Respiration is carried on by the expansion and contraction of the lungs; the process and the rate at which it proceeds are controlled by a (1) ________ in the brain.
Air is moved, into and out of the lungs by the process of breathing, technically called (2) ________ . This consists of a steady cycle of (3) ________ (inhalation) and (4) ________ (exhalation), separated by a period of rest. The cycle begins when the (5) ________ stimulates the diaphragm to contract and flatten, thus enlarging the chest cavity. The resulting decrease in pressure within the thorax causes air to be pulled into the lungs.
The (6) ________ between the ribs aid in inspiration by pulling the ribs up and out. Muscles of the neck and thorax are used in addition for (7) ________. Expiration occurs as the breathing muscles relax, the lungs spring back to their original size, and air is forced out. Muscles of the rib cage and abdomen can be called on for (8) ________.
Breathing is normally regulated unconsciously by centers in the (9) ________. These centers adjust the rate and rhythm of breathing according to changes in the composition of the blood, especially the concentration of carbon dioxide.