Friday, August 1, 2008

Coronary Circulation

Picture from Lippincott Williams & Wilkins Atlas of Anatomy, 1st Edition.2008 And Explanation from Julian, DG, Cardiology 8th Edition, Elsevier Saunders. 2005.

The Coronary Arteries
There are two major coronary arteries -right and left (Plate 4-22). The right coronary artery arises from the right coronary sinus of Valsava and runs down in the groove between the right atrium and the right ventricle. In Most hearts, its branches supply the sinus node, the atrioventricular node and bundle, the right ventricle and the inferior part of the left ventricle. The left coronary artery, which arises from the left coronary sinus of Valsava, soon divides into two large branches: The anterior descending branch which runs down between the two ventricle anteriorly, and the left circumflex branch which passes around in the groove between the left atrium and the left ventricle. The anterior descending artery supplies the interventricular septum and the anterior wall of the ventricle. The circumflex supplies the lateral and posterior aspect of the left ventricle. The major vessels traverse the external surface of myocardium, sending branches perpendicularly into the muscle mass. There are normally many small anastomoses between the coronary arteries, but these are of no functional importance. When an area of the heart become ischaemic, the anastomoses enlarge and then provide a collateral blood supply to the affected muscle which is often vital for its survival.

Plate 4-22. Anterior surface of the heart showing coronary arteries. The left coronary artery has two terminal branches: the anterior interventricular branch (also called the left anterior descending) and the circumflex branch. Postero-inferior surface of the heart showing coronary arteries. The right coronary artery gives off a right marginal branch and a large posterior interventricular branch. Near the apex, the posterior interventricular branch may anastomose with the anterior interventricular branch of the left coronary artery. (click image to enlarge)

The arteries divide to form arterioles and capillaries similiar to those elsewhere in the body, and the venules and veins join to form larger venous channels. Virtually all the blood from the left coronary artery eventually drains into the coronary sinus; that from the right coronary artery drains mainly into the anterior cardiac veins. From these veins the blood passes into the right atrium.
The blood flow in the coronary arteries resembles that in other regions in being dependent on the blood pressure and on the vascular resistence of the arteries and arterioles. A distinctive feature of the coronary circulation is that the arteries are compressed by the contracting myocardium during systole so that the resistence to flow at that time is sharply increased. Consequently, coronary blood flow occurs mainly during diastole. Flow is largely determined by the calibre of the small coronary arteries. Certainly, the aortic diastolic pressure is also determinant of coronary flow, but, according to Poiseuille's equation, flow is dependent directly on pressure differences but related to the fourth power of the radius. Therefore, a doubling of aortic diastolic pressures doubles the coronary flow, whereas a doubling of the radiusof the coronary arteries leads to 16-fold increase in flow. In health, many variations in coronary blood flow are mainly due to changes in impedance in the small coronary arteries; these dilate in response to metabolic signals from the myocardium, but they are also under the influence of neurohormonal agents. A further influence is the flow-related release of nitric oxide. Variations in tone also occurs in the large coronary arteries but these affect blood flow only if these vessels are narrowed by disease or if they are extreme (spasm).
In the normal resting heart, almost all of the oxygen is extracted during passage through the capillaries; coronary sinus blood is therefore almost completely desaturated. Unlike other organs, the heart cannot call upon a venous oxygen reserved when faced by increased demands, and is largely dependent upon the ability of the coronary arteries to increase their diameter.

(click image to enlarge)

The Coronary Veins
The coronary veins drain mainly into the coronary sinus, which drains directly into the right atrium (Plate 4-24). There are some small veins that drain directly into the heart chambers. Generally, these drain into the right side of the heart.