io8 HUMAN PHYSIOLOGY 



upon blood pressure, we cannot regard it as a mere process 

 of filtration of the blood fluid through the walls of the urinary 

 tubules. This assertion is based upon the following; facts : 



1 . The composition of the urine differs quantitatively from 

 that of proteid-free blood plasma. Many substances (e.g. 

 urea) are much more abundant in the urine than in blood. 



2. The harmful effect of ligaturing the renal artery cannot 

 be explained on the theory of mere filtration. 



Renal secretion is, therefore, dependent upon the special 

 activity of the gland-cells. These cells are temporarily 

 paralyzed by the stoppage of circulation (compression of 

 renal artery), so that they are not able to secrete. Diuretics, 

 in so far as they do not affect blood pressure, stimulate the 

 cells toward greater activity. 



According to the theory accepted at present, renal secre- 

 tion takes place as follows: The cells of the Bowman capsule 

 secrete chiefly water, while the cells of the convoluted tubules 

 secrete the solid constituents of urine, and this secretion is 

 concentrated as it passes through the urinary tubule by the 

 absorption of water from it. 



This view is based on the fact that sodium-sulphincligotate, 

 injected into the blood, is excreted by the kidneys, and is 

 found, during its excretion, only in the inner part of the cells 

 of the convoluted tubules and lower down in the lumen of 

 the urinary tubules, while it is never found in the cells of the 

 capsule. 



Most of the substances excreted in the urine are not formed 

 in the kidneys but in other organs, and are carried by the 

 blood to the kidneys to be picked out by them. Some sub- 

 stances can also be made in the parenchyma of the kidneys, 

 e.g. the hippuric acid. That energetic oxidations take place 

 in the kidneys is proven by the facts that the blood in the 

 renal veins is venous, and that the secreted urine is often 

 warmer than the blood flowing to the kidneys. 



Beside the end-products of metabolism, there are also- 

 excreted substances which are indeed normal constituents of 

 the body but which, for some reason, have accumulated in 



