302 THE MECHANICS OF THE HEART 



Clearly, the function of the ventricles is to develop the pressure 

 necessary to drive the blood through the vascular system. They 

 impart kinetic energy to the blood, and naturally, as the resistance 

 in the general circuit is much greater than that in the pulmonary cir- 

 cuit, the left ventricle must produce a much higher pressure than the 

 right. We have previously seen that the changes in intra-auricular 

 pressure are propagated outward into the veins, where they appear in 

 v the form of the physiological venous pulse. In a similar manner, the 

 intraventricular pressure makes itself felt throughout the arterial 

 system, where it forms the basis of the arterial pulse, because each 

 ventricular discharge raises the pressure in these channels above that 

 prevailing during the diastolic period of the heart. This topic will 

 be dealt with in greater detail in a subsequent chapter. 



The cardiac output per unit of time varies directly with the fre- 

 quency of the contraction and the power of filling of the ventricles. 

 Approximate values may be obtained in several ways, namely by: 



(a) Calculation from the total amount of blood present in the body. 

 (6) Measuring the capacity of the chambers of the excised heart. 



(c) Determining the volume-curve of the beating heart by means of the cardi- 

 ometer. 



(d) Calibrating the aortic blood-stream of the normal or excised heart with the 

 aid of a stromuhr, or current measurer. 



In addition, Zuntz has devised an indirect method for determining 

 this factor which depends upon a comparison of the amounts of oxygen 

 in the respiratory air, and the differences in the oxygen content of the 

 arterial and venous blood. To illustrate, a horse weighing 360 kilos 

 uses 2733 c.c. of oxygen in a minute, and its arterial blood contains 

 10.33 per cent, more oxygen than its venous blood. Thus, as every 

 100 c.c. of pulmonary blood are charged with 10.33 c.c. of oxygen, and 

 as in all 2733 c.c. of this gas are consumed in a minute, the total quan- 

 tity of blood traversing the lungs must amount to: 



100 X2733 

 1633- : 



Assuming a cardiac frequency of 50 in a minute, each contraction of the 

 right ventricle must yield about 50- c.c. of blood. Moreover, as the 

 left ventricle works in perfect unison with the right, this figure must 

 also represent the aortic discharge. Plesch states that the cardiac 

 output amounts to 59 c.c., this value being based upon gasometric 

 experiments upon man. By using the absorption of nitrous oxid 

 gas as an index, Krogh 1 has found values ranging between 40 and 120 

 c.c. in accordance with the frequency of the heart. This author also 

 states that muscular exercise increases the output very markedly. 

 By following a similar analytical procedure, Boothby 2 has obtained 

 an average value of 60 c.c. 



1 Skand. Archiv fur Physiol., xxvii, 1912. 



2 Am. Jour, of Physiol., xxxvii, 1915, 383. 



