Cardiac Cycle

The Cardiac Cycle

  • The cardiac cycle is a sequence of events that happens every time the heart beats. It includes diastole and systole phases.

Diastole Phase

  • Diastole is the period of relaxation of the heart muscle when both the atria and ventricles fill with blood.
  • This phase comprises atrial diastole, where the blood flows passively from the atria into the ventricles, and then ventricular diastole, where the ventricles continue to fill up with blood.

Systole Phase

  • Systole is the phase of contraction of the heart muscle that forces blood out of the heart.
  • It includes atrial systole, where the atria contract to complete filling of ventricles, and ventricular systole, where the ventricles contract to pump blood out of the heart and into the arteries.
  • The left ventricle contracts to propel oxygen-rich blood into the aorta, distributing it throughout the body, while the right ventricle propels deoxygenated blood into the pulmonary artery, towards the lungs for oxygenation.

Coordination of the Cardiac Cycle

  • The cardiac cycle is coordinated by a group of specialised cells known as the sinoatrial node (SAN) or the pacemaker, located in the wall of the right atrium.
  • The SAN generates electrical signals autonomously, which are conducted through the heart muscle fibres, causing them to contract in the correct sequence.
  • The atrioventricular node (AVN), located in the lower right atrium, delays the electrical signal to allow complete filling of ventricles before they contract.

Measuring the Cardiac Cycle

  • The complete cardiac cycle is typically illustrated in an ECG (electrocardiogram), from one heartbeat to the next, measured from the start of one QRS complex (indicating ventricular contraction) to the following one.
  • The cycle length, or duration of a heartbeat, can vary based on several factors, including physical activity and stress level. A healthy adult at rest typically has a cycle length of about 0.8 seconds.
  • Each phase of the cardiac cycle corresponds to a specific portion of the ECG waveform, allowing healthcare professionals to monitor the heart’s electrical and mechanical activity.