to help in this regard. Several drugs are available to prevent the airways from becoming clogged with mucus. Lung transplants have become increasingly common for people with cystic fibrosis. About 50 percent of adults and 80 percent of children who receive lung transplants live longer than two years.

Emphysema

Emphysema is a respiratory disease marked by breathlessness that is brought on by the enlargement of the alveoli in the lungs. It is the most common cause of death from respiratory disease in the United States. Emphysema occurs mainly among people who are fifty years of age or older. Heavy cigarette smoking is the primary cause of the disease, although a few cases are caused by an inherited defect.

When a person inhales cigarette smoke, that person's body releases substances that are meant to defend the lungs against the smoke. These substances can also attack the cells of the lungs. Normally, the body prevents such action by releasing other substances. In smokers and those with the inherited defect, no such prevention occurs. Lung tissue is then damaged in such a way that it loses its elasticity. Bronchioles collapse, trapping air in the alveoli. Unable to contract efficiently and move air out, the alveoli overexpand and rupture. The alveoli blend together, forming large air pockets from which air cannot escape. This cuts down the surface area for gas exchange.

As the disease progresses, coughing and shortness of breath occur. Exhaling becomes difficult. Over several years, the extra work of exhaling can cause the chest to enlarge and become barrel-shaped.

Emphysema is a serious and long-term disease that cannot be reversed. The body cannot repair the damage to the lungs. Ultimately, the disease can lead to respiratory failure. If emphysema is detected early, medications may be given to help relax and open air passages, thus reducing some of the symptoms. Mild exercise may be ordered to help strengthen muscles involved in

breathing. An individual suffering from emphysema must stop smoking immediately or no treatments will be effective at all.

Lung cancer

Lung cancer develops when cells of the lung tissues become abnormal and grow uncontrollably, forming tumors. It is the leading cause of death from cancer among both men and women in the United States. Approximately 160,000 people die from the disease each year.

Tobacco smoking is the leading cause of lung cancer. Ninety percent of lung cancers can be prevented by giving up tobacco. Smoking marijuana cigarettes is considered yet another risk factor for lung cancer. These cigarettes have a higher tar content than tobacco cigarettes. In addition, they are inhaled very deeply. As a result, the smoke is held in the lungs for a longer time.

Other causes of lung cancer include exposure to asbestos, toxic chemicals, radioactive minerals, environmental pollution (such as auto exhaust fumes), and a family history of lung cancer.

Lung cancers tend to spread very quickly to other parts of the body. Early symptoms to watch for include a cough that does not go away, chest pain, shortness of breath, persistent hoarseness, swelling of the neck and face, significant weight loss, unexplained fever, bloody or brown-colored spit or phlegm, and recurrent lung infections.

The most common treatment options for lung cancer include surgery, chemotherapy, and radiation. During surgery, surgeons may remove a small part of the lung, one lobe of the lung, or the entire lung. The extent of the surgery depends on how much of the lung is affected. After the cancer has been removed, the physician may recommend chemotherapy (using a combination of drugs to kill any remaining cancer cells and shrink any tumors) or radiation therapy (using X rays or other high-energy rays to kill any remaining cancer cells and shrink any tumors) or a combination of both. Either or both of these methods may be used to shrink the tumor before surgery is attempted.

Almost 50 percent of lung cancer patients survive if the cancer is detected before it has had a chance to spread to other organs and it is treated appropriately. Only 15 percent of lung cancers, however, are found at this early stage. The best way to prevent lung cancer is not to smoke at all or to quit smoking if one has already started. Secondhand smoke from other people's cigarettes should also be avoided.

Pneumonia

Pneumonia is an infection of lung tissues. It can be caused by nearly any organism known to cause human infections. This includes bacteria, viruses, fungi, and parasites. Pneumonia is the sixth most common disease leading to death in the United States. It is also the most common fatal infection acquired by people who are already hospitalized.

Pneumonia develops when the several types of immune substances in the respiratory tract are weakened to the point where invading organisms can take over. Once they do so, an infection develops in the normally sterile environment of the lungs. Symptoms include fever, cough, chest pain, shortness of breath, and increased respirations (number of breaths per minute). Many people cough up sputum (commonly known as spit) streaked with pus or blood.

MOVING AIR BY OTHER MEANS

Breathing is not the only process by which humans move air in and out of the lungs. Some processes or actions are reflexes initiated to clear air passages. Others are indications or extensions of emotional states. The following is a list of these common actions and how they are brought about:

Coughing: Reflex that removes irritants from the mucous membrane of the pharynx, larynx, or trachea. A deep inhalation is suddenly followed by an exhalation with the glottis temporarily closed. After pressure has built up, the glottis suddenly opens and an explosive exhalation is directed out of the mouth.

Crying: Action brought on by an emotional state. Inhalation followed by a number of short exhalations while the glottis is open and the vocal cords vibrate.

Hiccuping: Sudden inhalations when the diaphragm abruptly contracts. Sound produced when glottis closes suddenly to stop the inhalation. May be caused by an irritation of the diaphragm or the nerves that serve the diaphragm.

Laughing: Action brought on by an emotional state that produces basically the same air movements as crying.

Sneezing: Reflex that removes irritants from the mucous membrane of the nasal cavity. Action is the same as in coughing, except that oral cavity is closed off by the uvula (fleshy projection hanging from the soft palate) and the explosive exhalation is directed out of the nose.

Yawning: Reflex whose purpose or stimulus is not fully known. Tiredness, boredom, and seeing another person yawn all seem to trigger yawning. Very deep inhalation with jaws wide open brings tidal air to all alveoli.

The invading organism causes symptoms by provoking the body to launch an overly strong immune response in the lungs. That response, which should help fight the infection, instead damages lung tissue and makes it more susceptible to infection. Capillaries in the lungs become leaky and the fluid seeps into the alveoli. Mucus production in the lungs is increased. The alveoli further fill with fluid and debris from the large number of white blood cells being produced by the body to fight the infection. The amount of oxygen delivered to the rest of the body is then decreased.

Before the discovery of penicillin antibiotics, bacterial pneumonia was almost always fatal. Today, antibiotics are very effective against bacterial causes of pneumonia when given early in the course of the disease.

Tuberculosis

Tuberculosis (TB) is a potentially fatal infectious disease caused by a bacterium that can affect almost any part of the body. However, it is mainly an infection of the lungs. Although TB can be treated, cured, or even prevented if persons at risk take certain drugs, scientists have never come close to wiping it out. Popularly known throughout history as consumption, TB currently affects an estimated 8 to 10 million people worldwide each year. Roughly 3 million people die from the disease each year.

TB is usually contracted by inhaling air sneezed or coughed by someone who is suffering from the disease. Once inhaled, the bacterium reaches the alveoli. Actual tissue damage in the lungs is not caused directly by the bacterium, but by the reaction of a person's tissues to its presence. The infection may progress until large areas of the lung have been destroyed.

An infected person may at first feel vaguely ill or develop a cough blamed on smoking or a cold. A small amount of greenish or yellow sputum (spit) may be coughed up upon arising in the morning. In time, more sputum is produced that is streaked with blood. Chest pain, a low-grade fever, night sweats, loss of weight, difficulty breathing, and weakness are symptoms in advanced cases.

Individuals with TB can be treated at home with a combination of prescribed drugs. If the drugs are not effective, surgery to repair the damaged lung or remove part or all of it may be performed. If the disease is diagnosed early and prompt treatment is given, the recovery rate for TB sufferers is very good.

TAKING CARE: KEEPING THE RESPIRATORY SYSTEM HEALTHY

As people age, elastic tissue throughout the body begins to break down. The lungs begin to lose their elasticity, and the ability to ventilate the lungs (breathe in) becomes more difficult. In addition, many of the defenses set up to protect the respiratory system become less efficient, leaving the system open to infections.

These effects of aging can be delayed or even minimized by taking care of not only the respiratory system but the body as a whole. The following all play a vital part in keeping the body healthy and its immune response functioning at peak efficiency: proper nutrition, healthy amounts of good-quality drinking water, adequate rest, regular exercise, and stress reduction.

All forms of air pollution have some harmful effect on humans, especially on the respiratory system and its parts. For example, prolonged exposure to carbon monoxide can cause heart and respiratory disorders. The oxides of both sulfur and nitrogen attack the human respiratory system. At low concentrations, they can leading to an irritated throat and impaired breathing. At higher concentrations, they can lead to emphysema, bronchitis, and lung cancer. Although it is almost impossible to be free from air pollution in modern urban areas, steps can be taken to reduce the amount of pollutants breathed in. Avoiding polluted areas (if possible) and wearing a mask while working in dusty or dirty places are two such steps.

The single most important thing an individual can do to protect and preserve the respiratory system is to not smoke. Tobacco smoking is perhaps the single worst activity an individual can do in regards to health. In addition to nicotine, a powerful drug that affects the heart and blood vessels, tobacco smoke contains carbon monoxide. Carbon monoxide is a well-known toxic gas that reduces the ability of hemoglobin in red blood cells to carry oxygen to all the cells in the body.

Tobacco smoke also contains tars and other chemicals that damage the delicate cells in the mucous membrane lining the respiratory tract. Cilia projecting from that membrane are either paralyzed or destroyed by cigarette smoke. Pollutants and other particles, which then cannot be removed, settle in the lungs. The extra mucus produced in response to an irritated respiratory tract provides an ideal breeding ground for harmful bacteria.

Many illnesses or disorders result from smoking tobacco. If smoking is continued over a period of time, those illnesses become progressively worse. Chronic bronchitis, emphysema, and lung cancer are a few of the serious disorders that can result from smoking. All can lead to death.

FOR MORE INFORMATION

Books

Bryan, Jenny. Breathing: The Respiratory System. Minneapolis, MN: Dillon Press, 1993.

Parker, Steve. The Lungs and Respiratory System. Austin, TX: Raintree/Steck-Vaughn, 1997.

Roca, Nuria Bosch, and Marta Serrano. The Respiratory System. New York: Chelsea House, 1995.

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. The Respiratory System. New York: Twenty-First Century Books, 1994.

Smolley, Laurence A., Debra Fulghum Bruce, and Rob Muzzio. Breathe Right Now: A Comprehensive Guide to Understanding and Treating the Most Common Breathing Disorders. New York: Norton, 1998.

WWW Sites

Anatomy of the Respiratory System
http://www.jeffersonhealth.org/diseases/pulmonary/anatomy.htm
Site from the Jefferson Health System, a collective of member hospitals and healthcare organizations in the greater Philadelphia region. Includes information on the anatomy of the respiratory system, and includes a discussion of how the various parts work. Also includes links to diseases and disorders of the respiratory system.

Cyber Anatomy: Respiratory System
http://tqd.advanced.org/11965/html/cyber-anatomy_resboth.html
Site provides detailed information on the respiratory system that is geared for students in grades 6 through 12.

Human Respiratory System
http://www.stemnet.nf.ca/~dpower/resp/main.htm
Site provides links that give information on the exchange of gases in the human body. Also has a link that details the structure of the respiratory system.

Respiratory System
http://hyperion.advanced.org/2935/Natures_Best/Nat_Best_Low_Level/Respiratory_page.L.html
Site offers an extensive discussion of the respiratory system, its organs, and various parts. Also includes a discussion of the breathing process (the high band width selection offers more illustrations).

The Respiratory System
http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookRESPSYS.html
Site presents a detailed chapter on the respiratory system from the On-Line Biology textbook.

Your Gross and Cool Body—Respiratory System
http://www.yucky.com/body/index.ssf?/systems/respiratory/
Site presents facts and answers questions about the respiratory system and its various parts.