How Does the Heart Pump Blood? The Cardiac Cycle
Learn how the heart pumps blood during the cardiac cycle, from diastole and atrial systole to ventricular systole, pressure changes and valve movements.
The heart pumps blood by repeatedly filling its chambers and then contracting to push blood out. This sequence is the cardiac cycle. Changes in pressure open and close valves, keeping blood moving in one direction.
Filling: diastole
During diastole, the heart muscle relaxes. Blood returns to the right atrium from the body through the venae cavae and to the left atrium from the lungs through the pulmonary veins. As pressure in the atria becomes higher than pressure in the ventricles, the atrioventricular (AV) valves open and blood flows into the ventricles. Much of the filling happens before the atria contract.
Atrial systole
The atria contract, pushing the remaining blood into the ventricles. Systole means contraction; diastole means relaxation. Atrial systole happens before ventricular systole, so the ventricles fill before they pump. Both atria contract together, and both ventricles contract together; the two sides of the heart pump blood to different destinations.
Ventricular systole
The ventricles contract and their pressure rises. This closes the AV valves, preventing blood from flowing back into the atria. When ventricular pressure exceeds the pressure in the arteries leaving the heart, the semilunar valves open. The right ventricle pumps blood to the lungs through the pulmonary artery; the left ventricle pumps blood to the body through the aorta. The left ventricle has a thicker muscular wall than the right, producing the higher pressure needed to send blood around the body; the right ventricle sends blood to the lungs.
The ventricles then relax. Their pressure falls, so the semilunar valves close to prevent blood flowing back from the arteries. Once atrial pressure is higher than ventricular pressure, the AV valves open and filling begins again.
Exam mistake to avoid: valves do not actively pump blood or open because the heart tells them to. They open and close because pressure differs on either side.
