884 PHYSIOLOGY 



whole of the blood would accumulate 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 contraction 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 output of the heart. 



(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 corresponding 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 is due 

 to the fact that the heart is taking up fluid from the venous side and 

 pumping it into the arterial side. The pressure on the latter side must 

 rise so long as the rate at which the fluid is put into the arterial system 

 by the heart is greater than that by which it escapes through the peripheral 

 resistance. Arterial pressure therefore is a resultant of the two effects : 



(a) The amount of blood entering the arterial system from the heart ; 



(6) The amount of blood leaving the arterial system through the 

 peripheral resistance. 



It is evident that the pressure will be altered by altering either of the 

 two factors peripheral resistance or output of the heart. The cardiac 

 output will depend on the amount of blood contained in the heart at the 

 beginning of each contraction, on the strength with which the heart beats, 

 and on the number of contractions which the heart gives in any given 

 period of time. The filling of the heart at the beginning of each, beat is 

 in its turn dependent on the amount of blood which is available to fill the 

 cavities and therefore on the pressure in the great veins. Increased fre- 

 quency of heart-beat need not therefore necessarily increase the total 



