Definition of Lungs

Lungs – The paired organs in the chest that bring oxygen-bearing air into the body and expel wastes in the form of exhaled gases, primarily carbon dioxide.

Structure of Lungs and Function

The right lung has three lobes and the left lung has two lobes. An indentation between the left lung’s two lobes, called the cardiac notch, cradles the heart. The lungs and heart, along with their supporting structures, fill the thoracic cavity (chest). The heart pumps deoxygenated blood to the lungs via the pulmonary arteries and receives oxygenated blood back from the lungs via the pulmonary veins, circulating the body’s entire blood volume through the lungs once every minute.

The trachea (windpipe) carries air from the throat into the lungs, branching into the right and left bronchus to deliver air to the right and left lung, respectively. Each bronchus further subdivides into mainstem bronchi going to each lobe of the lung and into progressively smaller bronchial branches within the lungs. The smallest branches are the bronchioles which terminate in the alveoli, grapelike clusters of tiny sacs where the oxygen-carbon dioxide exchange takes place. A weblike mesh of capillaries (tiny blood vessels) covers each alveolus. Each lung contains about 300 million alveoli, which gives lung tissue a spongelike appearance.

Each lobe of the lung consists of multiple segments, anatomically and physiologically distinct. A bronchial structure-bronchi, bronchioles and alveoli along with supporting nerves, arteries, and veins-supplies each segment. The three lobes of the right lung contain 10 segments; the two lobes of the left lung contain 8 segments. This structural and functional compartmentalization aids the efficiency of the lung as well as helps protect it in the event of injury (either traumatic or due to disease), enabling portions of the lung to function when others are damaged or diseased.

Lung tissue contains elastin, a substance that, as the name implies, gives the lung tissue elasticity. The lungs have no ability to move on their own but rather function as a pair of synchronized bellows that stretch and rebound with contraction and relaxation of the diaphragm and the intercostal muscles (the muscles between the ribs). Contraction of these muscles expands the chest, and the lungs stretch to fill the space which pulls air into the lungs. When these muscles relax, the chest returns to its normal size and the lungs rebound, pushing air back out of the lungs. Each combination of inhalation and exhalation constitutes a respiratory cycle. The lungs complete 15 to 20 respiratory cycles each minute in a healthy adult.

Health Conditions that Affect The Lungs

PNEUMONITISPneumocystis carinii

How Lungs work

Oxygen-carbon dioxide exchange, the process of getting oxygen into and removing carbon dioxide from the blood, is the primary purpose of the lungs and is a function of physics in which molecules follow the path of least resistance. During inhalation the air pressure within the alveoli is less than the air pressure outside the lungs.

Oxygen molecules pass across the thin alveolar membrane and into the capillaries to enter the bloodstream. During exhalation the air pressure within the alveoli is greater than the atmospheric air pressure, inducing carbon dioxide molecules to cross from the capillaries into the air in the alveoli.

For further discussion of the lungs within the context of pulmonary structure and function please see the overview section “The Pulmonary System.”


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