The human respiratory system consists of a pair of lungs and a series of air passages leading to the lungs. The entire mechanism of breathing is ruled by two processes inhalation, taking in of oxygen-rich air and exhalation, giving out of the carbon-dioxide-rich air. The air which we breathe in and out of the lungs varies in its pressure. So basically when there is a fall in air pressure the alveolar spaces falls and the air enters the lungs (inhalation) and when the air pressure within the alveoli exceeds atmospheric pressure the air is blown from the lungs (exhalation). The flow rate of air is in proportion to the magnitude of the pressure difference.Now in the process of inspiration, there would be a contraction of muscles attached to the ribs on the outer side which pulls out the ribs and results in the expansion of chest cavity. Later, the diaphragm, contracts, moves downwards and expands the chest cavity resulting in the contraction of the abdominal muscles. The expansion of the chest cavity produces a partial vacuum which sucks air into the lungs and fills the expanded alveoli. The expiration process is considered once after the gaseous exchange takes place in the lungs and the air is expelled out. This expulsion of air is called expiration. During this process, muscles attached to the ribs contract, the muscles of the diaphragm and the abdomen relaxes which leads to a decrease in the volume of the chest cavity and increases the pressure of the lungs, causing the air in the lungs to be pushed out through the nose.