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.
There are certain abnormal constituents of urine, of which the two most important are albumen and sugar. The former is found only when there is some disturbance of the kidneys, ureters, or bladder, and its presence usually denotes some change in the cells lining the urinary tract. It may occur in congestion of the kidney as well as in disease. Sugar is found only in diabetes, the amount varying with the severity of the disease. In jaundice certain bile pigments are present in the urine, giving it a dark brown color and to the foam a greenish-yellow color. Even normal urine has some sediment upon standing, consisting of cells from the urinary tract and mucus. In very acid urine after standing a heavy sediment, whitish or pinkish, i.e., brick dust, in color, is thrown down. It does not necessarily denote disease, but shows the urine is acid and concentrated. In alkaline urine there is a sediment due to phosphates.
Rupture of the kidney occurs but is not so serious as rupture of the liver or spleen because the kidney is situated outside of the peritoneum. It necessitates the removal of the kidney, however, and when for any reason one kidney is removed the other increases in size and does double work to compensate for the loss. Removal of both kidneys means death. Sometimes the kidney becomes loose and moves about, a condition known as floating kidney. Perinephritic abscess is abscess in the loose fatty tissue about the kidney.
Fig. 61.—The urinary organs
viewed from behind.
The Ureters, one for each kidney, are tubes the size of a goose quill and about fourteen inches long, extending from the hilum of the kidney to the base of the bladder. They have three coats, an internal mucous, a muscular, and an external fibrous coat, this last being continuous with the cortex of the kidney and the fibrous tissue of the bladder. In the female the ureters may be felt through the wall of the vagina as they come into the bladder. In tubercular disease of one kidney the ureter becomes inflamed and enlarged and through the vagina feels almost like a lead pencil, a sure diagnostic sign.
The Bladder and Urethra.—In their course to [the bladder] the ureters pass from the abdominal into the pelvic cavity, but before describing the pelvis itself it will be well to complete the account of the urinary organs by considering the bladder and urethra. The bladder is the reservoir for the urine and has muscular walls lined with mucous membrane. A peritoneal coat covers the upper surface and is reflected to the walls of the abdomen and pelvis. It is situated back of the os pubis, the front bone of the pelvis, with its base or fundus directed downward and backward. Normally it is in the pelvis, but when much distended it mounds up into the abdominal cavity, where it can be felt in front as a tumor. It rests on the rectum in the male and on the cervix in the female and is held in place by numerous ligaments. When empty it may be Y-shaped, but it becomes oval when distended. Its capacity is about one pint.