breathing

Movement of air into and out of the air passages of an animal, brought about by muscle contraction. It is a form of gas exchange. Breathing is sometimes referred to as external respiration, for true respiration is a cellular (internal) process. In a mammal, breathing involves the action of the muscles of the diaphragm and the intercostal muscles (between the ribs). When a mammal breathes in, the diaphragm muscles contract, which lowers the diaphragm. The external intercostal muscles contract, which raises the ribs. Lowering the diaphragm and raising the ribs increases the volume of the thorax. This lowers the pressure of the air inside the lungs in the thorax. The pressure is now lower than atmospheric pressure so air flows into the air passages and the lungs inflate.

Lungs are specialized for gas exchange but are not themselves muscular, consisting of spongy material. In order for oxygen to be passed to the blood and carbon dioxide removed, air is drawn into the lungs (inhaled) by the contraction of the diaphragm and intercostal muscles; relaxation of these muscles enables air to be breathed out (exhaled). The rate of breathing is controlled by the brain. High levels of activity lead to a greater demand for oxygen and an increased rate of breathing.