The large size of the renal arteries and veins permits so rapid a transit of the blood through the kidneys that the whole of the blood is purified by them. The secretion of urine is rapid in comparison with other secretions, and as each portion is secreted, it propels that which is already in the tubes onwards into the pelvis of the kidney. Thence, through the ureter, the urine passes into the bladder, into which its rate and mode of entrance has been watched. The urine does not enter the bladder at any regular rate, nor is there a synchronism in its movement through the two ureters. In a recumbent posture the urine collects for a little time in the ureters, then flows gently, and if the body is raised, runs from them in a stream till they are empty. Its flow is increased in deep inspiration, or straining, and in active exercise, and in fifteen or twenty minutes after meals.

Substances taken into the stomach pass very rapidly through the circulation. It does not take longer than one minute for ferrocyanide of potassium to pass through. Vegetable substances pass in from sixteen to thirty-five. Neutral alkaline salts with vegetable acids, which were generally decomposed in transitu, made the urine alkaline in twenty-eight to forty-seven minutes. But the time of passage varied much; and the transit was always slow when the substances were taken during digestion.

There are really two distinct parts in the kidney—the actively secreting part, the epithelium of the secreting tubules; and what maybe called a filtering part, the malpighian bodies.

The specific gravity of urine is 1020—that is, the average human urine. Urine varies—in the morning before breakfast it is darker, urina sanguinis; urine secreted shortly after the introduction of any considerable quantity of fluid into the body, urina potus; and the urine evacuated immediately succeeding a solid meal of food, urina cibi. The last kind contains a larger quantity of solid matter than either of the others, the first and second being largely diluted with water.

Specific gravity: The morning urine is best calculated for analysis. The average healthy range may be stated at 1015 in the winter to 1025 in the summer, and variations of diet and exercise may make a great difference. In disease, the variations may be greater; sometimes descending in albuminaria to 1004, and frequently ascending in diabetes, when the urine is loaded with sugar, to 1050, or even to 1060.

The whole quantity of urine secreted in twenty-four hours is subject to variations according to the amount of fluid drunk, and the proportion of the latter passing off from skin, lungs, and alimentary canal. The average quantity voided in twenty-four hours by healthy male adults from twenty to forty years of age amounts to 52½ fluid ounces.

The chemical composition of urine. The average quantity of each constituent of the urine in 1,000 parts is:

Water (O H2), 967
Urea (C O N2 H4),14.239
Uric acid (C5 N4 H4 O3), .468
Coloring matter, mucus, and animal extractive matter,10.107
Salts. Sulphates (soda, potash), 8.185
Bisulphates (lime, soda, magnesia, ammonia),
Chlorides (sodium, potassium),
Silica, etc., Traces.
1,000.000

Urea is the principal solid constituent of the urine, forming nearly one-half of the whole quantity of solid matter. It is also the most important ingredient, since it is the chief substance by which the nitrogen of decomposed tissue and superfluous food is excreted from the body.

The salts excreted by the kidneys in 24 hours are: