THE BLOOD PRESSURE 997 



in an animal are not all -on one plane, and if the animal is in a vertical 

 position the hydrostatic pressure of the column of blood between 

 the heart and the dependent parts of the body may distend the veins 

 to such an extent that the whole of the blood is taken up in these veins 

 and none returned to the heart. The fact that after stoppage of the 

 heart the pressure is positive at all parts of the vascular system in the 

 animal with open thorax shows that there is actually a mean systemic 

 pressure, i.e. under normal circumstances, when the animal is in 

 a horizontal position, all parts of the system are slightly distended. 

 Direct measurement shows that this mean systemic pressure is about 

 10 mm. Hg. The smallness of this figure shows moreover that under 

 the influence of gravity alone the pressure will be easily reduced 

 to nothing at all in the upper parts of the body. In a man in the 

 vertical position, in the absence of the nervous reactive mechanism 

 which we shall consider later on, the whole of the blood would accumu- 

 late in the abdomen and lower parts of the body, and the circulation 

 would come to a standstill. On the other hand, the pressure may be 

 altered in any part of the vascular system in any of the following 

 ways : 



(1) Alteration of capacity of the total system either by contrac- 

 tion of walls of the vessels or by pressure on them from without. 



(2) Alteration of the total volume of the circulating fluid. 



Either of these two factors would affect in the first place the mean 

 systemic pressure. The distribution of pressure, i.e. the relative 

 pressure in the arteries and veins, will be determined by 



(3) Alteration in the heart beat. 



(4) Alteration in the peripheral resistance and therefore in the ease 

 with which the blood can, escape from arterial to venous side. 



In any change either in arterial or venous pressure at least two of 

 these factors are involved. Every constriction of arterioles causes 

 not only an increase in the peripheral resistance but also a diminished 

 capacity of the whole system, so that the arterial pressure is raised at 

 the same time as the mean systemic pressure. Nearly always such 

 a change will involve as its immediate consequence some correspond- 

 ing alteration in the heart-beat, so that at least three factors will 

 co-operate in the production of the rise or fall of blood pressure. We 

 shall have occasion to deal with many examples of these complex 

 conditions when we are discussing the reactions of the vascular system 

 as a whole. 



THE DEPENDENCE OF ARTERIAL PRESSURE ON OUTPUT 



OF HEART 



The importance of the heart-beat in determining arterial pressure 

 is connected with its output in a given time. The arterial pressure 



