1010 PHYSIOLOGY 



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 fj 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 c.c. 



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 in this 

 particular experiment, 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 main- 

 tained constant, the total coronary flow was 56 c.c. per minute. Artificial 

 respiration was then discontinued, and during the succeeding minutes the 

 coronary circulation was 61, 72, 150, 180. The circulation then failed. Car- 

 bonic acid produces also an increase in the flow through the coronary arteries, 

 but it is impossible with the highest attainable percentages of carbon 

 dioxide in the blood to effect such an increase in the coronary flow as is 

 observed during asphyxia. The dilatation of the coronary vessels, which 



