Respiration is a process in which oxygen is taken in and carbon dioxide is given out of the body. The cells use oxygen to produce energy and carbon dioxide is generated in the process. The human lung is the main organ in the respiratory system, which performs two important functions: inhalation and exhalation.
The whole process can be described as follows:
(1) When we breathe in, the air enters our nose and passed into the nasal cavity. From there, it moves to our wind pipe, which then divides into two tubes called the bronchi.
(2) The bronchi open into lungs, where they divide and branch into numerous fine tubes, each ending in an air-sac.
(3) The air-sac is called alveoli, which is a spongy perforated body, through which blood and air pass over each other and exchange of gases takes place.
(4) The oxygen is able to pass through the thin walls of the air-sacs into the blood vessels in the lungs, and then in the rest of the body, where it is used up by all the cells.
(5) On the other hand, carbon dioxide from the blood is passed into the air-sacs, from where it is breathed out of the body.