The inevitable consequence of the metabolic processes characterizing living creatures is that various by-products are formed in the body that must be got rid of for the reason that they not only are useless to the organism but may become decidedly harmful if retained.

Since there are many kinds of animals, there is a variety of devices for accomplishing this universal function. Excretions should not be confused with glandular secretions, such as saliva, milk, tears, mucus, enzymes, and hormones, which are of service to the organism. The mechanism of sewage disposal is the excretory apparatus.

The substances eliminated by excretion may be in the form of gases, solids, or liquids. Lungs and gills furnish the principal mechanism for the excretion of the gas carbon dioxide, resulting from the respiratory oxidation of carbohydrate and fat foods, while the digestive tube is the avenue of escape for the solid refuse from ingested food. Although the latter may not be regarded as true excretion, since the solids evacuated have never been incorporated as a part of the body, it is nevertheless an indispensable part in the process of the disposal of waste. Both of these methods of elimination have already been considered in the foregoing chapters upon the digestive and respiratory apparatus.

In addition to these two methods of disposal of the unusable products of the body, there is also a constant excretion, or sloughing off, of cellular material from the epithelial surfaces of the body, both from the outer exposed surface, and also from the lining of various tubes and ducts which have access directly or indirectly to the outside.

The present chapter is concerned primarily with the disposal of liquid waste by means of the urinary apparatus which is ordinarily referred to as the “excretory system.”

As a matter of fact liquid, or water in various guises, is disposed of in the animal organism through several different channels. It is thrown off from the lungs and the sweating skin of mammals as vapor; from the digestive tract as the fluid component of the feces; and above all from the kidneys in the form of urine. It comes to the kidneys from the blood, charged with salts in solution, both organic and inorganic, together with a variety of other chemical substances, as well as cell wreckage of various sorts.

Probably the simplest urinary apparatus of excretion is the contractile vacuole in protozoans, which periodically expels its liquid contents, accumulated from the surrounding substance of the cell to the outside.

In the bloodless flatworms (planarians) excretion is accomplished through a system of branching ducts that ramify throughout the body and join before emptying their excretory contents to the outside. The numerous extreme tips of this hollow branching system end blindly in swollen knobs, called flame cells, because in the cavity within them there is a tuft of cilia whose flamelike flickering motion forwards the collected waste liquid on its outward way from the surrounding tissues.

In animals having a body cavity, drainage tubes, or nephridia, are introduced connecting the cavity with the outside. When nephridia occur in invertebrates they are typically paired and independent of each other, but in vertebrates they are more or less massed together into definite organs of excretion, known as kidneys. Primarily nephfidia open at one end into the body cavity and at the other, either independently or indirectly, through a common connecting duct to the outside. This furnishes a means of escape for the coelomic fluid in the body cavity, which receives contributions by way of the blood from all parts of the body. Such an arrangement is of particular significance in many invertebrates, for example annelid worms, although decreasing in importance among vertebrates. On account of the lessening usefulness of the coelomic fluid in the absence of an open blood system and the elaboration of a closed blood system, the liquid wastes of vertebrates are collected directly by the blood stream rather than after finding temporary sanctuary in the coelom.

The excretory system of vertebrates may be described, therefore, as fundamentally made up of nephridia, more or less completely emancipated from the original direct connection with the body cavity, but nevertheless in intimate osmotic contact with blood capillaries by means of which waste materials are collected from the blood stream and transferred to the outside through ducts of exit.

The nephridial tubes of the kidneys have structurally much in common with the sweat elands of the mammalian skin that have been described as minute supplementary kidneys. Both are tubes with walls of excreting cells in close juxtaposition to capillaries, and have the power of abstracting waste materials from the blood stream. When it is remembered that there are over 2,000,000 sweat glands in the skin of an ordinary human being, and that end to end they constitute over twenty-five miles of glandular tubing, according to Macfie, it will be realized that these microscopic structures are by no means insignificant understudies to the kidneys in the disposal of liquid waste materials from the body.

For purposes of general description the urinary apparatus of a typical vertebrate may be considered under three headings: (1) kidneys; (2) urinary ducts; and (3) bladders.