The urine is a watery solution containing many waste products, especially urea. It is generally amber in color, varying in shade with circumstances, with an aromatic, characteristic odor when fresh. It is acid in reaction and has a specific gravity of about 1020, though this too varies with circumstances. Besides water, which is its chief constituent, it contains urea, uric acid, organic acids, urates, inorganic salts, including sodium chloride and phosphates of calcium and magnesium, a certain amount of ammonia, and certain pigments. Its acidity is due to acid sodium phosphate in solution but varies with the food, and in disease the urine may become alkaline when passed. After standing a few hours in a warm place it decomposes and becomes alkaline. The quantity, which is normally three pints or fifteen hundred cubic centimeters in twenty-four hours, varies with the amount of fluid drunk, the amount of perspiration, etc. The amount secreted depends chiefly, however, upon the flow of the blood through the kidneys; the greater the flow of blood, the larger the amount of urine formed; and the blood flow is determined by blood pressure and by vasomotor action. Secretion also seems to be increased by the presence of urea, which apparently serves as a stimulant to the kidney cells. The excretion of waste materials takes place by three main channels, the lungs, skin, and kidneys, and the materials are of four kinds, _urea_, _carbon dioxide_, _salts_, and _water_. The lungs carry off carbon dioxide and water chiefly, the skin these and inorganic salts, while the kidneys eliminate practically all the urea as well as inorganic salts and water. When the kidneys are not working the skin carries off much urea. In fact, a close relationship exists between the kidneys and the skin in the matter of excretion. Thus, with increased perspiration in warm weather comes decreased urine, while in cold weather the blood is sent in and the urine increased in amount. To incite action of the kidneys drugs known as _diuretics_ may be used. These act in two ways, by stimulating the kidney cells directly and by acting on the general circulation or nervous system. Any emotional or nervous excitement increases the flow of urine.