The lungs are a large spongy organ that sits behind the rib cage and are responsible for exchanging oxygen for carbon dioxide in red blood cells, expelling the carbon dioxide in the breath, and expanding to take oxygenated air back in. The lung is divided into thousands of air filled sacs which are surrounded by capillaries. Human lungs are so efficient that in a healthy human being, over 99% of the carbon dioxide is exchanged with oxygen before the blood is returned to the heart.

Because the lungs contain no muscle, the blood supply to the capillaries in the lungs is almost totally devoid of oxygen. The lungs depend on other muscles, particularly the diaphragm and the pectoral (chest) muscles, to expand the chest and essentially change the air pressure in the body to allow the lungs to expand and contract.

Unlike other bodily tissues, the lungs are surrounded with air, which allows the lungs to expand into the cavity. It is vitally important to allow this space to remain filled with air. If the space fills with a liquid (such as with pulmonary edema or bleeding into the space), the lungs will not be able to expand and the patient will suffocate.

In a healthy human body, the respiration rate, or the number of times the lungs expand and contract in a minute, is about 13-17. This rate rises when a person is exercising, and drops when the person is at rest or asleep. Very rapid breathing without exercise can lead to a deficiency of carbon dioxide in the lung, causing unconsciousness due to hyperventilation.

Lungs operate efficiently with the normal 20-25% oxygen that is contained in air. Patients in distress may be given pure oxygen when the patient has difficulty breathing, or the circulatory system is otherwise impaired. However, giving a patient pure oxygen may damage the lungs over a long period of time.

Pneumonia describes any condition where the air sacs in the lung fill with fluid. This prevents the lung from exchanging oxygen with the air and, in serious cases, will result in the suffocation of the patient.

There are several diseases that affect the lungs. Among the most common are lung cancer, bronchitis, tuberculosis, and pulmonary fibrosis. Smoking is known to be a major factor in the development of lung diseases, but other agents, such as asbestos and mineral dust are also known to cause lung diseases.

It is possible to transplant a lung when the original lung is diseased or damaged. However, a person only needs about half of their optimal lung capacity in order to live a fairly normal life.

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