ABSTRACT
Air flows in and out of the lungs when the air pressure is greater or less than external atmospheric pressure. The pressure in the lungs is varied by changes in the volume of the thorax effected by contraction and relaxation of the muscular diaphragm at the base of the thorax (contraction of the diaphragm lowers it and so increases thorax volume); contraction of intercostal muscles swings up the rib-cage to increase the thoracic cavity volume further. (In newborn babies, the ribs are at right angles to the vertebral column, so breathing is wholly diaphragmatic.)
Air is forced out of the lungs as the muscles relax. Usually only about 10% of the air in the lungs is exchanged at each breath (the tidal volume), but up to 70-80% can be exchanged by deep breathing (Fig. 1). The resting tidal volume in most men and women is about 500 cm3. When the deepest possible breath is inspired the excess over the resting tidal volume is the inspiratory reserve volume: about 1.9 liters in women and 3.3 liters in men. At the end of a forced expiration, the air expelled over the tidal volume is the expiratory reserve volume: in women about 0.7 liters, in men about 1.0 liter (Fig. 1). The tidal volume and the inspiratory and expiratory reserve volumes represent the vital capacity of the lungs. There is a residual volume of air left in the lungs (about 1.1 liters in women, 1.2 liters in men) which cannot be expelled (vital capacity + residual volume = total lung capacity) (Fig. 1).
