We all know instinctively that we must breathe. But the question arise is, why is it necessary for all living plants and animals to breathe? The reason is  a very simple one. By breathing we take in air that provides oxygen.Oxygen is necessary for the survival of living being . When we exhale the air , it changes. As it makes its way through our system, some of the oxygen is used up, while the amounts of carbon dioxide and water were increased. Nature always keeps up a constant supply of oxygen for our breathing. In fact , from year to year, there is little change in the amounts of oxygen and carbon dioxide  we breathe out into the air is taken in by plants. They exhale out the oxygen which we take in for our survival.
                    Breathing or the respiration , is made up of two parts : external and international breathing .External breathing is that procedure what most of us  think of when we refer to breathing. It consists of inhaling(taking in air) and exhaling(taking out of air) or inspiration and expiration. Inspiration is taking in air through the mouth, the nose, or both to the lung. Expiration means the process of sending out air again through those same channels i.e mouth to lung , but air that has had about one fifth of its oxygen exchanged for an equal amount of carbon dioxide by the process of breathing. This happens in the lungs. Internal breathing is the opposite of external. The oxygen which is taken from the air in the lungs is carried to the tissues of the body by the red cells in the blood of living being. In the tissues this oxygen burn certain food product, making them available for use by the body. The blood will take away the waste products, including water and carbon dioxide also. These are returned in the blood through blood vessels and finally to the lungs for exhaling.
                       Since we know that an exchange of gases(oxygen and carbon dioxide ) takes place both in the lungs and in the tissues, a large surface is needed to make this possible. For example , the linings of the lungs of an adult persons are equal to a area of about 36 feet square. This is larger than the ceiling of  house. Almost all area is kept in reserve, since we might need more than 8 or 10 times as much air during work as we need to refresh at rest. If we need more oxygen, we breathe more deeply and deeply or more quickly. Different creatures breathe at different rates and volume,depending on the oxygen they need and they utilize. A new born baby breathes about once a second, but at the age of fifteen, breathing is about 20 times a minute. An elephant breathes 10 times in a minute; a dog ,25 times in a minute.
                        Breathing is the process which is running all the energy  of the living being.This is the process which can be under  control by only human being.Some practitioner can stop breathe for the several minutes without difficulties. This is the result for their continues practice. Now a days due to the increase of the pollution, the air get polluted by different harmful gases. We can clearly find the difference between the air of densely populated town and the air of the Mountain region. Fresh air is the source of energy in the life.