574 



VETERINARY PHYSIOLOGY 



or peptone when injected into the blood are not excreted, 

 although in the frog with the vessels unligatured they 

 appear in the urine. 



These observations seem to show that the Malpighian bodies 

 have to do chiefly with the filtering off from the blood plasma 

 of water and of solids held in true solution, and that their 

 activity depends upon the rate of blood-flow through them, and 

 upon the pressure in the glomerular capillaries. That such a 

 purely physical process is involved seems to be indicated by the 

 fact that a free flow of urine (diuresis) can be induced, without 

 increasing the chemical changes in the kidneys, as indicated by 



Fkj. 227.— To show the relationship between the production of urine and the 

 consumption of oxygen by the kidney under the influence of Ringer- 

 Solution and of Sodium Sulphate. The black area indicates the amount 

 of urine secreted, the thin line the consumption of oxygen. (Barckoft.) 



the oxygen consumption, by injecting into the blood hypertonic 

 solutions of various salines, such as NaCl, which dilute the blood 

 and increase its volume (fig. 227). 



The reason why the formation of urine stops when the 

 arterial pressure falls to about 50 mm. Hg seems to be due to 

 the fact that the filtration pressure must be well above the 

 osmotic pressure of the colloids of the blood which are not filtered 

 off. By determining the difference between the osmotic pressure 

 of blood serum and of the filtrate from it through a semi-perme- 

 able membrane Starling concluded that the osmotic pressure 

 of the blood proteins is about 30 mm. Hg, and that therefore the- 

 filtration pressure must be above this. When this osmotic 



