Exchange of Air
The amount and movement of air and expired gases in and out of the lungs are controlled by expansion and recoil of the lungs. The lungs do not actively expand and recoil themselves. Rather they are acted upon to do so in two ways by downward and upward movement of the diaphragm to lengthen and shorten the chest cavity and
(2) by elevation and depression of the ribs to increase and decrease the back to front diameter of the chest cavity (figure given below)
Normal quiet breathing is accomplished almost entirely by movement of the diaphragm . During inspiration, contraction of the diaphragm creates a negative pressure (vacuum) in the chest cavity and air is drawn into the lungs. During expiration diaphragm simply relaxes the elastic
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When the glottis is open and no air is flowing into or out of the lungs the pressure in all parts of the respiratory tree is the same all the way to the alveoli and is equal to the atmospheric pressure. To cause inward flow of air during inspiration . the pressure in the alveoli must fall to a value slightly below atmospheric pressure. During expiration , alveolar pressure must rise above atmospheric pressure.
Exchange of Respiratory Gases
With ventilation oxygen diffuses from the alveoli into the pulmonary blood and carbon dioxide diffuses from the blood into the alveoli. The process of diffusion allows for the movement of oxygen and carbon dioxide through the alveolar capillary membrane. At rest, the partial pressure of oxygen in the alveoli is about 60mmHg greater than that in the pulmonary capillaries. Thus oxygen diffuses into the pulmonary capillary blood. Similarly, carbon dioxide diffuses in the opposite direction. This process of gas exchange is so rapid as to be thought of as instantaneous .
Control of