8.3.3—Cardiac cycle and valves

One cardiac cycle is one heartbeat: atrial systole, ventricular systole and diastole. Muscle contraction decreases chamber volume and raises pressure; relaxation increases volume and lowers pressure. Valves respond to these pressure differences to keep flow one-way.

1. Atrial systole: atrial muscle contracts → atrial volume falls and atrial pressure rises above ventricular pressure → atrioventricular (AV) valves open → blood is pushed into the relaxed ventricles. Semilunar valves remain closed.
2. Ventricular systole: ventricular muscle contracts → ventricular volume falls and ventricular pressure rises above atrial pressure → AV valves close, preventing backflow into the atria. When ventricular pressure exceeds pressure in the aorta and pulmonary artery, semilunar valves open and blood is ejected.
3. Diastole: atria and ventricles relax → ventricular pressure falls below arterial pressure → semilunar valves close, preventing blood returning from the arteries. The atria fill from the vena cava and pulmonary veins; when atrial pressure rises above ventricular pressure, AV valves open and blood flows passively into the ventricles.
4. Repeat: filling restores chamber volume, the pressure sequence begins again with atrial systole, and blood continues to move without a pause between cycles.

Valve rule: a valve opens when pressure behind it exceeds pressure in front; it closes when the pressure relationship reverses. The closure prevents backflow rather than actively pushing blood.

Do not assign valve states from “systole” alone without checking which chamber pressure is changing. This card covers mechanical pressure and valves; SAN, AVN, bundle of His and Purkyne conduction belong to 4614. No image generated or bound.