THE CAUSATION OF THE HEART-BEAT 967 



THE NUTRITION OF THE HEART 



In the frog's heart the muscle fibres are supplied directly by the blood 

 within the cavities, the spongy ventricular wall permitting the access of 

 blood between the fibres. In the mammalian heart the muscular tissue is 

 nourished through the coronary arteries, which break up into a meshwork 

 of capillaries around all the fibres. 



The flow of blood through the coronary circulation may be measured either in 

 the whole animal or in the heart-lung preparation by introducing a cannula through the 

 wall of the right auricle into the coronary sinus and collecting the blood from the latter 

 outside the body. Another method is to feed a heart from the aorta through the 

 coronary arteries with blood and collect the total outflow from the cut pulmonary artery. 

 By a comparison of these two methods it is found in the dog that the blood flow through 

 the coronary sinus forms about three-fifths of the total blood passing through the 

 coronary arteries. It is therefore possible to measure the flow through the coronary 

 sinus in the heart-lung preparation under varying conditions of pressure and output. 

 The figures so obtained multiplied by f will represent approximately the total flow 

 through the coronary circulation. 



Blood enters the coronary arteries from the aorta both during systole 

 and diastole, though it is probable that the systole of the ventricles exercises 

 a direct effect in increasing the resistance to the flow of blood through the 

 heart and squeezes out the contained blood into the coronary veins. This 

 may be one reason why the flow of blood through the coronary system is 

 greater in a beating heart than in a heart which is quiescent. The most 

 important factor in determining the flow through the coronary vessels is the 

 arterial pressure. The marked effect of this factor is shown in the following 

 Table : 



Heart weight, 107 gms., total output per minute, 1400 cc. 



Arterial Coronary circulation 



pressure per minute 



60 50 



100 90 



128 124 



166 208 



190 500 



We see from this Table that the heart muscle is supplied with blood 

 in proportion to its needs, since its work and its respiratory exchanges 

 increase continuously with the rise of arterial resistance. Indeed, under 

 the severe test of contracting against an average pressure of 190 mm. Hg. 

 over one-third of the whole blood leaving the heart was passing through its 

 muscular walls, one gramme of muscular tissue being irrigated with 5 c.c. 

 of blood per minute. Another important factor in determining the coronary 

 flow is the effect of the metabolites produced by the contracting heart-muscle 

 itself. This is well shown when the heart is asphyxiated. Thus in one 

 experiment while the arterial pressure was maintained -constant, the total 

 coronary flow was 56 c.c. per minute. Artificial respiration was then dis- 

 continued, and during the succeeding minutes the coronary circulation was 

 61, 72, 150, 180. The circulation then failed. Carbonic acid produces 



