450 VETERINARY PHYSIOLOGY 



mercury, while the diastolic pressure is only about 70 mm. 

 The difference between these, of course, gives the pulse 

 pressure. 



(3) Factors controlling Arterial Pressure. 



(1) The force of the heart and (2) the degree of peripheral 

 resistance both modify the arterial pressure, and normally 

 these so act together that any disturbance of one is com- 

 pensated for by changes in the other. Thus, if the heart's 

 action becomes increased and tends to raise the arterial 

 pressure, the peripheral resistance falls and prevents any 

 marked rise. Similarly, if the peripheral resistance is 

 increased, the heart's action is diminished, and the rise in 

 the pressure is checked. Under certain conditions, however, 

 this compensatory action is not complete, and changes in 

 the arterial pressure are thus brought about. 



(3) The volume of blood has a comparatively small 

 influence on the arterial pressure (1) because by changes in 

 the degree of contraction of the peripheral vessels, the 

 volume of the vascular system may be adapted to the 

 volume of blood contained, and (2) because there is a very 

 free exchange of water between the blood and the tissues 

 through the walls of the capillaries. Hence, after a 

 haemorrhage, the volume of the blood is rapidly restored, 

 and hence, after transfusion of salt solution, the fluid rapidly 

 passes out of the vessels. But if an excessive loss of blood 

 occurs, or if a large quantity of blood stagnates in any 

 region and is thus put out of effective circulation, the vessels 

 may not be able to adapt themselves, and the arterial 

 pressure may fall. From observations made during the 

 Great War it appears that in man a loss of 40 per cent, of 

 the blood is generally accompanied by a fall of the systolic 

 pressure to about 80 mm. Hg. 



When the pressure falls in this way it may be restored 

 by injecting into a vein a sufficient amount of some fluid 

 which has no prejudicial action on the blood and which 

 does not too readily transude out of the capillaries. In 

 man, gum arable in 6 per cent, solution has been used. 



