The Cardiac Cycle

Each individual pump of the heart represents one heart beat. Cardiac Cycle

The cardiac cycle represents the mechanical events of one heartbeat. 0.8 seconds 72 times a minute

The cardiac cycle consists of two phases that represent the contraction and relaxation of the heart muscle. Diastole 0.5sec relaxation Systole 0.3sec contraction

Cardiac Diastole - the heart relaxes for 0.5 seconds, during which the sequence of events in images A & B occurs. A. The atria fill with blood while the atrioventricular (bicuspid & tricuspid) valves are closed. B. The atrioventricular valves are pushed open by rising atrial pressure and the ventricles start to fill with blood. The semi-lunar valves are closed throughout.

Atrial Systole - First the SA Node initiates an electrical impulse that travels across the atrial walls. This is followed by a wave-like contraction across the myocardium of each atrium. The effect of the atrial systole is to force all remaining blood past the atrioventricular valves (bicuspid and tricuspid) into the lower chambers. The semi-lunar valves (pulmonary and aortic valves are closed during this activity). As shown in image C. C. The Atria contract, forcing remaining blood down into the ventricles.

Ventricular Systole - The electrical impulse passes from the AV Node to the Purkinje Fibres. Once ventricular myocardium is stimulated, a second wave-like contraction spreads across the ventricular walls, whist the atrioventricular valves remain closed. Pressure inside the ventricles continues to rise. The ventricles continue to contract until the continued increase in pressure pushes open the semi-lunar valves. The result is that blood flows into the pulmonary (lungs) and systemic (around the body) circulatory systems. As shown in image D. D. The ventricles contract and atrioventricular valves close. Ventricular contraction forces open the semi-lunar valves, so that blood is ejected into pulmonary artery and aorta.

Cardiac Impulse Myogenic sino-atrial node (SA) AV node Bundle of His Purkinje fibres

Structures involved in the conduction of the cardiac impulse