480 A TEXTBOOK OF PHYSIOLOGY 



Whatever the nature of the mechanism may be, the passage of 

 water seems to be carried out with a minimum amount of work on 

 the part of the kidney, as is shown by the fact that an intravenous 

 injection of 5 per cent, sodium chloride solution causes a diuresis 

 unattended by any increased oxygen absorption. The total osmotic 

 pressure of the blood is equal to seven atmospheres, and if water were 

 separated from the blood through a semi-permeable membrane, work 

 would have to be done to overcome this pressure. If the water and salts 

 of the blood, and not the urea, sugar, etc., were separated, the osmotic 

 pressure overcome would be considerably higher than the blood - 

 pressure. After dilution of the blood with 0-9 per cent. NaCl solution 

 to provoke diuresis, it was found that the kidney secreted urine when 

 the blood -pressure was lowered even to 18 mm. Hg. 



Water flows from the ureter when circulated through the blood- 

 vessels of the dead kidney. In contiguity lie the renal vessels, 

 glomeruli, and the tubules, and it is probable that the membranes 

 which separate these allow leakage in the poisoned or dead kidney. 

 It becomes then an indifferent matter whether the water takes the 

 channel of the tubules or venules. -The kidney substance imbibes 

 the water, becoming converted, so to speak, into a bog or morass. 

 When a 0-75 per cent, solution of NaCl is perfused through the excised 

 kidney of the ox, the filtrate varies very slightly from the perfused 

 fluid, and stoppage of the renal vein increases its amount. In the 

 living kidney, the stoppage of the renal vein arrests the secretion of 

 urine. 



It has been suggested that one function of the glomerulus is to 

 act as a pulsating mechanism placed at the commencement of the 

 tubule. Undoubtedly, with each heart -beat the urine is driven 

 forward out of the tubules into the pelvis of the kidney. The whole 

 kidney expands with systole and shrinks on diastole, and not only 

 blood is expressed from the renal veins, but urine from the collecting 

 tubules by each sj^stolic expansion. The pulsatile expansion of the 

 kidney is necessary for the normal secretion of urine. 



The Nature of the Glomerular Secretion. At the present time it 

 is impossible to say what is the exact nature of the secretion of the 

 glomerulus. Probably the mechanism is such that water and salts, 

 especially chlorides, pass through with great ease. In cases where 

 by injury or operation the tubules in the medulla have been largely 

 destroyed, and the glomeruli in the cortex left intact, a much more 

 watery urine is secreted. 



The proteins and the sugar of the blood are held back, by the 

 glomerular membranes, since normal urine contains only traces of 

 these bodies. When the glomeruli are damaged, these bodies may 

 pass through into the urine, especially the blood -proteins. Direct 

 leakage then takes place. It is a matter of doubt whether any of the 

 nitrogenous constituents pass into the urine at the glomerulus. It 

 may be concluded that its main function is the separation of water 

 and salts. 



In the frog, the renal portal vein is the main blood-supply of the 



