BLOOD SUPPLY TO THE HEART 



I 52 I 



The posterior branches are shorter, up to 15 mm; they 

 supply the posterior one-third of the septum and 

 anastomose with the anterior branches. In dogs, how- 

 ever, the anastomoses are deficient and the large 

 anterior septal artery's superior and inferior divisions 

 supply the central portion of the upper two-thirds of 

 the septum including the moderator band and lower 

 His bundle (36, 64, 246, 290, 399). The more distal 

 bundle branches are supplied by the penetrating 

 vessels. 



coronary blood volume. Available information re- 

 garding coronary blood volume (artery through 

 coronary vein content) is incomplete and quite ap- 

 proximate. In humans, at postmortem, average values 

 in both sexes range from about 2 to 6 ml per 100 g 

 heart weight (310). In the arrested dog and cat heart 

 and isolated beating dog heart, values approximate 6 

 to 8 ml per 100 g heart muscle (129). 



Myocardial Arterioles and Capillaries 



As the superficial arteries penetrate the myo- 

 cardium, they bifurcate or trifurcate disproportion- 

 ately so that the parent vessel and diameter grow 

 gradually smaller while the daughter vessels narrow- 

 rapidly, terminating in the capillary network (294, 

 295). The deeper arterioles lose the internal elastic 

 membrane and subendothelium present in the more 

 superficial layers and contain a single layered intima 

 and a media one to two muscle layers thick. As the 

 arteriole narrows, its muscularis becomes discontinu- 

 ous and the muscle cells decrease in frequency with 

 increasing distance from the arteriole. This latter 

 vessel, the metarteriole, is continuous at its distal end 

 with the simple endothelial tube characterizing the 

 capillary. A group of one or more, usually three, mus- 

 cle cells at the proximal end of a capillary constitutes 

 a sphincter and denotes the precapillary. 



Recent studies have suggested that the myocardial 

 capillaries are not all functional at all times as was 

 previously believed (294, 295). It has been shown that 

 the metarterioles and precapillary sphincters can 

 close off the capillary lumen. Thus, during sphincteric 

 contractions, the nucleus of the endothelial cell under- 

 lying the sphincter becomes rounded and is forced 

 into the lumen of the vessel thereby occluding it. Dur- 

 ing relaxation and in those regions where there are no 

 sphincters, the nucleus is flattened along the wall and 

 the lumen is open. The demonstration of nerve fibers 

 accompanying the vessels in the areolar connective 

 tissue and terminally "splaying'' to surround the 

 myocardial cells and sphincters, and the absence of 



any such supply to "true" capillaries, lends support 

 to a changing dynamic state of capillary patency and 

 function. Moreover, the demonstration of anastomotic 

 connections between arterioles, metarterioles, pre- 

 capillaries, and venules in both man and dogs suggests 

 arteriovenous shunting as an integral component of 

 the myocardial capillary circulation. 



In the newborn human and rabbit there is approxi- 

 mately one myocardial capillary per four myocardial 

 fibers, corresponding to 4,000 capillaries per mm 2 of 

 tissue (382). In the human adult the ratio of capillaries 

 to fibers approaches 1:1, while the capillary concen- 

 tration approximates 3,000 to 4,000 per mm 2 of tissue, 

 both values being fairly constant over a wide age span 

 (6, 125, 382). The capillary diffusing area per cm 3 of 

 tissue averages 1,145 cm2 m children, and 1,184 cm2 

 in adults (6). An analysis of tissue from various ven- 

 tricular areas reveals similar capillary densities and 

 surface areas for the human left ventricle, right ven- 

 tricle, and papillary muscle, whereas the interventric- 

 ular septum shows a decrease in both these parameters. 

 While the maximum diffusing distance is calculated 

 to be 8 /u in all of the above areas, that to the con- 

 ducting system proper is appreciably greater. 



In contrast to the septal myocardium, there is a 

 scanty capillary supply to the A-V node and His 

 bundle in sheep and cattle (125). Capillaries and 

 conducting fibers are not intimately connected and 

 are often separated by wide spaces of connective tissue. 

 In the His bundle, capillaries are located outside the 

 dense band of fibers with the central nuclei far from 

 the source of blood. Other investigations in dogs and 

 humans have shown a well-developed system of sinu- 

 soids anastomosing with capillaries, veins, and arteries 

 which traverse the annulus fibrosus and supply the 

 A-V node and common bundle (365, 366). 



Exchange of metabolites in myocardial capillaries 

 has received anatomic amplification and clarification 

 by electron microscopic techniques (iii, 270, 286). 

 The endothelial cells form a continuous capillary and 

 arteriolar lining without any evidence of intercellular 

 or intracellular pores. Many vesicles or caveolae are 

 concentrated under the cell membranes facing both 

 the capillary lumen and pericapillary spaces, and are 

 believed to represent continuous invagination and 

 pinching off of the plasma membrane which then 

 crosses the cell and liberates nutrients, metabolites, 

 and other materials (1 1 1, 270, 286). Injected colloidal 

 gold particles have been photographed concentrating 

 along the luminal side, engulfed and transported 

 across the cell in vesicles, and finally, phagocytized by 

 macrophages in the pericapillary spaces. This trans- 

 port mechanism has been variously termed "pino- 



