Diaphragm
The diaphragm is the primary muscle of inspiration. It is a thin, dome-shaped sheet of musclethat inserts into the lower ribs. When it contracts, it pushes downward and spreads out,increasing the vertical dimension of the chest cavity and driving up abdominal pressure. Thisincrease in pressure drives the abdominal contents down and out, which in turn increases thetransverse size of the chest cavity.
Because the diaphragm is covered by the inferior surface of the parietal pleura, when itcontracts it pulls the pleura with it. This lowers thepleural pressure, which causes thealveolar pressure to drop, which, in turn, causes air to flow into the lungs. During quietexpiration, the diaphragm passively relaxes and returns to its equilibrium position. However,during exercise, expiration becomes an active process-- the abdominal muscles contract to raiseabdominal pressure, which pushes the diaphragm upward and forces air out of the lungs.Duringquiet breathing, the diaphragm moves a centimeter or two up and down, but during exercise, itcan move more than 10 cm.
The diaphragm is supplied by the phrenic nerve from cervical segments 3,4, and 5.
See Also:
