Coronary arterial blood passes through the capillary beds; most of it returns to the right atrium through the coronary sinus. Of the coronary arteries, epicardial arteries are largest (2 to 5 mm in diameter), large arterioles are medium in size (1.0 to 0.5 mm in diameter), and small arterioles are smallest (<0.1 mm in diameter). Some of the coronary venous blood reaches the right atrium via the anterior coronary veins. In addition, vascular communications directly link the myocardial vessels with the cardiac chambers; these communications are the arteriosinusoidal, arterioluminal, and thebesian vessels. The arteriosinusoidal channels consist of small arteries or arterioles that lose their arterial structure as they penetrate the chamber walls, where they divide into irregular, endothelium-lined sinuses. These sinuses anastomose with other sinuses and with capillaries, and they communicate with the cardiac chambers. The arterioluminal vessels are small arteries or arterioles that open directly into the atria and ventricles. The thebesian vessels are small veins that connect capillary beds directly with the cardiac chambers and also communicate with the cardiac veins. All the minute vessels of the myocardium communicate in the form of an extensive
Posterior descending branch of right coronary artery • Fig. 17.29 Illustrations of the anterior and posterior surfaces of the heart, depicting the location and distribution of the principal coronary vessels.
plexus of subendocardial vessels. However, the myocardium does not receive significant nutritional blood flow directly from the cardiac chambers.
The primary factor responsible for perfusion of the myocardium is aortic pressure. Changes in aortic pressure generally evoke parallel directional changes in coronary blood flow. This is caused in part by changes in coronary perfusion pressure. However, the major factor in the regulation of coronary blood flow is a change in arteriolar resistance engendered by changes in the metabolic activity of the heart. When the metabolic activity of the heart increases, coronary resistance decreases; when cardiac metabolism decreases, coronary resistance increases (see
Blood flow in the heart is autoregulated. If a cannulated coronary artery is perfused by blood from a pressure-controlled reservoir, perfusion pressure can be altered without a change in aortic pressure and cardiac work. The relationship between initial and steady-state blood flow is shown in the experiment depicted in
Fig. 17.30 . This is an example of autoregulation of blood flow, which is mediated by a myogenic mechanism in large and small arterioles ( ). The metabolic activity of cardiac muscle in small arterioles and the endothelium modulate autoregulation. The coronary circulation adjusts serial resistances within the microvasculature thereby adapting blood flow to O2 requirements. Blood pressure is kept within narrow 0.2 0.4 0.6 0.8 1 Time (sec) •Fig. 17.30 Pressure-Flow Relationships in the Coronary Vascular