Final answer:
The coronary arteries fill during the diastole phase of the cardiac cycle when the heart muscle is relaxed and the ventricles are filling with blood.
Step-by-step explanation:
The coronary arteries fill during the diastole phase of the cardiac cycle. During diastole, the heart muscle is relaxed, allowing the chambers of the heart, particularly the ventricles, to fill with blood. It is during this filling time that the epicardial coronary arteries, which are the surface arteries of the heart, receive blood flow as the heart itself requires oxygen and nutrients to function effectively. The coronary veins, in contrast, are responsible for taking deoxygenated blood away from the heart tissue back to the chambers. The process of ventricular diastole occurs after the atria have contracted (atrial systole) and just before the ventricles begin their contraction (ventricular systole). The ventricles are typically filled with approximately 70-80 percent of their capacity due to inflow during diastole, which includes the period when the coronary arteries are filling. It's important to note that an electrocardiogram (ECG) can be utilized to record the electrical activity of the heart and can help in identifying the different phases of the cardiac cycle, including diastole.