During ventricular diastole the atria contract, opening the atrioventricular valves (tricuspid on the right, mitral on the left) and expelling blood from the atria into the ventricles. Back pressure in the aorta and pulmonary arteries forces those valves closed. The back pressure in the aorta due to systemic vascular resistance and elastic recoil forces blood into the coronary circulation while the ventricular muscle is relaxed.
Contraction of the ventricles in systole forces the atrioventricular valves closed and opens the aortic and pulmonic valves. The edges of the atrioventricular valves are tethered to the papillary muscles by chordae tendineae, preventing prolapse back into the atria. The deoxygenated blood from the right ventricle is propelled to the lungs and the oxygenated blood from the more muscular left ventricle is sent into the systemic vessels via the aorta. While the ventricles contract, the atria receive blood from the body on the right and from the lungs on the left.
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