176 THE VASCULAR MECHANISM. 



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 contents of which are at intervals, lay 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 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 grammes 

 as the quantity, in man, ejected at each stroke at a pressure of 250 mm. 1 of 

 mercury, which is equivalent to 3.21 metres of blood, this means that the 

 left ventricle is capable at its systole of lifting 180 grammes 3.21 metres 

 high, i. e., it does 578 gramme-metres of work at each beat. Supposing the 

 heart to beat 72 times a minute, this would give for the day's work of the 

 left ventricle nearly 60,000 kilogramme-metres. Calculating the work of 

 the right ventricle at one-fourth that of the left, the work of the whole heart 

 would amount to 75,000 kilogramme-metres, which is just about the amount 

 of work done in the ascent of Snowdon by a tolerably heavy man. 



A calculation of more practical value is the following: Taking the 

 quantity of blood as T ^ of the body- weight, the blood of a man weighing 

 75 kilos would be about 5760 grammes. If 180 grammes left the ventricle 

 at each beat, a quantity equivalent to the whole blood would pass through 

 the heart in 32 beats, i. e., in less than half a minute. 



THE PULSE. 



128. We have seen that the arteries, though always distended, undergo 

 at each systole of the ventricle 'a temporary additional distention, so that 

 when a finger is placed on an artery, such as the radial, an intermittent pres- 

 sure on the finger, coming and going with the beat of the heart, is felt, and 

 when a light lever is placed on the artery, the lever is raised at each beat, 

 falling between. 



This intermittent expansion which we call the pulse, corresponding to 

 the jerking outflow of blood from a severed artery, is present in the arteries 

 only, being, except under particular circumstances, absent from the veins 

 and capillaries. The expansion is frequently visible to the eye, and in some 



1 A high estimate is purposely taken here. 



