14a FROM FISH TO PHILOSOPHER 



by way of the two ureters into the bladder. (Figure 10.) 



The total filtering surface in these two million 

 glomeruli is estimated to be about 0.76 sq. meters, or 

 nearly half the average body surface area (1.73 sq. 

 meters). The filtration rate in man (neglecting sex dif- 

 ferences, since all aspects of renal function are slightly 

 greater in males than in females) averages about 125 cc. 

 per minute, or 180 liters (roughly 190 quarts) per day. 

 To supply this enormous quantity of filtrate, the kidneys 

 require nearly 1200 cc. of blood per minute or 1700 

 liters (1800 quarts) per day. This amounts to one-fifth 

 of the total blood pumped out of the heart, though the 

 two kidneys represent only one-half of one per cent of 

 the total body weight— the kidneys get forty times as 

 much blood per unit weight as does, on the average, the 

 rest of the body. 



The 180 Hters of glomerular filtrate formed each day 

 contain some 1100 grams (2.5 poimds) of sodium chlo- 

 ride, of which only 5 to 10 grams are excreted in the 

 urine— 95 per cent is reabsorbed by the tubules. Some 

 425 grams (nearly a poimd) of sodium bicarbonate and 

 145 grams of glucose are filtered, and more than 99 per 

 cent of both are reabsorbed. Also filtered, only to be 

 reabsorbed, are substantial quantities of potassiimi, cal- 

 cium, magnesium, phosphate, sulfate, amino acids, 

 vitamins, and many other substances valuable to the 

 body. 



If the two million tubules in the two human kidneys 

 were connected end to end, they would stretch for nearly 

 fifty miles. There is no reason for surprise, therefore, that 

 by the time this filtrate has emerged from its convolu- 

 tions into the renal pelvis and bladder it is reduced 

 from 180 Uters to an average of 1.5 liters per day of 

 final urine in which waste products— and other sub- 

 stances present in the blood in excess— have been con- 

 centrated, some of them a hundredfold or more. 



The total extracellular fluid (plasma plus interstitial 

 fluid)— or what Claude Bernard called the mtemal en- 



