23.7: Coronary Circulation

The heart, an organ critical to survival, gets nourishment not from the blood it pumps but from a separate circulation system known as coronary circulation. This is the shortest circulation in the body and is responsible for supplying the heart with the nutrients it needs to function effectively.

Coronary circulation begins at the base of the aorta, where two main arteries arise—the left and right coronary arteries. These arteries encircle the heart in the coronary sulcus and supply the arterial blood the heart requires.

The left coronary artery moves toward the left side of the heart and divides into two major branches. The left anterior descending artery, or LAD, follows the anterior interventricular sulcus and delivers blood to the anterior two-thirds of the interventricular septum and the anterior walls of both ventricles. The circumflex artery, on the other hand, supplies the left atrium and the posterior walls of the left ventricle.

The right coronary artery also divides into two branches on the right side. The right marginal artery serves the myocardium of the lateral right side of the heart. In contrast, the posterior descending artery, or PDA, runs in the posterior interventricular sulcus to the heart apex and supplies the posterior ventricular walls. Near the apex of the heart, this artery merges with the anterior interventricular artery. The branches of the right coronary artery supply the right atrium and almost all of the right ventricle.

It's important to recognize the significant variability in the origin of the PDA. For example, in 70-80% of individuals who are right-heart dominant, the PDA stems from the right coronary artery. In contrast, around 5-10% of the population is left-heart dominant, where the PDA originates from the left circumflex artery. Additionally, about 10-20% of people have a codominant circulation, with the PDA receiving blood from both the circumflex and right coronary arteries.

The coronary arteries provide intermittent, pulsating blood flow to the myocardium. They are located in the epicardium and send branches inward to nourish the myocardium. These arteries deliver blood when the heart is relaxed but are less effective when the ventricles are contracting as they get compressed by the contracting myocardium. Despite the heart accounting for just 1/200 of the body's weight, it requires about 1/20 of its blood supply, with the left ventricle receiving the most plentiful blood supply.

After passing through the capillary beds of the myocardium, the venous blood is collected by the cardiac veins, which roughly follow the paths of the coronary arteries. These veins join to form an enlarged vessel called the coronary sinus, which empties the blood into the right atrium. The coronary sinus has three large tributaries: the great cardiac vein in the anterior interventricular sulcus, the middle cardiac vein in the posterior interventricular sulcus, and the small cardiac vein running along the heart's right inferior margin. Several anterior cardiac veins also empty directly into the right atrium.