The cardiac cycle coordinates diastole, atrial systole, ventricular systole, valve movements and electrical conduction to produce one-way blood flow continuously.
Coverage 2013–2024 · Updated 15 Jul 2026
The cardiac cycle is a pressure-driven sequence initiated by the sinoatrial node: atrial systole, ventricular systole and diastolic refilling.
The SAN excitation spreads across atria, causing atrial systole. After a conduction delay, ventricles contract; left ventricular pressure closes the AV valve and then opens the aortic semilunar valve when it exceeds aortic pressure.
As ventricles relax, pressure falls, semilunar valves close and the AV valves reopen when atrial pressure exceeds ventricular pressure. During diastole the chambers refill before the next SAN impulse.
Systolic arterial pressure is the peak reached during ventricular ejection; diastolic pressure is the lower arterial pressure during ventricular relaxation. A reading is reported systolic over diastolic in mmHg.
Systole and diastole must be assigned to a chamber: atrial and ventricular systole are offset. Valve movements follow pressure gradients rather than causing the pressure changes.
This objective is assessed through essay response, commonly using Identify / State / Label.
Identify / State / Label / Outline / Describe / Explain
Build the answer around this relationship: The SA node initiates the heartbeat and atrial contraction.
Describing blood flow without linking it to chamber contraction or pressure changes.
Representative question
Explain the events of the cardiac cycle, including the heart sounds.
HL transport adds pressure and route systems. Tissue fluid forms by capillary pressure and returns by osmotic pull or lymph. Double circulation separates pulmonary and systemic routes. The heart creates directional pressure with chambers, septum, valves, and cycle timing. Plants add root pressure and phloem pressure-flow translocation.
