The cardiac cycle

The cardiac cycle refers to the pattern of contraction and relaxation of the heart during one complete heartbeat.

Contraction of the heart muscle is known as systole and relaxation of the heart muscle is called diastole.

The cardiac cycle has 3 stages:

  1. Atrial and Ventricular diastole (chambers are relaxed and filling with blood)
  2. Atrial systole (atria contract and remaining blood is pushed into ventricles)
  3. Ventricular systole (ventricles contract and push blood out through aorta and pulmonary artery)

Cross section of the heart showing how deoxygenated blood from the body enters the right atrium. The atria relaxed, oxygenated blood from lungs entres the left atrium

Functions of the heart

The 'lubb dubb' heart sounds heard through a stethoscope are the sounds of the atrioventricular (AV) valves and semilunar (SL) valves opening and closing.