It is concerned with the vital function of making available the much needed oxygen to the body. It would be well to understand here why we need oxygen so badly for remaining alive. Our life activity is ever based on biochemical processes which cannot go on without a continuous supply of energy. This energy is received from the food we eat, especially the carbohydrates, fats, and proteins. The end roducts of these constituents are supplied to each tissue by the blood. The energy stored in these products can be released for the purpose of life activity of any tissue, only through the process of oxidation. This is a process in which oxygen combines with the substance containing energy, and releases energy along with water vapour and carbon dioxide. This process has to go on continually in every living tissue of the body, and it is, therefore that we can not live without oxygen for more than a few minutes. The carbon dioxide produced in the process of oxidation is a poisonous gas, and must be removed from the body as quickly as possible. It is collected, as we have noted earlier, in the blood flowing in the capillaries. We shall now see how it is eliminated from the body through the respiratory system.Air from the atmosphere enters our body through the nose, and goes to the larynx or the sound box, and then to the wind pipe. The wind pipe further divides into two branches, one of which goes to the left lung, and the other to the right lung. They give rise to further branches and sub-branches in the lungs. At the ends of the minute sub-branches are borne the air sacks or cells (alveoli) which are surrounded by capillaries, through which the blood flows from the heart to the lungs, and back again to the heart. The walls of the air sacks are very thin. They allow gases to pass through in both directions. The oxygen from the air sacks is absorbed in the blood in the capi11aries, and the carbon dioxide and water vapour from the capillaries enters the air sacks. This gaseous exchange is a very important phenomenon concerning the respiratory system.The lungs are made of elastic tissues which expand and contract during respiration. They are contained in what is called the "thoracic cavity" which is pro­tected on all sides by the ribs. The base of this cavity is made by a dome shaped band of a muscle cal1ed the "diaphragm", which plays an important role in the mechanism of respiration. The cavity below the diaphragm is called the "abdominal cavity". The cavity accommodates organs like the stomach, duodenum, liver, pancreas, kidneys, and intestines.The act of respiration is composed of three parts, namely, inhalation, exhalation and pause. During inhalation the thoracic cavity increases in volume due to an elevation and extension of the ribs, along with a descent of the diaphragm downward. The lungs expand due to this, and air rushes in through the nose to fill the vacuum created thereby. The lungs then contract automatically after a while, due to their elastic recoil, expelling some amount of air during exhalation. This is foHowed by a state of pause, and then another round of inhalation and exhalation takes place. Car-bon dioxide and water vapour are got rid of through exhalation.