Breathing is the process of moving air into and out of the lungs.
Breathing involves the pressure gradient between the surrounding atmosphere
and the thoracic cavity. Since atmospheric pressure is relatively constant, breathing
depends upon changing the pressure within the thoracic cavity.
The lungs have a certain total volume called the total lung capacity. There is a
certain portion of air always present in the lungs, called the residual volume. If one
inhales as much air as possible and then exhales as much as possible, the volume
The breathing cycle includes an inhalation (or inspiration), and exhalation (or
expiration), and then a short rest period. The rate of respiration is the number of
breathing cycles per minute. The amount of air exchanged in a given period depends
upon the rate and depth of breathing, which are adjusted according to physiological
In ordinary, low-level activity, the breathing cycles are of the quiet type.
Occasionally, there will be a breathing cycle with a slightly greater volume exchange,
called the complementary cycle.
The volumes of air exchanged are much greater in forced breathing. The volume
depends upon the oxygen demand.
If one makes an exhalation effort but still holds the air inside the lungs, it is called
Valsalva's maneuver. If one suddenly releases the air, the result is a cough.
Costal breathing is accomplished by moving the rib cage as a whole. (para 7-10)
In costal inhalation, the lungs are expanded and inflated with air because of the
upward movement of the rib cage.
The "bucket handle" effect increases the transverse diameter of the rib cage.
The second type of movement increases the anterior-posterior diameter of the rib cage.
These increased diameters enlarge the volume of the thoracic cavity. Thus, the
pressure of the air inside decreases. The pressure difference forces air into the
respiratory passages and into the alveoli of the lungs. (para 7-12)
Costal exhalation is essentially the reverse of costal inhalation. The rib cage
moves downward as a whole. There is a decrease in the transverse and A-P
diameters. This increases the pressure inside so that it is greater than the pressure