When freshly voided, the urine of man in health is a clear straw-colored fluid, with a peculiar aromatic odor. The intensity of the color varies with the amount of solids - the color being a rough indication of the degree of concentration. On standing and cooling, a slight cloud of mucus often appears floating in the fluid. This comes from the lining membrane of the bladder, and it usually entangles a few flattened epithelial cells, which are the only organized structural elements found in it in health.

Diagram showing the relation borne by the blood vessels to the tubules of the kidney.

Fig. 175. Diagram showing the relation borne by the blood vessels to the tubules of the kidney. The upper half corresponds to the cortical, the lower to the medullary part of the organ. The plain tubes are shown separately on the right, and the vessels on the left. The darkly-shaded arteries send off straight branches to the pyramid and larger interlobular branches to the glomeruli, the efferent vessels of which form the plexus around the convoluted tubes.

The fresh urine has a distinctly acid reaction. This does not depend upon the presence of free acid, but upon the large amount of acid salts, particularly acid sodium phosphate, which it invariably contains. A strictly vegetable diet renders man's urine alkaline, and it is said to become less acid after meals. In the herbivorous mammalia the urine is normally alkaline so long as their digestion is going on, but when they are deprived of food for some time, it becomes acid, showing that the alkalinity depends upon their diet.

The specific gravity of urine varies greatly at different times, commonly, however, ranging between the figures 1015 - 1020. After copious drinking, abstinence from proteid food, and in cool weather, it may fall as low as 1003; and after prolonged abstinence from liquids, much animal food, and very active sweating, it may attain 1040.

The quantity of urine secreted is also very variable, that produced by an adult usually amounting to about 2 pints per diem (1000-1500 cc). The amount is increased by - (1) elevation of the general blood pressure, or the pressure in the arteries from any cause whatever; (2) contraction of the cutaneous vessels from cold; (3) copious drinking; (4) excess of nitrogenous diet; (5) the presence of soluble matter in the blood, such as sugar, salt, etc.; and, (6), the presence of urea as well as various medicaments, has a special action on the renal secretion, greatly increasing the amount of urine passed.

Although the quantity of urine differs so much under different circumstances, the amount of solids excreted by the kidneys in the 24 hours remains pretty much the same, being on an average over 1 1/2 oz. (50 grammes) for an adult man.

From this it is obvious that the height of the sp. gr. must vary inversely with the amount secreted, so that the more scanty the urine the higher we expect to find the percentage of solids.