kidney weighs about 125 to 170 grams. It is protected by a considerable amount of fat and supported by connective tissue and the peritoneum. Attached to the hollow side of each kidney is the dilated upper end of the ureter, forming the renal pelvis.
Structure
The kidney is composed of an external cortical and an internal medullary substance. The cortex or cortical substance is soft and granular and reddish brown. The urine is formed in the cortex. The medulla or medullary substance is a pyramid-shaped mass of tubes or tubules that drain the urine to the pelvis of the kidney. Blood enters the kidney through the renal artery and is distributed to the glomerulus (fig. 3-54).
The GLOMERULUS, lying in the cortex, consists of a tuft of capillaries. This tuft is surrounded by the glomerular capsule, which is a cuplike dilation of the end of the renal tubule. This combination is called a MALPIGHIAN BODY.
The renal tubule begins with the malpighian body, takes multiple turns, forming the proximal convoluted tubule, extends toward the hilum in the medullary portion to form the descending LOOP OF HENLE, doubles back on itself as the ascending loop of Henle, and goes through several more turns as the distal convoluted tubule. The structural and functional unit of the kidney is called a NEPHRON, of which there are about 1 million in each kidney. For this reason a large portion of the kidney may be destroyed without serious body damage. In addition, the loss or medical donation of one kidney does not seriously affect the body’s welfare if the remaining kidney is healthy.
Several of the nephrons terminate in one collecting tubule. Several collecting tubules unite to form a renal pyramid, which drains the urine into a branch (calyx) of the renal pelvis.
Figure 3-54.—Functional unit of the kidney.
Function
The kidneys are effective blood purifiers and fluid balance regulators. Besides maintaining a normal pH of the blood (acid-base balance), they keep the blood slightly alkaline by removing excess substances from the blood. For example, if the blood becomes too acid, they will remove acid in the form of salts; if the blood is too alkaline, they will remove alkaline salts.
The main function of the kidneys is to remove the nitrogenous waste products that result when products of proteins are broken up. They also remove excess sugar.
The second important function of the kidney is reabsorption of water, salts, sugar, and protein elements of the blood. This selective reabsorption keeps the blood at an acid-base balance and also at a constant concentration of water, salts, and proteins. This delicate balance is necessary for normal life processes. Controlled reabsorption accounts for the amount of urine that is finally passed from the kidneys. The glomerulus filters gallons of blood each day. It is estimated that 10,000 quarts of blood pass through the kidneys in 24 hours and about 80 gallons of glomerular filtrate are formed. All the water from this filtrate is reabsorbed in the renal tubules except that containing the concentrated waste products.
The amount of urine a normal person excretes varies from 1,000 to 1,500 ml per day, but a person can function normally by excreting only 500 ml per day. The amount of urine excreted varies greatly with temperature, water intake, and state of health. No matter how much water you drink, the blood will always remain at a constant concentration, and the excess water will be excreted by the kidneys. A large water intake does not put a strain on the kidneys as one might think, instead
