164 FROM FISH TO PHILOSOPHER 



Studies on the seal through the cycle of fasting and 

 feeding show that urine formation is intimately related 

 to feeding. As digestion proceeds and the requirement 

 for urea excretion increases after a meal, and as meta- 

 bolic water becomes available, the urine flow increases 

 to large values, only to decrease again to 10 per cent or 

 less of the maximal value 8 to 10 hours after the meal, 

 when the available metabolic water has been spent. The 

 composition of the urine is in no way remarkable as com- 

 pared with other mammals. The maximal osmotic con- 

 centration of the urine at minimal urine flow is about 

 5.6 times that of the plasma, and thus significantly 

 greater than in the dog (4.5) but very much less than 

 in the kangaroo rat (17 or higher). 



The increase in filtration rate in the fed animal is not 

 attributable to the opening up of inactive glomeruli, but 

 to an increase in function in all glomeruli. Thus the seal 

 maintains itself in water balance not by excessive con- 

 centration of the urine, as does the kangaroo rat, but 

 by reducing the quantity of solutes filtered through the 

 glomeruli. The cycle in urine flow following a meal par- 

 allels, and is largely caused by, change in the rate of 

 glomerular filtration, a phenom^ion not observed in man. 

 It is probable that this imique control of the renal cir- 

 culation is related to the fact that the seal is a diving 

 mammal. When it submerges to catch fish it must some- 

 times engage in vigorous swimming with no possibility 

 of respiration, and L. Irving and his collaborators have 

 shown that the seal and other diving mammals have de- 

 veloped an elaborate reflex that involves arrest of respi- 

 ration, slowing of the heart, and constriction of the blood 

 vessels in large areas of the body, thus reducing the 

 blood flow to all organs other than the heart and braui 

 and temporarily conserving oxygen; this oxygen debt is 

 paid off by increased respiration when the animal returns 

 to the surface. This diving reflex can be elicited by 

 merely holding the nostrils closed when the animal is 

 out of water. During this artificial dive the renal blood 



