The Secretion of the Urine.—In a perfectly normal being the problems of waste and repair are balanced to a nicety. As we know, the tissues of the body are bathed in lymph containing in solution the compounds that are necessary for their nourishment—proteids, carbohydrates, fats, salts, and gases. Waste follows in direct proportion to the activity of the tissues. The worn-out, effete material first finds its way into the lymph, and from it into the blood-stream, to be later eliminated from the economy, else deleterious results will follow their retention in the body. It is by the selective action of the cells of the various glands of the body that these useless substances are removed from the blood, and converted into such forms as can be readily excreted. In the main, the products to be removed are urea and the allied nitrogenous bodies—carbon dioxid, salts, and water. These organs are of vital importance, since nearly all of the waste-products containing nitrogen are eliminated in the urine.
The most easily understood function of the kidneys is the excretion of the urinary water; this varies in amount with the rapidity of flow through the renal vessels and to some extent on the blood-pressure.
Fig. 11.—Relation of kidneys to heart and great blood-vessels: A, Heart; B, B, kidneys; C, bladder (after Quain).
The separation of the solid substances of the urine form the next function of the kidneys, and these substances fall into two groups—inherently useful materials, which are in excess or which have served their purpose, and substances which are inherently harmful. The latter class embrace many end-products of metabolism, which we are accustomed to consider as normal constituents of the urine.
The analytic study of the urine is of great value to the physician and surgeon, because of the knowledge which it gives concerning the processes of metabolism occurring within the body. The nature and amounts of the various end-products of metabolism are carefully investigated as they occur in the urine, whether they be normal or pathologic.
The Urine.—The normal human urine recently passed is a clear liquid, of a straw color, with an average specific gravity of 1020, the specific gravity indicating the amount of solids contained in the urine. It is acid in reaction. The quantity for twenty-four hours is 50 ounces, or about 3 pints, depending on the amount of water ingested. During sleep the amount secreted is less than at other times. The amount of urine is decreased after profuse sweating, diarrhea, thirst, diminution in the blood-pressure, and after severe hemorrhage. When the body temperature is considerably higher than normal, the amount of urine is again diminished in quantity.
Urea forms nearly one-half of the solids in normal urine and nearly one-half of the urea is nitrogen. It is the principal representative of the waste of the nitrogenous tissues. The chief source of urea is from the metabolism of the muscles; the ingestion of a large amount of proteid food stimulates metabolism. The quantity of urea passed in the twenty-four hours is about 500 grains.
The uric-acid constituent is very small, and in the human urine scarcely reaches 0.03 per cent. of the solids. Lack of exercise leads to an increased formation of uric acid by a lessening of the oxidation of the tissues. In gout the amount of uric acid excreted is abnormally small, because it accumulates in the blood and tissues. The brick-dust deposit sometimes seen in the urine is composed chiefly of the urate of sodium. The average daily amount of uric acid passed in the human urine is about 7 grains. The excretion of uric acid may be increased by drinking copious drafts of water.
Toxicity of the Urine.—After the removal of both kidneys the animal dies of uremia; that is, there is an accumulation of urinary products in the blood. The removal of one kidney is not necessarily fatal, and women have so lived very comfortably for many years. A human being excretes enough poisonous material by the kidneys in two days to cause death.