CHAP, iv.] THE VASCULAR MECHANISM. 253 



The Work done. 



138. We can measure with approximative exactness the in- 

 traventricular pressure, the length of each systole, and the number 

 of times the systole is repeated in a given period, but perhaps the 

 most important factor of all in the determination of the work of 

 the vascular mechanism, the quantity ejected from* the ventricle 

 into the aorta at each systole, cannot as yet be said to have been 

 accurately determined ; we are largely obliged to fall back on 

 calculations having many sources of error. The general result 

 of some of these calculations gives about 180 grms. (6 oz.) as the 

 quantity of blood which is driven from each ventricle at each 

 systole in a full-grown man of average size and weight, but this 

 estimate is probably too high. 



In the dog the quantity has been experimentally determined, by 

 allowing the heart to deliver its contents through one branch of the 

 aorta, all others being ligatured or blocked, into a receiver, the con- 

 tents of which are at intervals, by an ingenious contrivance, returned 

 to the right auricle. The time taken to fill the receiver and the 

 number of beats executed during that time being noted, the average 

 quantity ejected at a beat is thus given. It is found to vary very 

 widely. 



Various methods have been adopted for calculating the average 

 amount of blood ejected at each ventricular systole. The simplest 

 method is to measure the capacity of the recently removed and as yet 

 not rigid ventricle, filled with blood under a pressure equal to the 

 calculated average pressure in the ventricle. On the supposition that 

 the whole contents of the ventricle are ejected at each systole this 

 would give the quantity driven into the aorta at each stroke. The 

 other methods are very indirect. 



It is evident that exactly the same quantity must issue at a beat 

 from each ventricle ; for if the right ventricle at each beat gave 

 out rather less than the left, after a certain number of beats the 

 whole of the blood would be gathered in the systemic circulation. 

 Similarly, if the left ventricle gave out less than the right, all 

 the blood would soon be crowded into the lungs. The fact that 

 the pressure in the right ventricle is so much less than that 

 in the left (probably 30 or 40 mm. as compared with 200 mm. 

 of mercury), is due, not to differences in the quantity of blood in 

 the cavities, but to the fact that the peripheral resistance which 

 has to be overcome in the lungs is so much less than that in the 

 rest of the body. 



It must be remembered that though it is of advantage to speak 

 of an average quantity ejected at each stroke, it is more than 

 probable that that quantity may vary within very wide limits. 

 Taking, however, 180 grms. as the quantity, in man, ejected at 



