Your heart feeds itself with blood only when it relaxes
Every organ in your body receives a surge of oxygen-rich blood when the heart contracts. The heart muscle itself is the lone exception. During systole, the squeezing muscle squeezes shut its own internal arteries, while the opened aortic valve flaps partially block blood flow into the coronary entrances. Only during diastole, when the ventricles relax, does fresh blood finally rush through the coronary arteries to nourish the working cardiac tissue.
The Paradox of the Pumping Muscle
In the broader systemic circulation, arterial perfusion follows a simple pattern: when the left ventricle contracts during systole, it generates high pressure that propels a wave of oxygenated blood through the aorta and outward to every peripheral organ. Brain tissue, kidneys, skeletal muscles, and abdominal viscera all experience their primary burst of arterial inflow during this systolic peak. Because these organs sit downstream from the heart, their internal tissue pressures remain significantly lower than the incoming arterial driving pressure, allowing blood to flow through their capillary beds unimpeded during ventricular contraction.
The heart itself breaks this rule due to its own mechanical architecture. The muscular walls of the left ventricle must generate tremendous internal tension to overcome systemic vascular resistance and force open the aortic valve. As the thick bands of ventricular muscle tighten, they exert direct physical pressure on the blood vessels woven through them. This compressive force—known as intramyocardial tissue pressure—rises so high during peak systole that it exceeds the pressure of the blood inside the coronary arteries, momentarily squeezing the intramural microvessels shut and halting forward flow.
Anatomical Origins at the Root of the Aorta
To understand how the cardiac muscle receives nourishment despite this mechanical roadblock, one must look at the exact anatomical origin of the coronary circulation. The right and left coronary arteries do not branch off the aorta downstream in the chest; instead, they emerge from small openings called coronary ostia located at the very base of the ascending aorta, within the aortic sinuses just above the cusps of the aortic valve.