- Cardiac Cycle.

Cardiac Cycle.

The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. (1)

A single cycle of cardiac activity can be divided into two basic phases - diastole and systole.

Each beat of the heart involves five major stages.

1. The first two stages, often considered together as the "ventricular filling" stage, involve the movement of blood from atria into ventricles. 
2. The next three stages involve the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle). (1)

Five stages of cardiac cycle:

1. Late diastole (filling phase): Heart relax, semilunar valves close, AV valves open. 
2. Atrial systole: Atrial contracts, AV valves still open, blood flow from atrial to ventricle,Atrial contraction normally accounts for about 10% of left ventricular filling when a person is at rest because most of ventricular filling occurs prior to atrial contraction as blood passively flows from the pulmonary veins, into the left atrium, then into the left ventricle through the open mitral valve.
3. Isovolumic ventricular contraction: ventricle contract, AV valves close so there is no vlume change (isovolumic).
4. Ventricular ejection (ejection phase): semilunar valves open so blood ejected from ventricle.
5. Isovolumic ventricular relaxation: ventricles stop contract and start to relax, ventricular pressure goes down and aorta pressure push semilunar valves closed.

This video goes through the concepts involved in the cycle, explaining the phonocardiogram, electrocardiogram (ACG/EKG), ventricular volume and pressure, atrial pressure and aortic pressure.

References:

1. Guyton, A.C. & Hall, J.E. (2006) Textbook of Medical Physiology (11th ed.) Philadelphia: Elsevier Saunder ISBN 0-7216-0240-1