Lungs

Anatomy

Definition: Vital organs that allow gas exchange between an organism and its environment.

Anatomy or system affected: Chest, respiratory system

Specialties and related fields: Environmental health, exercise physiology, oncology, pulmonary medicine, vascular medicine

Structure and Functions

Lungs are vital organs that allow gas exchange between an organism and its environment. Most animals have lungs, although many fish do not. Efficient gas exchange with the environment is critical for larger organisms because oxygen is required for the last step in a series of cellular chemical reactions which processes nutrients from food. These reactions, called aerobic respiration, provide most of the energy that maintains life. Furthermore, as these reactions proceed, parts of larger carbon molecules are removed. Carbon dioxide is produced as a by-product and must be removed from the body. Hence, oxygen and carbon dioxide must be exchanged.

Small aerobic organisms can simply absorb the oxygen from air or water across their moist membranes or skins. The oxygen travels from where it is more concentrated to where it is less concentrated, a process called diffusion. The carbon dioxide inside the cells also diffuses across the membrane in the opposite direction to the environment. Larger organisms, however, have relatively less outside surface area and require special structures for their gas exchange. Various types of gills, swim bladders, and lungs are all examples of ways to absorb more oxygen and release more carbon dioxide.

This article focuses on one of these specialized structures: the lung. The lung is found in air-breathing land creatures. It allows oxygen to enter the blood and carbon dioxide to be removed. Form reflects function: The lung provides large amounts of moist surface area, close to many small blood vessels for gas exchange. Humans have a joined pair of lungs suspended in the chest cavity. The two lungs are somewhat different in size: The left lung is divided into two lobes, while the right has three lobes. This difference reflects the fact that the left side of the chest cavity has less room because of the position and shape of the heart.

The pathway to the lungs begins with the nose. The air entering each nostril is temporarily divided among three pathways (nasal conchae) and then warmed and moisturized by contact with a mucous membrane containing many blood vessels. Bacteria and particles get caught on the sticky mucus on this membrane. If objects pass this point they can be trapped by mucus lower in the tract and be swept out by waving cilia, hairlike fibers extending from cells of the airway that move in unison to push particles backward. Large particles that irritate the mucous membranes can cause a sneeze, which may eject the offending particle at speeds up to 100 miles (160 kilometers) per hour.

The air then continues to the pharynx (throat), where the nasal passageways and the mouth meet, and moves into the larynx, the organ that produces the voice. Swallowing pulls the larynx upward, allowing the epiglottis to flip over the opening and prevent food from entering this part of the airway. This movement of the larynx during swallowing can be felt by light touch with the fingers.

The trachea, or windpipe, follows. It is 11 centimeters long in humans and made rigid by rings of cartilage. The inside of the trachea has cilia and also produces mucus. As the trachea approaches the lungs, it branches into two bronchi, which enter sides of the lungs at a midpoint between top and bottom. The walls of the bronchi contain cartilage rings and smooth muscles. Irritation in the larynx, the trachea, or the bronchi may cause coughing. Coughing is a reflex, like sneezing, that attempts to cast out impurities.

The bronchi continue to branch until they contain only smooth muscle; at this point, they are called bronchioles. The smooth muscle can contract or relax to allow the diameter of the bronchioles to adjust. Hence, the airflow can be changed according to the needs of the body. The pathways inside the lungs resemble an upside-down tree. Millions of cilia line the bronchial “tree” and constantly beat to remove particles. Each bronchial tube branches into several alveolar ducts. Each duct ends with a grapelike cluster of sacs called alveoli. The irregular branching that has led to this point ranges from eight to twenty-five divisions, with an average of twenty-three. Each alveolus has walls that are only one cell thick. Because of the large number of these air sacs, the lungs are very light in weight.

Gas exchange occurs in the alveoli. These structures are closely associated with the body's smallest blood vessels, the capillaries. Oxygen dissolves into the moisture on the vast surface of the alveoli. It then crosses the thin tissue of the lungs and moves into the capillaries to enter the blood. Carbon dioxide moves in the other direction to the lungs. Direction is maintained by the principle of diffusion: Flow is always from a higher to a lower concentration. The surface tension of the watery film inside the alveoli can cause a problem in gas exchange. Water molecules have a strong attraction for one another and can cause the alveoli to collapse to a smaller volume, reducing the surface area available for gas exchange. Fortunately, among the regular cells of the lining of the alveoli is found a second type of cell, called the type II cell. Type II cells produce surfactant, a mixture of chemicals that lowers the overall surface tension in the alveoli by separating the water molecules. Therefore, the alveoli stay fully inflated.

Roaming white blood cells called macrophages are a final defense against foreign objects at the alveolar level. Macrophages protect the lungs by attacking and eating bacteria and particles. They can be found elsewhere in the body performing the same function.

The pleural membrane is a double covering, one layer lining the outside of the lungs and the other lining the inside of the chest cavity. These two layers, which are really the same membrane, move over each other as breathing occurs, reducing friction. If air enters the space between the double membrane, however, the lung will collapse, a condition known as pneumothorax.

Air enters the entire airway by expansion of the chest cavity, or thorax. The cavity can be thought of as a box in which the top cannot be moved upward but the bottom and the sides may move outward. The arched diaphragm muscle at the base of the cavity contracts to lower the bottom of the box. Muscles between the ribs, called intercostals, contract to elevate the chest. The ribs, which slant downward when relaxed, move outward. This expansion pushes the walls out, increases the volume of the chest cavity, and lowers its internal pressure, causing air to be pushed into the lungs. Exhalation results when the muscles relax and allow the natural recoil of the lungs to expel the air.

Young children breathe differently than do older children or adults. Babies and toddlers have ribs that are nearly horizontal. They depend mainly on the descent of the diaphragm muscle for breathing. By two years of age, the ribs have moved to the adult position and rib muscles increase in importance. In addition, a sexual difference in breathing has been observed. Females tend to rely mainly on rib movement, while males tend to use both rib and diaphragm movement, with an emphasis on the diaphragm.

The medulla oblongata of the brain and the reticular formation of the brain stem help to regulate breathing. Physical activity produces more carbon dioxide and affects the rate of respiration. The normal relaxed breathing rate is about twelve to twenty times a minute. A person resting in bed may inhale eight liters of air per minute, while a runner may reach fifty liters per minute. If a person relaxes and falls into a very shallow rhythm, a yawn attempts to break the pattern. A yawn is a deeper breath that causes more gas exchange.

Disorders and Diseases

The lungs are the only major internal organs exposed to the outside environment, and they tend to show the effects of both age and type of use. A child's lungs are pink, but with age this color becomes darker and mottled because of particles that are trapped inside the macrophages of the lung. The lungs of city dwellers, factory workers, and coal miners show the greatest effects because of the poor quality of the air being inhaled. Understanding the pathologies of the lungs is linked to understanding the function of the lung itself.

For example, smoking cigarettes or cigars and exposure to air pollution are known to cause chronic bronchitis. The repeated irritation of the bronchi by pollutants causes the linings of the air tubules to thicken, closing down the airways. Muscles contract, and the secretion of mucus increases. Poor drainage may lead to pneumonia. Smoking tobacco can also lead to cancers of the lung, mouth, pharynx, and esophagus. Tobacco smoke may contain as many as forty-three carcinogenic (cancer-causing) chemicals. Lung cancer usually begins with changes in the lining of the bronchi among the cells with cilia and those that produce mucus. The long-term irritation of smoking eventually destroys these cells faster than the bronchi can replace them. Abnormal cells, without cilia or the ability to produce mucus, begin to take their place. These cells offer less protection and, as irritation and replacement continues, may become cancerous. In the United States, lung cancer is the leading cause of cancer deaths in both men and women, and evidence has revealed the danger of secondhand smoke as well. Smoking also leads to greater risk of various other lung diseases.

Pneumonia is a general term for any inflammation that produces a fluid buildup in the lungs. The excess fluid makes breathing difficult by blocking the alveoli. The cause of the inflammation can be bacterial, viral, fungal, or chemical. For example, Legionnaires' disease is a type of pneumonia caused by a bacterium that lives in air conditioners, humidifiers, and other water-storage devices. Because it causes a lack of oxygen in the body, pneumonia can be fatal if it is not controlled: More than 50,000 deaths attributed to pneumonia occur in the United States each year. The very young and the very old are in the most danger, especially if they have already been weakened by other illnesses. Since the discovery of antibiotics, however, the recovery rate from pneumonia has improved.

Bronchitis is an inflammation of the mucous membrane of the bronchi that often follows a cold. A telltale symptom is a deep cough that eventually brings up gray or greenish phlegm. Bronchitis may be viral or bacterial; if the cause is bacterial, antibiotics can help in recovery. Chronic bronchitis can result from repeated attacks of bronchitis and is aggravated by smoking and air pollution.

Another lung disease is emphysema, which is usually caused by smoking. This condition is often seen in advanced cases of chronic bronchitis. In emphysema, the alveoli overinflate and break. Nearby alveoli are damaged and merge into larger units, leaving less surface area for gas exchange. Therefore, less of the air coming into the lungs comes into contact with the membrane. The increase in dead air space requires deeper breaths to obtain oxygen, and the lungs suffer further damage. Because the air sacs are permanently broken down, the damage is irreversible. Individuals with both emphysema and chronic bronchitis have chronic obstructive pulmonary disease (COPD), which is a progressive illness that makes it difficult to breathe. Smoking is the leading cause of COPD, although long-term exposure to pollutants, chemical fumes, and dust can also cause the disease.

Tuberculosis is a highly contagious bacterial infection that damages the lungs and can spread to the kidneys and the bones. Immunization, screening for exposure, and antibiotics have controlled the number of cases found in countries with modern medical care systems. Worldwide, however, tuberculosis remains a major danger, and more than one million lives are still lost to this disease every year.

Asthma involves a hyperactive response of the airways. During an attack, the smooth muscles of the bronchi and bronchioles contract, and excess mucus is produced. In other cases, the airways may become inflamed and swollen. The cause may be allergies or other stimuli. Asthma is rarely fatal but interferes with normal functioning, as breathing becomes difficult. The blockage of breathing can be reversed with proper medications. Asthma does not lead to emphysema.

Respiratory distress syndrome occurs in about 50,000 premature infants every year. These infants have not yet developed the ability to produce sufficient surfactant in their alveoli to prevent collapse. The importance of surfactant can be illustrated by the difficulty of a baby's first breath. To inflate the alveoli requires up to twenty times the force of a normal breath. Without surfactant, the alveoli would collapse again and the next breath would be just as difficult. In 1990, a surfactant treatment derived from calf lungs became available. Treatment of premature babies with this surfactant before symptoms develop has resulted in an 88 percent survival rate. Continuous positive airway pressure (CPAP) or assisted ventilation is also used to treat respiratory distress syndrome.

Cystic fibrosis is a severe genetic problem in which the mucus produced in the airways (and the gastrointestinal tract) is abnormally thick. This thick mucus interferes with gas exchange, causing the heart to work harder and the valves to be damaged. As a result, the lung may collapse. Serious infections are more likely to occur. Progress on curing cystic fibrosis is being made: Researchers working in this area have located the gene that causes the condition. Medical advances have greatly increased the life expectancies of people with cystic fibrosis.

If air is allowed to enter between the pleural membrane, the lungs will instantly collapse. The two lungs are independent enough so that one lung can be collapsed for healing while the other performs the gas exchange for the body. Furthermore, each lung subdivides into its lobes and then into ten bronchopulmonary segments. Each of these segments is a structural unit that can be removed surgically if diseased.

Perspective and Prospects

The ancient Greeks established the first understandings of lung function. They rightly accepted that life depended on air but overgeneralized that air carried all disease. Empedocles of Agrigentum (ca. 500–430 BCE) demonstrated that air was a real substance by filling a wineskin with it. Empedocles erred, however, in explaining the mechanism of breathing. He compared the body to a pipe and thought that the movement of air in and out of the lungs caused vital air to move in and out of pores in the body's skin.

The writings of Galen of Pergamum (129–ca. 199 CE) came to dominate Western medicine until the Renaissance. In his physiology, Galen attempted to connect the function of the lungs with the blood. He believed, however, that the liver produced a “vegetative” blood that traveled to the vena cava and then took different pathways. Some then flowed to other veins to nourish the whole body for growth. The rest entered the right side of the heart. Some of this substance entered the pulmonary artery into the lungs to allow impurities to be exhaled. The rest filtered to the left side of the heart through imagined pores in the septum.

In Galen's complicated scheme, the lungs were not only for exhaust: Vital air was inhaled there to be modified. The heart then pumped the modified air through the pulmonary vein to its left side. Here the air joined the blood to become “vital spirit,” which traveled by arteries to warm the whole body. The brain converted this vital spirit into “animal spirit,” distributing it by the nerves to cause movement and sensation. Galen did not know that blood traveled from one side of the heart to the other by moving through the lungs. He believed that the lungs acted as a reservoir of air for the heart. Galen also thought that breathing cooled the heart.

William Harvey (1578–1657) studied the position of valves in the veins and realized that Galen's vegetative blood traveled backward. Harvey then argued for a single blood that must go through the lungs to reach the other side of the heart. Blood travels in a circle, he bravely suggested. He was supported when the new microscopes discovered the necessary small vessels that connect arteries to veins.

Antoine-Laurent Lavoisier (1743–1794) noted that the lungs take in oxygen and that carbon dioxide is exhaled. He concluded that a slow combustion must occur in the lungs to warm the blood, while opponents noted that the lungs are not warmer than other parts of the body. By the 1790s, the idea was accepted that the lungs exchange Lavoisier's gases with the blood. Many believed that blood was the essence of life. In the 1850s, however, Georg Liebig and Hermann von Helmholtz showed that muscle tissue uses oxygen and releases carbon dioxide and heat. It was finally realized that the cells are the location of Lavoisier's slow fire of respiration and that the blood is the carrier of gases between the cells and the lungs.

Key Terms

aerobic respiration: the chemical reactions that use oxygen to produce energy; some small organisms do not use oxygen and are called anaerobic

alveoli: small, thin-walled sacs at the end of the airways; most gas exchange with the blood occurs here

cellular respiration: the chemical reactions that produce energy in the cell; these reactions can be aerobic or anaerobic

cilia: hairlike structures on cells that sweep mucus containing bacteria and foreign particles out of the airways

diffusion: the constant motion of molecules that tends to spread them from places of high concentration to those of lower concentration; gases move across the alveoli by diffusion

gas exchange: the movement of oxygen and carbon dioxide across the membrane of the lungs; other gases, such as nitrogen, may also cross the membrane

mucus: a thick, clear, slimy fluid produced in many parts of the body; in the lungs, mucus catches foreign material and provides lubrication to allow smooth airflow

respiration: the exchange of gases in breathing or the cellular chemistry that involves the same gases in the cell and produces energy

Bibliography

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