1.9 Cardiac Cycle Foundation
The cardiac cycle foundation explains how the heart pumps blood through coordinated contractions and relaxations of its chambers.
Cardiac Cycle Foundation is the body of fundamental principles describing the repeating sequence of mechanical events that occur within the heart during a single heartbeat, encompassing the coordinated phases of atrial and ventricular contraction and relaxation that together produce the filling and ejection of blood. It establishes the systematic division of the cycle into distinct phases, the pressure and volume relationships that characterize each phase, and the valvular events that separate one phase from the next.
Phases of the Cardiac Cycle
Atrial Systole
Atrial systole refers to the phase in which the atria contract, completing the final portion of ventricular filling by actively propelling the remaining blood volume from the atria into the ventricles before ventricular contraction begins.
Ventricular Systole
Ventricular systole encompasses the period of ventricular contraction, divided into an initial phase of isovolumetric contraction, during which pressure rises without a change in volume, followed by a phase of ejection as blood is expelled into the great arteries.
Ventricular Diastole
Ventricular diastole represents the period of ventricular relaxation and filling, beginning with isovolumetric relaxation and proceeding through rapid and then slower phases of passive ventricular filling prior to the onset of the next atrial systole.
Pressure and Volume Relationships
Pressure Changes Across the Cycle
Throughout the cardiac cycle, pressure within the atria, ventricles, and great vessels rises and falls in a characteristic pattern, with ventricular pressure exceeding atrial and arterial pressure during ejection and falling below both during filling.
Volume Changes Across the Cycle
Ventricular volume increases progressively during diastolic filling and decreases sharply during systolic ejection, with the difference between maximum and minimum volume corresponding to the stroke volume ejected with each beat.
Isovolumetric Phases
Distinct isovolumetric phases occur both at the onset of systole and at the onset of diastole, during which ventricular pressure changes substantially while volume remains constant due to the simultaneous closure of both inflow and outflow valves.
Valvular Events and Their Timing
Atrioventricular Valve Function
The mitral and tricuspid valves open during ventricular diastole to permit filling and close at the onset of ventricular systole, preventing the backflow of blood into the atria as ventricular pressure rises.
Semilunar Valve Function
The aortic and pulmonary valves remain closed during ventricular diastole and isovolumetric contraction, opening only once ventricular pressure exceeds arterial pressure, and closing again as ventricular pressure falls below arterial pressure at the end of ejection.
Coordination of Valvular Timing with Cycle Phases
The precise timing of valve opening and closure delineates the boundaries between the distinct phases of the cardiac cycle, ensuring that blood moves in one direction through the heart and preventing regurgitant flow between chambers.