The two lungs are spongy and highly elastic organs of respiration in the pulmonary cavities of the thorax, where the aeration of blood occurs.
Description
Each lung has an irregular conical shape with a blunt top, called the apex, extending into the root of the neck. They have concave bottoms resting on the arc of the diaphragm, a mostly concave inner mediastinal surface that follows the lines of the pericardium, and a convex outer (costal) surface. The right lung is larger than the left, and consists of three lobes (upper, middle, and basal or lower). The left lung consists of two lobes, an upper and a basal, or lower, lobe.
Each lung consists of an exterior plasma coat comprised of an organ coat which folds back to make an interior lining for the chest cavity. The inner lung contains sub-serous areolar tissue with elastic fibers interspersed over the surface of the organ. The parenchyma, or functional part of the organ, is composed of secondary lobules (alveolar ducts) that differentiate into primary lobules (alveoli) consisting of blood vessels, lymphatics, nerves, and an alveolar duct that connects with air space.
The lung, as it relates to inspiration and expiration, has two distinct zones in which the lung passages convey air to the alveolar sacs. The zones relate to the two functions of these passages. One is for conducting air, and the other is for respiration. The parts of the conducting zone do not participate in gas transfer, rather they convey air to and from the respiratory zone. All of the parts of the respiratory zone can take part in gas transfer. However, the uppermost branches, such as the respiratory bronchioles, participate in respiration only in times of exertion.
The conducting zone starts at the trachea and branches out to the bronchi. The bronchi differentiate into bronchioles and then into terminal bronchioles. The respiratory zone starts after the terminal bronchioles at the respiratory bronchioles. These differentiate into the alveolar ducts, which terminate at the alveolar sacs. The lungs consist mainly of the tiny air containing alveolar sacs.
Function
The lung is the sole means of gas exchange in respiration. Air is brought into the body through the mouth or nose and trachea to the lung. There oxygen diffuses from the airspace of the alveoli into the blood stream and carbon dioxide diffuses from the blood into the alveoli's airspace.
The alveoli are small hollow sacs. Their ends connect to the lumens of the airways. The air adjacent to surfaces of the alveolar wall are lined by a single cell layer of flat epithelial cells called type I alveolar cells. In between type I cells are type II cells. They are thicker, and secrete a fluid called surfactant. In the alveolar walls this fluid and connective tissue fills the interstitial space and is interspersed with capillaries. In some places the interstitial space is nonexistent and the epithelial cell membranes are in direct contact with the capillaries. The blood in the capillaries is separated from the air by a single layer of flat epithelial cells. The surface area in a single alveoli is roughly the size of a small basketball court due to the undulating terrain of the type I and II epithelial cells. There are around 300 million alveoli in the adult male. Thus, there is a large surface area where the air and the blood stream are in close proximity. This large surface area is necessary for gas exchange to easily occur. The respiratory system also needs a continual supply of fresh air, which is supplied by the process of breathing.
The process of breathing is aided by the position of the lungs in the thorax (chest). The thorax is a closed chamber that extends from the neck muscles to the diaphragm. The diaphragm is a dome shaped sheet of skeletal muscle that separates the thorax from the abdomen. The sides of the thorax are bounded by connective tissue around the spine, ribs, intercostal muscles, and sternum.
A completely enclosed sac consisting of a thin sheet of cells, called the pleura, surround each lung. Between the pleura and the lung is interstitial fluid. As the diaphragm expands and contracts the intra-pleural pressure placed on the lungs causes the lung to inflate and deflate. Breathing allows a fresh supply of air and oxygen to enter the lung upon inflation and carbon dioxide to exit the lung upon deflation. It also causes a change in the pressure of the lung.
The epithelial surface from the conducting zone to the respiratory bronchioles is lined with cilia that continually beat in the direction of the pharynx. There are epithelial cells and glands on this surface that secrete mucus. This mucus catches particulate and bacterial matter, and the material (and mucus) is slowly moved by the cilia toward the pharynx. There it is either swallowed or coughed up as sputum. The epithelial layer also secretes another viscous fluid that allows the cilia to move mucus easily out of the lung.
Toxic substances can inhibit ciliary action. Agents like cigarette smoke can paralyze the cilia for extended periods of time. This inhibits the movement of mucus and particles out of the lungs. The suspension of this process can inhibit gas exchange and eventually cause prolonged oxygen deficiency.
Respiration
Respiration is the process by which the body takes in oxygen and emits carbon dioxide. The following is a summary of the steps of respiration:
ventilation
interchange of CO2 and O2 between alveolar air and blood in lung capillaries
transport of CO2 and O2 through the bloodstream
interchange of CO2 and O2 between blood in lung capillaries and alveolar air by diffusion
use of O2 and production of CO2 by cells in metabolism
Ventilation is the interchange of air between the atmosphere and the alveoli by bulk flow. Bulk flow is the movement of air from a region of high pressure to one of low pressure. Bulk flow may be thought of as occurring between the outside air, the air in most of the lung, and the air in the alveolar sacs. Flow of some gases (especially oxygen and carbon dioxide) also occurs between the alveolar air and the blood. It is important to note that the pressure of individual gases is different in different types of air. For example, air going into the lungs is rich in oxygen and low in carbon dioxide. Air leaving the lungs is rich in carbon dioxide and low in oxygen. The different concentrations (or pressures) of individual gases are known as the partial pressures, and the partial
pressure of each individual gas adds up to the total pressure of the gas.
When air is inspired (taken in), it has a higher partial pressure of oxygen than the air already in the lung, and a lower partial pressure of carbon dioxide. Therefore, inspired air allows oxygen to flow from the area of highest pressure (inspired air) to the alveolar sacs (that have a lower partial pressure of oxygen), and into the bloodstream. The same inspired air has a low partial pressure of carbon dioxide, so carbon dioxide leaves the bloodstream (where it has a high partial pressure), enters the alveolar air (where the pressure is lower), and is passed onto the inspired air (where the partial pressure is even lower). Thus, carbon dioxide gas and oxygen gas both move from areas of highest pressure to lowest pressure in an attempt to reach a pressure (or concentration) equilibrium. This process is called gas exchange. After gas exchange has taken place, the air is expired, or expelled to rid the body of air that has a high concentration (partial pressure) of carbon dioxide gas. Then the process begins again.
Lung expansion and contraction
The concept of bulk flow (explained above) and Boyle's law explain the expansion and contraction of the lung. Boyle's law states that, at constant temperature, an increase in the volume of a container (lung) lowers the pressure of a gas, and a decrease in the container (lung) volume raises the pressure. Thus, when the volume of the lung expands, the pressure inside the lung is lowered, and when the volume of the lung contracts, the pressure inside the lung rises.
Inspiration occurs when the muscles of inspiration increase the volume of the thoracic cavity. The decrease in pressure in the cavity causes the lungs to expand to fill the cavity, which lowers the pressure inside the lung. Since air flows from areas of high pressure to low pressure, air fills the lungs to equalize the air pressure inside the lungs with the outside air, and inspiration occurs. The difference between the internal pressure in the lung and the pressure of the outside air is called the transpulmonary pressure.
During expiration, the muscles of inspiration relax, and the lung contracts. The decreased volume causes increased pressure inside the lungs, which results in air being expired, or expelled. In normal adults, expiration does not require any effort.
Role in human health
The lungs ability to extract oxygen from the atmosphere and supply it to the body's tissues is essential for metabolism and therefore for life. Disease and disorder can interfere with the body's normal function and slow a normally healthy person. Serious interference with the lung's function can cause hypoxia and even death.
Common diseases and disorders
Asthma is an intermittent disease characterized by a chronic inflammation of the airways, causing smooth muscle contraction in the airway. The causes vary from person to person and can include allergies, viral infections, environmental pollutants, mold, dust, dander, cigarette smoke, overexertion, and naturally released bronchiorestrictors. Ingested items such as food coloring, preservatives, and medications can trigger an attack.
Chronic obstructive pulmonary disease (COPD) refers to emphysema, chronic bronchitis, or a combination of the two. This category of disease is one of the major causes of death and disability in the world. These diseases restrict ventilation and the oxygenation of the blood.
Chronic bronchitis is characterized by excessive mucus production in the bronchi and chronic inflammatory changes in the small airways. The accumulation of mucus and thickening of inflamed airways obstruct the flow of air. It is primarily a result of cigarette smoking, although pollution may also play a role.
Emphysema is a major cause of hypoxia and is characterized by the destruction of the alveolar walls, and the atrophy and collapse of the lower airways. The lungs self-destruct through the secretion of proteolytic enzymes by white blood cells. Cigarette smoke stimulates the release of harmful enzymes and destroys the enzymes that normally protect against proteolysis. The proteolytic enzymes cause the breakdown of the alveolar walls. The damaged alveoli fuse and a gradual decrease in the surface area available for gas exchange results. Emphysema increases the work of breathing and, when severe enough, causes hypoventilation (inadequate venti lation). The obstruction caused by the collapse of the lower airways is accompanied with destruction of the lung's elastic tissues and the eventual collapse of the air ways.
Pneumonia is normally caused by bacterial or viral infection. It can be triggered by the inhalation of toxic chemicals, chest trauma, yeast, rickettsiae, and fungi. It is the inflammation and compaction of the lung parenchyma. The alveolar spaces fill with mucus, inflam matory cells, and fibrin.
Tuberculosis is caused by the infection of Mycobacterium tuberculosis. It can affect most organs but is most commonly found in the lungs. The bacteria cause lesions to be formed on the lungs and spread to other tissues. Pulmonary tissue in motion will be chroni cally affected and may eventually be destroyed, if left untreated. The erosion of lung tissue into the blood ves sels can result in life-threatening hemorrhages.
Interstitial space—The spaces found within organs and tissues.
Metabolism—A series of chemical and physiological changes in the body that either build larger molecules out of smaller molecules (anabolism) or break down larger molecules into smaller ones (catabolism).
Parenchyma—The active portion of an organ that fulfills its function (as opposed to purely structural portions of the organ).
Proteolysis—The breaking down of proteins by cleaving or hydrolyzing peptide bonds (the bonds connecting amino acids within the protein).
BOOKS
Bullock, John, et. al. National Medical Series for Independent Study—Physiology. Third ed. Williams & Wilkins, 1995.
Vander, Arthur et. al. Human Physiology—the Mechanisms of Body Function. Eighth ed. McGraw-Hill, 2001.
ORGANIZATIONS
The American Lung Association. 1740 Broadway, New York, NY, 10019. 212-315-8700. <http://www.lungusa.org/>.
