SECRETION 357 



amount of filtrate is less under the increased ureter pressure, so that one can only 

 compare the proportions of the different constituents and the experiments do not 

 show that there was in fact any absorption of sulphate or urea. 



If an animal receives no sodium chloride in the food for several days, the serum 

 still contains nearly the whole of its normal amount, but the urine practically none. 

 Very nearly the whole of that filtered through in the glomeruli must be reabsorbed 

 in the tubules. 



As already pointed out, the filtration process tends to cause a loss of food 

 materials, so far as these are non-colloidal, as indeed those of the blood are. 

 Although a part of these may be held in adsorption equilibrium, even glucose 

 itself, as pointed out above (page 57), a certain quantity must escape in proportion 

 to the amount of the filtrate. In fact, small amounts of glucose and amino-acids 

 are normally present in the urine. Nishi (1910), however, brings evidence that 

 there is absorption of sugar in the tubules of the cortex. Even when excess of 

 glucose is present in the blood, it is found that the medulla of the kidney contains 

 none, although it is present in the cortex. If diuresis is produced, glucose is 

 present in both parts. The obvious explanation of the results is that most of that 

 present in the glomerular filtrate is reabsorbed in the tubules, except when the 

 current is too rapid to allow sufficient time Some experiments by Easier (1906) 

 support this view. Sugar solution was run into the ureter of one side under a 

 pressure of 26 mm. of water and was found to be present in the urine of the 

 opposite side. 



In experiments of this kind, however, it must be remembered that unless we assume 

 complete impermeability of the tubule cells to the particular substance in question, diffusion 

 must take place to some extent, if the concentration is greater in the lumen of the tubules than 

 in the blood vessels. 



For the reason last mentioned, most of the earlier experiments with dyes are 

 capable of interpretation either on the hypothesis of absorption or of secretion. 

 This objection does not seem to hold for those of Ghiron (1913), who injected a 

 small amount of aniline-blue or Congo-red into a vein, while observing with the 

 microscope the surface of the living kidney of the mouse (for the method, see 

 Ghiron's paper of 1912). It was seen that a pale blue or red glomerular filtrate 

 first appeared in the convoluted tubules. This would have the same concentration 

 in dye as that of the blood, so that no dye would pass through the cells of the 

 tubules by mere diffusion, since the concentration would be the same on both 

 sides. But it was seen that the border of the cells next the lumen was the first 

 to become filled with particles of dye, which gradually passed towards the side of 

 the capillaries. So that the cells evidently absorbed material from the lumen and 

 passed it back to the blood. 



The, Normal Process. We arrive then at the following conception of the 

 normal process of renal activity in the higher land animals, as was sketched in 

 outline above. By a retention of the pure filtration process of the lower animals, 

 a filtrate is first made, which contains all the non-colloidal constituents of the 

 blood in the same concentration as therein. But, if this were to be sufficient 

 to carry away the whole of the waste products, which are present in very 

 low concentration in the blood, an enormous loss, both of water and of valuable 

 constituents, would be entailed. To meet this, a mechanism has been developed, 

 by which not only a great part of the water is reabsorbed, but also a large 

 proportion of the valuable salts, such as sodium chloride, and also organic food- 

 stuffs, such as glucose and amino-acids. At the same time there is an active 

 secretion of waste products, such as urea and uric acid, by the cells of the tubules, 

 into the glomerular filtrate bathing them. With these, foreign salts injurious 

 to the organism are excreted. The existence of this latter process renders the 

 filtration of such large quantities of water unnecessary; but it is important to 

 remember that the actual work done is measured by the difference between the 

 osmotic pressures of the constituents of the blood and urine, irrespective of the 

 way in which the actual concentration is brought about. 



