72 



THE MECHANISM OF THE CIRCULATION. 



side of the circulation. Thus the venae cavse have a constant low head of 

 pressure, from which the heart draws its supply, and the diastolic pressure 

 within the heart is kept constant during the enormous variations of arterial 

 pressure which are brought about by vaso-constriction. 



The variations in vena cava pressure under different circumstances can 

 be seen in the following experiments carried out on dogs : 1 — 





If at any moment the right heart be unable to empty itself, owing to 

 obstruction in the pulmonary circulation — as, for example, in Valsalva's 

 experiment — the large veins and the liver form a reservoir for the congested 

 blood. In those animals which remain long under water — such as the seal, 

 otter, and diving birds — large dilatations of the vena cava inferior occur. 

 During the time of impeded respiration accumulation of venous blood takes 

 place in the vena cava without an unusual distension of the right heart.' 2 



If the left heart be unable to completely empty itself, and its output per 

 second decrease owing to great increase in peripheral resistance, the pulmonary 

 veins and capillaries will then, in the first place, form a reservior for the con- 

 gested blood ; and when the pulmonary obstruction tells back on the right heart, 

 the liver and systemic veins form a second line of defence. It is to be con- 

 cluded that the venous side forms a low-pressure reservoir for the circulatory 

 system, by which the diastolic pressure of the heart can be maintained constant. 



The seat of peripheral resistance lies in the small arteries. That this is so 

 is shown by the enormous fall of arterial pressure which takes place when 

 the splanchnic arterioles are dilated by section of the splanchnic nerves. 

 Normally, the sum of the resistances which, at each moment, oppose the 

 flow from the aorta is maintained at the same value. When one vascular 

 area dilates, another constricts, and thus the arterial pressure is maintained con- 

 stant. Owing to the vasomotor mechanism the necessary blood quantum is 

 greatly reduced, for, since the blood can be sent to any organ that requires it, 

 all the available space in the vascular system need not be filled. A high- 

 pressure main runs to all the organs of the body. By means of the vasomotor 

 nerves the stream can be turned on here or there, and any part flushed with 

 blood, while the supply to the remaining parts is proportionately diminished. 



The Elasticity of Arteries. 



The coefficient of elasticity of an excised strip of an artery is 

 not a constant factor, for with the extension it increases. The curve 



1 Bayliss and Starling, Journ. Physiol., Cambridge and London, 1894, vol. xvi. 



2 Allen Thomson, "Todd's Encyclopedia," 1836, p. 679. 



