390 



ESSENTIALS OF PHYSIOLOGY. 



water and salts, the remaining urinary constituents being secreted by 

 the tubules. 



Ludwig believed that the whole of the urine was formed by nitra- 

 tion through the glomeruli as a fluid identical in composition with 

 the blood plasma minus its proteins, and that in its passage down the 

 lumen of the tubules much of the water and some of the salts were 

 reabsorbed, so that the composition of urine, as it left the kidney, 

 differed greatly from that of blood plasma. Since blood plasma 

 contains about 0'02 to 0*05 per cent, of urea, whereas urine contains 

 2 per cent, urea, this theory demands that at least 60 litres of fluid 

 should be filtered off by the glomeruli, of which all but 1*5 litres are 

 reabsorbed ; it is no longer accepted. 



Heidenhain regarded both the tubules and glomeruli as possessing 

 a secretory function, the latter secreting water and salts by a selective 

 and vital process. This view is still accepted by some authorities, 

 whereas others believe that the glomeruli form by filtration a fluid 

 identical in composition with the blood plasma minus its proteins, and 

 that the tubules secrete into this fluid, as it passes along them, water 

 and other urinary constituents. The latter theory is really a slight 

 modification of Bowman's theory 



The question as to whether the formation of urine takes place by a 

 process of filtration or of secretion can be answered by ascertaining 

 whether the conditions under which it is formed conform to those 

 known to hold for filtration or secretion elsewhere. Tn filtration, the 

 amount of filtrate varies directly with the difference of pressure on the 

 two sides of the filtering membrane, and it usually contains the same 

 percentage of crystalloids as the fluid undergoing filtration. When a 

 true secretion, such as that of saliva, takes place, the pressure of the 

 saliva in the ducts may rise higher than that of the blood, and the 

 amount of secretion is, within wide limits, independent of the blood 

 pressure ; moreover, the composition of the secretion differs greatly from 

 that of the blood. Further, during secretion, the secreting cells perform 

 work and take up more oxygen from the blood. 



In the discussion of this question it is convenient to consider 

 separately the functions of the glomeruli and of the tubules. 



THE FUNCTION OF THE GLOMERULI. 



In the mammalian kidney, it is impossible to obtain separately the 

 urine formed by the tubules and glomeruli respectively, though tKere 

 is evidence that when the flow of urine is profuse it is derived mainly 

 from the glomeruli. If urine is simply filtered through the walls of 



