When do the coronary arteries fill with blood?

The coronary arteries fill with blood during diastole, specifically when the aortic valve closes. When the left ventricle is in the process of contracting, blood is being ejected into the aorta under high pressure, which causes the aortic valve to open. This phase does not allow for significant blood flow into the coronary arteries because they are being compressed by the contracting myocardium.

Once the left ventricle relaxes and the aortic valve closes, the pressure in the aorta can cause blood to flow back toward the heart, filling the coronary arteries. This process is essential for supplying oxygenated blood to the heart muscle itself. Thus, it is during the closing of the aortic valve that the conditions become favorable for the coronary arteries to receive blood.