kidney weighs about 125 to 170 grams. It is pro- tected by a considerable amount of fat and sup- ported 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  cor- tical  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, con- sists of a tuft of capillaries. This tuft is surrounded by the glomerular capsule, which is a cuplike dila- tion of the end of the renal tubule. This combina- tion  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 Figure 3-54.—Functional unit of the kidney. 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 por- tion  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 col- lecting tubule. Several collecting tubules unite to form a renal pyramid, which drains the urine into a  branch  (calyx)  of  the  renal  pelvis. 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 ex- cess  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  ac- counts  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 con- taining  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  concen- tration, 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 3-46