2-5
2-5. MECHANICS OF RESPIRATION
a. The act of breathing, the cycle of inspiration and expiration, is repeated about 16-20 times per minute in a resting adult. Breathing is regulated by the respiratory center in the medulla of the brain. The level of carbon dioxide (CO) in the circulating blood is one of the major influences upon the respiratory reflex. The respiratory center is sensitive to changes in blood composition, temperature, and pressure, and will adjust the rate and depth of breathing to accommodate the body's needs.
b. The physical conditions that control the flow of air into and out of the lungs are referred to as the mechanics of ventilation. Air flows from an area of higher pressure to an area of lower pressure. During inspiration, contraction of the diaphragm and intercostal muscles increases the size of the thoracic cavity.
This causes the pressure within the thoracic cavity to become less than that of the atmosphere, and air is drawn through the air passages into the alveoli. During normal expiration, relaxation of the same muscles will cause the thoracic cavity to decrease in size, thereby increasing the pressure within the thoracic cavity to that which is greater than atmospheric pressure. Air will then flow out of the lungs into the atmosphere.
