26o 



VETERINARY PHYSIOLOGY 



in the small arteries. This distribution of arterial pressure 

 might be plotted out as in fig. 122, Ar. 



Veins. If the pressure in any of the small veins in a 

 medium vein, and in a large vein near the heart be measured, 

 it will be found 



1st. That the venous pressure is less than the lowest 

 arterial pressure. 



2nd. That it is highest in the small veins, and becomes 

 lower in the larger veins. In the great veins entering the 

 heart it is lower than the atmospheric pressure during the 

 first part of each ventricular diastole (tig. 122, V. ). 



Capillaries. The pressure in the capillaries must obviously 



I6O 



V. 



20 



FIG. 122 Diagram of the Distribution of Mean Blood Pressure throughout the 

 Blood Vessels. Ar., the arteries ; C., the capillaries ; V., the veins. 



be intermediate between that in the arteries and in the 

 veins. 



It is not so easily measured, but it may be approximately 

 arrived at by finding the pressure which is required to empty 

 the capillaries e.y. to blanch a piece of skin. 



The pressure in any part of a system of tubes depends upon 

 two factors 



1st. The force propelling fluid into that part of the tubes. 



2nd. The resistance to the outflow of fluid from that part 

 of the tubes. 



The pressure in the Arteries is high, because with each 

 beat of the heart about 80 grms. of blood are thrown with the 



