Answer: A — Rapid Ejection

Reason

The cardiac cycle has 4 phases:

Phase	Valvular Event	What Happens
Phase 1 (Diastole)	AV valves open	Ventricular filling (rapid → slow → atrial kick)
Phase 2 (Systole)	AV valves close	Isovolumetric contraction — all valves shut, pressure builds
Phase 3 (Systole)	Semilunar valves open	Rapid ejection → then slower ejection
Phase 4 (Diastole)	Semilunar valves close	Isovolumetric relaxation — all valves shut again

The semilunar valves (aortic + pulmonary) open at the start of Phase 3 — Rapid Ejection. This occurs once ventricular pressure during isovolumetric contraction (Phase 2) rises high enough to exceed aortic/pulmonary arterial pressure, forcing the semilunar valves open and allowing blood to eject rapidly.

Why the distractors are wrong:

B (Slow ejection): The semilunar valves are already open during slow ejection — they opened at the start of Phase 3.
C (Isovolumetric contraction): All four valves are closed during this phase — pressure is rising but hasn't yet exceeded outflow vessel pressure.
D (Isovolumetric relaxation): Semilunar valves have just closed to begin this phase; AV valves haven't opened yet either.

The correct answer is A — Rapid Ejection.

— Medical Physiology (Boron & Boulpaep), Table 22-1, p. 745

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Wiggers diagram cardiac cycle pressure volume phases

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Here is a comprehensive explanation of the Cardiac Cycle — the topic of this MCQ.

The Cardiac Cycle

The cardiac cycle is the complete sequence of events from one heartbeat to the next. At 75 beats/min, one cycle lasts ~800 ms — systole (~300 ms) and diastole (~500 ms). It is governed by 4 phases, defined by the opening and closing of two sets of valves.

The Two Valve Sets

Valve Type	Valves	Location
AV (inlet) valves	Mitral (left), Tricuspid (right)	Between atria and ventricles
Semilunar (outlet) valves	Aortic (left), Pulmonary (right)	Between ventricles and great arteries

Key principle: Valves open and close passively based on pressure gradients — they open when upstream pressure > downstream pressure, and close when the reverse occurs.

The 4 Phases of the Cardiac Cycle

Phase 1 — Ventricular Filling (Diastole)

AV valves: OPEN | Semilunar valves: CLOSED
Ventricular pressure falls below atrial pressure → AV valves open
Blood flows passively from atria into ventricles
Sub-phases:
- Rapid filling (early): large pressure gradient, fast flow
- Diastasis (mid): slow trickle, pressures nearly equalized
- Atrial contraction (end): P wave on ECG; atrial "kick" adds ~20% of stroke volume at rest (up to 40% during exercise)
Ends when the ventricle begins to contract and ventricular pressure exceeds atrial pressure → AV valves close → produces S1 heart sound

Phase 2 — Isovolumetric Contraction (Systole)

ALL valves: CLOSED
Triggered by QRS complex on ECG
Ventricular muscle contracts → pressure rises rapidly
Volume does not change (no blood in or out) — hence "isovolumetric"
Ends when ventricular pressure exceeds aortic/pulmonary pressure → semilunar valves open
This is the phase of maximum energy expenditure with no ejection

Phase 3 — Ventricular Ejection (Systole) ← Semilunar valves are OPEN here

AV valves: CLOSED | Semilunar valves: OPEN
Sub-phases:
- Rapid ejection (early): pressure peaks, ~70% of stroke volume ejected quickly — semilunar valves open at the START of this sub-phase
- Slow (reduced) ejection (late): pressure begins to fall, flow slows
Stroke volume = ~70 mL (out of end-diastolic volume of ~130 mL)
Ejection fraction (EF) = SV/EDV = ~55–65%
Ends when ventricular pressure falls below aortic pressure → semilunar valves close → produces S2 heart sound (dub)

Phase 4 — Isovolumetric Relaxation (Diastole)

ALL valves: CLOSED
Ventricular muscle relaxes → pressure drops rapidly
Volume unchanged (no blood in or out)
Ends when ventricular pressure falls below atrial pressure → AV valves open → cycle repeats

Summary Table

Phase	AV Valves	Semilunar Valves	What Happens
1. Ventricular Filling	Open	Closed	Blood fills ventricles
2. Isovolumetric Contraction	Closed	Closed	Pressure builds, no flow
3. Ventricular Ejection	Closed	Open	Blood ejected to arteries
4. Isovolumetric Relaxation	Closed	Closed	Pressure drops, no flow

Pressure-Volume Loop (Wiggers Diagram concept)

Pressure-volume loop of the left ventricle showing all 4 phases, valve events, stroke volume, and isovolumetric periods

Reading the loop clockwise:

Bottom right → top right: Isovolumetric contraction (pressure rises, volume fixed)
Top right → top left: Ventricular ejection (volume falls as blood is pumped out)
Top left → bottom left: Isovolumetric relaxation (pressure drops, volume fixed)
Bottom left → bottom right: Diastolic filling (volume rises, low pressure)

Cardiac Cycle Phases in Context

Diagram of the heart during systole contraction phase (C), ejection phase (D), diastole relaxation phase (E), and filling phase (F)

C = Systole, contraction phase (isovolumetric contraction)
D = Systole, ejection phase (semilunar valves open)
E = Diastole, relaxation phase (isovolumetric relaxation)
F = Diastole, filling phase (AV valves open)

Heart Sounds

Sound	Cause	Timing
S1 ("lub")	AV valve closure	Start of systole (end of Phase 1)
S2 ("dub")	Semilunar valve closure	Start of diastole (end of Phase 3)
S3 (pathological)	Rapid ventricular filling sound	Early diastole
S4 (pathological)	Stiff ventricle during atrial kick	Late diastole

Key Exam Points to Remember

Isovolumetric contraction & relaxation = ALL valves closed; volume unchanged
Semilunar valves open at the start of rapid ejection (Phase 3) — the MCQ answer
AV valves open at the start of ventricular filling (Phase 1)
S1 = AV closure; S2 = semilunar closure
Ejection fraction ~55–65% is normal; <40% indicates systolic dysfunction

— Medical Physiology (Boron & Boulpaep), Table 22-1 & pp. 745–746 | Color Atlas of Human Anatomy Vol. 2, pp. 86–87

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