64 Physiology of the Kidney 



all mammalian forms, but the human kidney differs only in 

 details from that organ in the dog, cat, and rabbit. It is not 

 surprising that in function the human kidney has its closest 

 homologue in the kidneys of the great apes, who can claim 

 with man a common ancestor back somewhere in the Mio- 

 cene. 



Examining the pattern of the human kidney, we must 

 not be surprised to find that it is far from a perfect organ. 

 In fact, it is in many respects grossly inefficient. It begins 

 its task by pouring 125 cc. of water into the tubules each 

 minute, demanding for this extravagant filtration one third 

 of all the blood put out by the heart. Out of this veritable 

 Niagara of water, 99 per cent must be reabsorbed again. 

 This circuitous method of operation is peculiar, to say the 

 least. At one end, the heart is working hard to pump a large 

 quantity of water out of the body; at the other end the tub- 

 ules are working equally hard to defeat the heart by keeping 

 99 per cent of this water from escaping. Thus heart and 

 kidney are literally pitched in constant battle against each 

 other — our lives depend on neither one of them ever win- 

 ning out. Nature frequently opposes two forces against each 

 other in order to maintain a steady state, but the opposition 

 in this instance takes on an aspect of sheer extravagance. 

 Paradoxically, the kidney has to do its greatest work when 

 it excretes the smallest quantity of urine; as the urine flow 

 increases it does less and less work, and if the urine flow were 

 to increase to the collossal figure of 125 cc. per minute — 170 

 liters per day — the kidney, in respect to the excretion of 

 water, would be doing no work at all. 



In consequence of the circuitous pattern of the filtration 

 and reabsorption of water, nearly half-a-pound of glucose 



