During which cardiac phase do the coronary arteries primarily fill with blood?

The primary filling of the coronary arteries occurs during early diastole. This phase follows cardiac systole when the heart muscle is contracted and blood is ejected from the ventricles. As the ventricles relax, there is a decrease in intraventricular pressure, which allows blood flow from the aorta into the coronary arteries.

During early diastole specifically, the aortic valve closes and the pressure in the aorta remains higher than that in the coronary arteries, facilitating the flow of blood into the coronary circulation. This process is crucial because the myocardium relies on this blood supply for oxygen and nutrients.

In contrast, during systole, the pressure in the myocardium is high and the coronary arteries are compressed, which reduces blood flow to them. Late diastole is primarily concerned with the filling of the ventricles rather than the coronary arteries. Isovolumetric relaxation occurs just before diastole proper begins, and during this phase, the coronary arteries are not predominantly filling either.

Understanding this filling mechanism is vital for both diagnosing and managing various cardiac conditions, particularly those involving ischemia and myocardial perfusion.