The living body suffers constant loss of substance in the performance of its functions, and to make up for this loss and to obtain materials for its other requirements, it receives nutrition in the form of food, water and oxygen. Food to be utilised has to be digested, water and soluble salts are absorbed unchanged, and oxygen, as we have just seen, is taken into the system by the lungs from the air.

Regarding the body as practically made up of water, nitrogenous matter, fat, and mineral matter; we find that the water and soluble salts which have served their purpose, or which are in excess of the requirements of the system, can be removed (excreted) without change. Water is discharged chiefly from the kidneys, lungs and skin; and mineral matters from the kidneys and to a slight extent from the skin, especially in the case of common salt. The kidneys act as filters which have selective power in removing waste material along with water from the blood. This impure water (urine) is carried by two tubes (the ureters) from the kidneys into the bladder, from which it is finally discharged through the urethra. The insoluble mineral matters of the body, though fairly stable, gradually undergo change by becoming converted into soluble salts. Broken up nitrogenous matter yields compounds which are removed along with the urine, and fat which by uniting with the oxygen brought to the tissues by the blood, is changed into grape sugar and finally into carbonic acid and water; the carbonic acid being given off by the lungs into the atmosphere. The fat which has accumulated in the tissues undergoes the same changes, when it becomes broken up, as the fat which results from the disintegration of nitrogenous matter. A portion of the waste fat, as we all know, is excreted unchanged in perspiration and in other secretions. As many of the waste materials have a poisonous influence on the body; it is necessary for the maintenance of health that the organs (lungs, kidneys, intestines, skin, liver, etc.) which excrete them, should be in perfect working order, and that the system should have a full supply of oxygen and water, which is necessary for dissolving out and carrying away hurtful products.

The skin, like the lungs, gives off carbonic acid, although in very much smaller quantities; and it absorbs a minute quantity of oxygen from the air. In man, the lungs expire about 200 times the amount of carbonic acid given off by the skin.