A 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.
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.
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 of air.
Over a period of time, quiet breathing may not totally satisfy the oxygen requirements of the body. Thus, we can observe a breathing cycle with a slightly greater volume exchange called the complementary cycle. It provides a little extra oxygen to make up the difference.
In forced breathing, the volumes of air exchanged are much greater than in quiet breathing. The actual volume exchanged depends upon the oxygen demand.
Holding of breath:
One can inhale a volume of air and hold it for a period. If one makes an exhalation effort but still holds the air inside the lungs, it is called Valsalva's maneuver (forced expiration against a closed glottis).
If one suddenly releases the air, terminating Valsalva's maneuver, the result is a cough. If the musculature of a patient's abdominal wall is paralyzed, the patient cannot execute the Valsalva's maneuver and cannot produce a cough.
During speech or vocalization, the breathing cycles overlap. That is, the subsequent cycle begins before the previous one is ended. The purpose of this is to maintain a continuous outflow of air.