108 FROM FISH TO PHILOSOPHER 



are carbohydrates such as glucose, xylose, lactose and 

 sucrose: as valuable foodstuffs, glucose and related car- 

 bohydrates have always been conserved by the organ- 

 ism, and it has never been necessary for the renal tubules 

 to excrete them into the urine. It was this observation 

 that in part supphed the lead from which inulin (also 

 a carbohydrate) came to be used for measimng the fil- 

 tration rate. Nor can the aglomerular kidney excrete pro- 

 tein: protein appears in the urine only in consequence 

 of excretion through the glomeruli. 



The aglomerular kidney forced the historically minded 

 physiologist to review the over-aU history of the kidney, 

 to ask how come and why. By putting this broad ques- 

 tion to nature he acquired a better view of the evolu- 

 tion of renal function, and hence a better theory of renal 

 function. And from the synthesis of this new knowledge, 

 which was the work of many men, came new methods 

 which now with confidence can measure, with an error 

 of only a few per cent, the filtration rate, the renal blood 

 flow, and a multiplicity of tubular operations, in a pa- 

 tient lying comfortably in bed. 



In addition to the partial or complete degeneration of 

 the glomeruli, the kidneys of the marine fishes show 

 other evolutionary changes traceable to a salt-water hab- 

 itat. The glomerulus, it will be recalled, was evolved as 

 a high-pressure filtering device, and to this end it was 

 supphed with arterial blood from the renal artery, this 

 blood being introduced into the glomerular capillaries 

 at a relatively high pressure in order to effect filtration. 

 As the blood flow through the glomeruli is reduced in 

 volume, the blood supplied to the tubules by the post- 

 glomerular circulation must be reduced proportionately, 

 and the renal-portal system becomes of increasing im- 

 portance in sustaining the fimction of the tubules. Tubu- 

 lar excretion neither involves nor requires the filtration 

 of any water: the solutes to be excreted pass from the 

 peritubular capillaries to the excretory tubule cells by 



