Mechanism of Breathing

Breathing is the rhythmic, repeated process of drawing air into the lungs (inspiration) and expelling it back out (expiration), and unlike the heartbeat, it is driven not by any muscle contracting directly within the lung tissue itself -- the lungs contain no muscle capable of actively expanding or contracting them -- but entirely by cyclical changes in the volume of the surrounding thoracic cavity, brought about by two sets of skeletal muscles: the diaphragm and the intercostal muscles.

The physical principle underlying the whole mechanism is a simple and general one: for a fixed quantity of an enclosed gas at a constant temperature, its pressure and its volume vary inversely -- if the volume of the space containing the gas increases, the pressure of that gas falls, and if the volume decreases, the pressure rises. Because the lungs are elastic and are mechanically coupled to the thoracic wall through the pleural membranes, any change in the volume of the thoracic cavity is transmitted almost directly to the lungs themselves, and the resulting change in pulmonary (intrapulmonary) pressure, relative to the constant atmospheric pressure outside the body, is exactly what drives air to flow in or out.

Inspiration begins when the diaphragm, normally relaxed and dome-shaped (curving upward into the thoracic cavity), contracts and flattens, pulling its dome downward and so increasing the vertical dimension of the thoracic cavity. At almost the same moment, the external intercostal muscles -- the outer of the two layers of muscle between the ribs -- contract, pulling the ribs upward and outward and so increasing the front-to-back and side-to-side dimensions of the thoracic cavity as well. Together, the contraction of the diaphragm and the external intercostal muscles enlarges the thoracic cavity in all three dimensions at once; because the lungs are pulled outward along with the expanding thoracic wall, the volume of the lungs increases correspondingly, and by the inverse pressure-volume relationship, the pulmonary pressure inside the lungs falls to a level slightly below atmospheric pressure. Air, always moving from a region of higher pressure to one of lower pressure, therefore flows in through the respiratory passage described in the previous section, down this pressure gradient, until the pulmonary pressure once again equals atmospheric pressure and inflow stops -- completing one inspiration.

Expiration, under normal, quiet breathing, is very largely a passive process, requiring no active muscular effort of its own. Once inspiration ends, the diaphragm and the external intercostal muscles simply relax; the diaphragm's dome springs back upward under its own elasticity and the recoil of the abdominal organs beneath it, and the rib cage falls back down and inward under the elastic recoil of the stretched lung tissue and thoracic wall themselves. This return to the resting position reduces the volume of the thoracic cavity, and with it the volume of the lungs, so that the pulmonary pressure rises to a level slightly above atmospheric pressure, driving air back out through the respiratory passage until the pulmonary pressure again equals atmospheric pressure and outflow stops. …