The Heart's Double Pump Action: How Atria & Ventricles Circulate Blood

The Cardiac Cycle: A Coordinated Double Pump

The heart functions as two side-by-side pumps: the right side moves deoxygenated blood to the lungs; the left side sends oxygenated blood to the body. Each beat involves synchronized atrial and ventricular contraction (systole) and relaxation (diastole).

1. Atrial Systole

SA node fires → atria contract → blood pushed through open AV (tricuspid/mitral) valves into ventricles. Completes ventricular filling (~20–30%).

2. Ventricular Systole

• Isovolumetric contraction: Ventricles contract, AV valves close (S1 heart sound), pressure rises.
• Ejection: Pressure exceeds aortic/pulmonary pressure → semilunar valves open → blood ejected (left → aorta; right → pulmonary artery).

3. Early Diastole (Ventricular Relaxation)

• Ventricles relax → pressure falls below arterial pressure → semilunar valves close (S2 heart sound).
• AV valves remain closed; ventricles fill passively from atria.

4. Atrial Diastole & Filling

Atria relax → receive venous return (SVC/IVC → right atrium; pulmonary veins → left atrium) → AV valves open → passive ventricular filling begins next cycle.

Continuous Cycle

This sequence repeats ~60–100 times/minute, ensuring uninterrupted circulation: systemic (left heart) and pulmonary (right heart) circuits in series.

The "double pump" design separates oxygen-poor and oxygen-rich blood streams, enabling efficient gas exchange and tissue perfusion.