998 PHYSIOLOGY 



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 ; 



(b) 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 frequency of heart-beat need not there- 

 fore necessarily increase the total output of the heart into the arterial 

 system. If the heart is beating with optimum rate and force it will 

 keep the venous system, at any rate that part nearest the heart, 

 practically empty, and it is not possible for it to obtain more blood 

 to put into the arterial side, however frequently it may beat. There 

 will be an optimum frequency of the heart-beat which will depend 

 on the state of filling of the great veins. The fuller these are the 

 more rapidly the heart may beat without diminution of total output. 

 On the other hand, in a normal animal with the heart beating at its 

 optimum rate and with effective contraction of its muscular walls, 

 while slowing the heart-rate will diminish the total output and therefore 

 the arterial pressure, increase in the frequency of the beat cannot raise 

 the arterial pressure to any appreciable extent, though the heart 

 may tend to wear itself out by beating at a greater rate than the 

 optimum. 



