If the air sacs, or alveoli, and airways of an average sized adult were laid flat, the total surface area available for the exchange of oxygen and carbon dioxide in the lungs would be roughly the size of a regulation badminton court, or about 750 sq. ft. According to the American Association of Respiratory Care, the alveoli, which provide this surface area, contain millions of tiny capillaries making gas exchange highly efficient. Essentially, the principal function of the lung is to bring in oxygen and release carbon dioxide, a process called respiration. A subset of this function is pulmonary circulation, or the process of moving oxygenated blood to the heart and into the systemic circulation.
Breathing
Moving air into and out of the lungs depends on the differences in pressure between the air outside the body, atmospheric pressure and the air inside the lungs. These differences in pressure occur because the volume of the chest cavity changes when the breathing muscles contract and relax. At rest, the volume of the chest cavity changes. The diaphragm, a muscular sheet that forms the floor of the chest, and the external intercostal muscles that lie between the ribs, both contract and relax. This process of breathing is a function of the lungs.
Air Blood Interchange
The thin walls of the air sacs, or alveoli, in the lungs are in extremely close proximity to the thin surface area of the capillaries in the lungs. According to the National Emphysema Foundation, air in the alveoli, and blood in the capillaries is separated only by the respiratory membrane, a thin structure made up of cellular layers located at the point where the alveolar and capillary walls meet. This proximity allows oxygen and carbon dioxide to pass easily between the alveoli and capillaries.
Oxygen and Carbon Dioxide Exchange
Gases tend to pass from areas in which they are highly concentrated to areas of low concentration. For this reason, oxygen passes out of the alveoli, where it is highly concentrated, into capillaries that have a low oxygen concentration. According to the Cleveland Clinic, the oxygen enters the red blood cells and binds to molecules of hemoglobin, an oxygen carrying substance. For the same reason, carbon dioxide passes out of the plasma in the capillaries and into the alveoli.
Non-Respiratory Functions
In addition to respiration or the oxygen and carbon dioxide transfer, the lungs also filter out microscopic particles that may form during systemic circulation. Additionally, the lungs provide a shock-protection of sorts to protect the heart.
