Phases of the Cardiac Cycle

Ten question-and-answer flashcards covering each labeled step (a–f) of the cardiac cycle, associated valve actions, pressure changes, key volumes, and heart sound S2.

What defines the Atrial Systole phase of the cardiac cycle?

This initial phase involves atrial contraction to complete ventricular filling before ventricular contraction begins.

What mechanical event marks the very start of the cardiac cycle (t = 0)?

The onset of atrial systole, when atrial contraction begins to “top-off” the relaxed ventricles with additional blood.

During atrial systole, what is the functional purpose of atrial contraction?

To add a small “top-off” volume of blood to the already-relaxed ventricles before they contract.

At roughly t ≈ 100 msec, which phase transition occurs in the heart?

Atrial systole ends and atrial diastole begins; the ventricles now contain the end-diastolic volume (EDV).

What key volume do the ventricles hold at the end of atrial systole?

End-diastolic volume (EDV).

What is the primary event during Ventricular Systole – Phase 1 (Isovolumetric Contraction)?

Ventricular pressure rapidly increases, leading to the closure of AV valves but without blood ejection yet, as semilunar valves remain closed.

What happens to the atrioventricular (AV) valves during ventricular systole phase 1?

Rising ventricular pressure pushes the AV valves shut, producing an isovolumetric contraction (no blood ejection yet).

Why is ventricular systole phase 1 called "isovolumetric" contraction?

Both AV and semilunar valves are closed, so volume in the ventricles remains constant while pressure rises.

What characterizes Ventricular Systole – Phase 2 (Ejection)?

Ventricular pressure exceeds arterial pressure, causing semilunar valves to open and blood to be ejected into the systemic and pulmonary circuits.

What pressure change triggers the opening of the semilunar valves?

Ventricular pressure exceeding arterial (aortic and pulmonary) pressure during ventricular systole phase 2, allowing blood ejection.