c. Vital Capacity. The vital capacity of the lung is the total amount of air that
can be exchanged during total filling and emptying of the lung. For example, if one
inhales as much air as one can and then exhales as much as possible, the volume
exhaled would be the vital capacity.
7-9. BREATHING CYCLES
A breathing (respiratory) cycle is a sequence in which the lungs are filled and
emptied to produce an exchange of the air in the lungs. The cycle includes an
inhalation of air (filling of the lung with air), then a rapid exhalation (emptying), and then
a short rest period. See Figure 7-2 for a representation of the "filling" of the lungs.
Figure 7-2. "Filling" of the lungs.
a. Volume Exchanges During Breathing. The amount of air exchanged in a
given period depends upon the rate and depth (volume) of breathing. Rate and depth
are adjusted according to physiological demand. The rate of respiration is the number
of breathing cycles per minute.
b. Some Types of Breathing Cycles.
(1) Quiet ("tidal") breathing. As one takes part in ordinary, low-level activity,
the breathing cycles are of the quiet type. This type involves only a minimal exchange