THE CIRCULATION 259 



heart it is lower than the atmospheric pressure during the 

 first part of each ventricular diastole. (Fig. 125, V.) 



Capillaries. The pressure in the capillaries must obviously 

 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.g. to blanch a piece of skin. 



II. ARTERIAL PRESSURE. 



The force of the heart and the degree of peripheral resist- 

 ance both modify the arterial pressure, and normally these so 

 act together that disturbance of one is compensated for by 

 changes in the other. Thus, if the heart's action becomes 

 increased and tends to raise the arterial pressure, the peri- 

 pheral resistance falls and prevents any marked rise. 

 Similarly, if the peripheral resistance is increased, the 

 heart's actidn is diminished, and no rise in the pressure 

 occurs. Under certain conditions, however, this compen- 

 satory action is not complete, and changes in the arterial 

 pressure are thus brought about. 



The volume of blood has a comparatively small influence 

 on the arterial pressure because the veins are so large that 

 they accommodate very varying amounts of fluid. 



Factors controlling Arterial Pressure. 



(a) Heart's action. The influence of this may be readily 

 demonstrated by stimulating the vagus nerve while taking a 

 tracing of the arterial pressure. The heart is inhibited, 

 less blood is forced into the arteries, and the pressure 

 falls. 



If, on the other hand, the accelerator nerve is stimulated, 

 the increased heart's action drives more blood into the 

 arteries, and the pressure rises. 



(6) Peripheral resistance. The resistance to outflow 

 from the arteries depends upon the resistance offered in the 

 small arteries, the walls of which are chiefly composed of 

 muscular tissue. When these fibres are contracted, the 

 lumen of the vessels is small and the resistance is great. 



