Respiration - Human Respiration In humans, as in the other vertebrates, the lungs are enclosed in the thorax, or chest. The rib cage gives shape to the thorax and provides the lungs with protection from traumatic injury. The muscular diaphragm is a dome-shaped partition at the base of the rib cage. When the ribs are expanded by the action of muscles that surround them, the volume of the thorax is increased. The diaphragm contracts, helping to increase the volume of the thorax. Within the thorax, the lungs are held close to the body wall by atmospheric pressure. When the thorax expands during inspiration (breathing in), the lungs also expand and become filled with air drawn through the nose and mouth. Relaxation of the diaphragm allows the opposing set of muscles to return the chest to its natural position, forcing the air from the lungs during expiration (exhalation, or breathing out). The air that enters the lungs contains approximately 21 percent oxygen and 0.04 percent carbon dioxide. By contrast, the air that leaves the lungs contains 14 percent oxygen and 4.40 percent carbon dioxide. (In both cases the nitrogen content is about 78 percent.) About 200 to 500 cc (12 to 30 cu in) of air are usually inhaled and exhaled in each breath. This quantity is called tidal volume. About 1500 cc (about 90 cu in) of additional air, called complemental air, can be inhaled on a deep inspiration. Another 1500 cc, called supplemental air, can be exhaled on a forced expiration. The sum of these three quantities is called the vital capacity. About 1500 cc of air always remains in the lungs and cannot be exhaled; this volume is called the residual, or alveolar air.