(1) During the process of breathing

(1) During the process of breathing, there is an alternating increase and decrease in thevolume of the chest, as a result the person inhales and exhales. A person inhaleswhen the dimensions of the chest increase. Once inside the lungs, the air providesoxygen to the blood and in turn it absorbs carbon dioxide and aqueous vapors.Exhalation occurs when all the muscles relax, the diaphragm presses upward, and,under the action of the weight of the chest and the elasticity of the lungs, air is forcedout of the body. Exhaling does not require muscular effort; it occurs as the result ofthe resiliency of the ribs and the muscular tissues and the elasticity of the lungs.(2) When breathing calmly a person produces an average of 12 - 13 respiratory cycles aminute. Consequently, one respiratory cycle lasts 4 - 5 seconds. If one traces therespiratory cycle, it is not difficult to note that the strained position of inhalation isreplaced very quickly by exhalation. The very next inhalation begins after arespiratory pause of 2 to 3 seconds, (figure 1-7) during which time the carbon dioxideaccumulates in the lungs. The duration off the respiratory pause is determined by theratio of oxygen and carbon dioxide in the air remaining in the lungs.