From the small intestines, the digested food containing glucose, amino acids and fatty acids, etc, reaches the cells of the body by blood circulation. In cells these products are utilised to produce energy and also used for repair of the tissues themselves.
This becomes possible because of chemical reaction involving their oxidation. The reaction releases adequate energy for the use of living organisms. In other words, the oxidation of glucose and other carbohydrates releases energy and this process works with inhaled oxygen.
In this process, the end-product, that is, carbon-dioxide is exhaled out of the body in respiration.
The chemical reaction by respiration, which occurs inside the cell is known as internal (or cellular) respiration. In cellular respiration the oxidation is rapid, producing heat, which burns off the hydrogen of carbohydrates in the glucose, leaving behind or releasing carbon, which is then eliminated as carbon-dioxide and water vapour.
In cellular respiration it is comparatively slower as by controlled combustion and energy is released bit by bit, some of which also remains stored in chemical bonds of some specific compounds to be utilised later when required.
The function of respiration is performed by the lungs. During the breathing-in process (inhalation), the body gets oxygen from the air, which gets into the lungs to be absorbed by the blood. During breathing-out (exhalation) carbon-dioxide and water vapours are breathed out.