356 PRINCIPLES OF GENERAL PHYSIOLOGY 



Histological evidence in other animals shows all stages of extrusion of granules 

 from cells of the tubules (see the observations of Metzuer in Altmann's paper, 1894, 

 p. 133, in the case of the embryo chick, and in the kitten, 1907, 2, p. 221 and 

 footnote on p. 222. Also Todaro, 1902, in Salpa). 



It is obviously more difficult to prove the secretion of urea, but possibly the 

 formation in the cells of vacuoles which discharge into the lumen of the tubules, as 

 described by Gurwitsch (1902), is connected with the process. No direct evidence 

 has been found, so far as I am aware, of excretion of glucose or sodium chloride 

 by the tubules. On the contrary, as we shall see, there is evidence that they are 

 absorbed. 



Absorption of Solutes by Tubules. In the frog, owing to the fact that the 

 glomeruli are supplied by blood directly from the aorta, while the tubules are 

 supplied from a separate renal portal vein, it is possible to investigate the two 

 systems separately. 



I do not propose to describe the earlier experiments of Nussbaum and others, since tlu-v 

 were to a certain extent inconclusive, on account of neglect of the fact that, while the ivnal 

 portal blood supplies the tubules alone, the arterial blood from the aorta, aft IT passing through 

 the glomeruli, supplies the tubules with oxygenated blood, so that cutting off the glonuTu hi 

 circulation at the same time caused death of the tubules from asphyxia. Those interested 

 will find a description of the experiments in the work of Starling (1912), or of Met/.nn 

 (1907, 2). 



The later experiments of Bainbridge, Collins, and Menzies (1913) have brought 

 out some points of interest, to which brief reference may be made. The urine of 

 the frog is normally of a lower osmotic pressure than the blood, or, if the kidney 

 is perfused with Ringer's solution, the salt concentration of the urine is lower than 

 that of the Ringer's solution. This state of affairs is brought about by the 

 tubules, since, when they are poisoned, the urine is always isotonic with the 

 solution perfused, that is, it is a pure glomerular filtrate. Now this activity of 

 the tubules may be due either to secretion of water or to absorption of salts. No 

 evidence could be obtained of the former except, perhaps, under the influence of 

 some diuretic agent such as urea. Sodium chloride must therefore be absorbed. 

 The frog, being essentially a water animal, is under no necessity of hoarding water 

 and, in fact, it has been stated that the urine secreted in twenty-four hours may 

 exceed the total weight of the body. It would seem possible, then, that the whole 

 of its excretory products could be got rid of by mere filtration ; but it is important 

 that the valuable substances, like sodium chloride and glucose, also in the filtrate, 

 should be retained. 



Experiments made by Cushny (1901) point in the same direction. In the later 

 stages of the diuresis brought about by injection of a mixture of sodium chloride 

 and sulphate, the proportion of chloride to sulphate in the blood was 0'493 to 

 0*191, whereas in the urine it was 0'094 to 2'0. The sulphate is much less readily 

 absorbed by the tubule cells than the chloride is, as by cells in general, and it is 

 evident that the fact favours the reabsorption of the valuable chloride. It is 

 possible that the foreign sulphate may actually be excreted by the tubules, but 

 there is no direct evidence of the fact. During the maximum of diuresis, the 

 concentrations of the two salts in the urine approach much more closely to tlni' 

 in the blood, although that of the sulphate is higher than in the blood, while that 

 of the chloride is lower. It is clear that the faster the liquid passes along the 

 tubules, the less opportunity is there for the activity of the cells of the tubules 

 effect changes in its composition, so that the more rapidly the urine is produced, 

 the more nearly is it isotonic with the blood. It is important to notice that, in 

 Cushny's experiments, the percentage of chloride in the urine was never higher 

 than in the blood. It would appear from some experiments by Loewi (1902) that 

 mere diffusibility is not the only controlling factor when poisonous salts are 

 concerned, since sodium iodide is excreted as effectively as sodium sulphate. 



Cushny also performed some experiments in which the kidney was caused to 

 secrete under an increased pressure in the ureter, so that the glomerular filtrate 

 remained longer in contact with the tubules. The results showed a greater 

 absorption of sodium chloride than of sulphate and urea. Of course, the total 



