Cardiac Cycle Illustrations and Diagrams
Periods and Phases
Background Information
The cardiac cycle is the sequence of mechanical and electrical events during a single heartbeat. When the heart beats 75 times per minute, one cardiac cycle lasts 0.8 seconds. The duration shortens as the heart rate increases and lengthens as the heart rate decreases. Because it is recurring, the end of one cycle immediately precedes and prepares the heart for the start of the next cycle.
The cardiac cycle consists of two primary periods: ventricular systole and ventricular diastole. Each of these periods contains specific phases that correspond to distinct mechanical events occurring in the heart’s chambers.
Periods of the Cardiac Cycle
The cardiac cycle consists of two main periods (parts or divisions): ventricular systole and ventricular diastole. Pacemaker cells in the heart walls regulate the timing and duration of these periods.
◉ Ventricular systole makes up about 1/3 of the cardiac cycle. During this time, the chamber walls contract and eject blood from the heart into large arteries that unite with the pulmonary and systemic circulatory systems.
◉ Ventricular diastole makes up the remaining 2/3 of the cardiac cycle. During this time, the walls of the chambers relax, and blood enters the heart through large veins from the pulmonary and systemic circulatory systems.
Phases of the Cardiac Cycle
Physiologists typically subdivide ventricular systole and diastole periods into several phases (or stages) to better study the events in the heart chambers and major blood vessels. The approach and number of phases vary, but generally resemble the scheme below.
Ventricular Systole:
◉ Ventricular Isovolumetric Contraction
The blood-filled ventricles start contracting during this phase, increasing the pressure in the chambers. When the ventricle pressures rise above the atria pressures, it closes the atrioventricular valves. Because all the heart valves are closed, blood cannot exit the ventricles, so the blood volume in these chambers remains unchanged or “isovolumetric.”
◉ Ventricular Ejection
The ventricular walls continue to contract, causing the blood pressure in these chambers to increase. When the ventricular blood pressures rise above those in the large heart arteries, it forces the semilunar valves to open, allowing blood to flow into the aorta and pulmonary trunk arteries. Initially, the blood flows rapidly out of the ventricles but slows as the phase continues.
Ventricular Diastole:
◉ Ventricular Isovolumetric Relaxation
The ventricles begin to relax, decreasing the blood pressure in the chambers. When the ventricular pressures fall below the pressures in the large arteries, it closes the semilunar valves. Because all the heart valves are closed, blood cannot enter the ventricles, so the blood volume in these chambers remains unchanged or “isovolumetric.”
◉ Passive Ventricular Filling
The pressure on the blood in the ventricular chambers decreases as the ventricular walls continue to relax. When the ventricular pressures fall below the atrial pressures, it forces the atrioventricular valves to open. Initially, blood in the atria rapidly enters the ventricles. Filling slows as blood from the heart’s large veins flows into the ventricles after passing through the atria.
◉ Atrial Systole
The previous phase nearly fills the ventricles with blood. To complete the filling process, the atria contract to actively inject additional blood into the ventricles just before they contract.
Illustrations
