Water (H20) is present in nearly all tissues in larger proportion than any other compound, making up about 70 per cent, of the entire body weight. The amount in each texture varies, the different tissues having widely different consistence.

Water is introduced into the body not only as drink, but a large quantity is also taken with our solid food. It is highly probable that in the chemical changes which take place in the tissues, some water is formed by the oxidation of the hydrogen of the more complex substances.

In the economy it acts as the universal solvent in the fluids of the body, and as the agent by means of which the chemical changes of the various organs can be accomplished.

Water leaves the body by the lungs as vapor, and by the skin, kidney, and many other glands, as the fluid in which the solids of their secretions are dissolved.

Inorganic acids occur in the body either combined, forming salts, in which condition we find several (sulphuric, phosphoric, silicic), or uncombined. In the latter state we have only two, viz.: -

Hydrochloric Acid, HC1, which is formed in the stomach, and plays an important part in gastric digestion.

Carbonic Acid Gas, C02, exists in most of the fluids of the body, having been absorbed by them from the tissues. The venous blood contains a considerable quantity, some of which is got rid of during the passage of the blood through the lungs. It is a waste product, which must be constantly eliminated from the body (see Respiration).