A person can live for some week without meal and he can live for some days without water but he dies within 3-6 minutes in the case if the breathing is blocked. Tissues of the body especially tissues of the heart and brain need oxygen every moment. Tissues become inactive within a few minutes in the absence of oxygen and heart throbbing gets stopped. So, it is said that oxygen is life.
Respiratory system inhales oxygen from the atmosphere. It supplies oxygen to each cell of the body and cells use oxygen and then it is exhaled in the form of carbon dioxide. Here it means to say that respiration is a reciprocity process between the cells and atmosphere air.
Respiration is a common form of two quite different procedures inspiration and expiration. A process in which the air is taken inside is called inspiration and the process in which the air is expelled outside is called expiration. In the process of respiration, taking breath inwardly and expelling breath outwardly is included.
In the respiration system, these both processes keep on continuing on different levels. The first process is between cells and alveoli and it is called external respiration or pulmonary respiration and the second process is between capillaries and tissues and it is called internal respiration or tissue respiration.
Respiratory organs situated in continuation from the nose to the pulmonary alveoli make respiratory tract. These are situated in this way-