220 URINE 



Here it will be seen that the amount of secretion is in- 

 fluenced not by the general blood-pressure but by the 

 degree of dilatation of the renal arterioles — that is to say, 

 by the local blood-pressure in the kidney. 



When the pressure in the ureter is raised by partially 

 clamping this vessel the rate of flow falls. This might, 

 however, be due not to the reduction in the filtration 

 pressure but to the obstruction of the veins consequent 

 upon the dilatation of the tubules. 



2. The Osjnotic Pressure of the Plasma Colloids. — The 

 proteins of the plasma exert an osmotic pressure of 25-30 

 mm. of mercury. AVhen the arterial blood-pressure is 

 reduced to 40 mm. the flow of urine ceases. Allowing for 

 a certain difference between the arterial pressure and the 

 pressure in the renal arterioles, these facts indicate that 

 the cessation of flow occurs because the filtration pressure 

 is neutrahsed by the osmotic pressure. In confirmation 

 of this explanation is the fact that when the ureter is 

 obstructed the pressure rises within it until it is about 

 30-40 mm. below arterial pressure. 



The protein content of the plasma may be reduced by 

 partially replacing the blood by a suspension of corpuscles 

 in Ringer's solution. When this is done there occurs a 

 copious diuresis which cannot be explained by the change 

 in the viscosity of the blood. Clearly the proteins by 

 their osmotic pressure restrain the tendency to filtration. 



3. The Consumption of Oxygen.- — This question has been 

 settled by Barcroft and Straub, who studied the gaseous 

 metabohsm of the kidney upon the injection of salts. The 

 result is shown in Fig. 29, from which it will be seen that 

 when Ringer's solution is injected there is a diuresis un- 

 accompanied by increased oxygen consumption. Under 

 these circumstances the energy is derived not from the 

 kidney but from the heart. 



There is therefore convincing evidence that physical 

 factors play a very important part in the formation of 

 urine. 



