The process of breathing involves drawing air into and expelling air from the lungs. The human respiratory system relies on a breathing method called negative-pressure breathing. Inhalation is brought about by the increase in the volume of the thoracic cavity. Such an increase causes the air pressure in the cavity to drop below atmospheric pressure and, thus, air to be drawn into the lungs. The contraction of the rib cage's muscles, which causes the cage to move up and out, and the contraction of the diaphragm, the muscular partition between the thoracic and abdominal cavities, downward into the abdominal cavity, effects the increased volume of the thoracic cavity. Exhalation is a passive process caused by the decrease in the thoracic cavity's volume. The relaxation of the rib cage's muscles and the diaphragm into their resting positions causes the air pressure in the lungs to rise above atmospheric pressure, thus forcing the air out of the lungs. Total breathing capacity, called vital capacity, is the sum total of the lungs' tidal air, the air drawn in and released by a single breath, and their reserve air, the extra air that can be forcibly drawn in and released from the lungs. The vital capacity of human lungs is about four liters.
