The expression 'cardiac cycle' refers to:

Correct Answer: D

Rationale: The cardiac cycle is the sequence of mechanical and electrical events between one heartbeat and the next , including systole and diastole (~0.8s at 75 bpm). A is too broad, B is cardiac output, C is ECG-specific, and D is heart sound-specific, not the full cycle.

Correct Answer: B

Rationale: The SA node (B) is the natural pacemaker, initiating impulses at 60-100 bpm. The AV node (C) delays, AV bundle (A) conducts, and Purkinje fibers (D) spread the signal.

Correct Answer: C

Rationale: Stroke volume (C) is the volume pumped per contraction (~70 mL). A is end-diastolic volume, B is end-systolic volume, D is cardiac output.

Correct Answer: D

Rationale: Capillaries are microscopic (A), exchange sites (B), and fed by arterioles (C), but blood flow is slow (D is false, ~0.3 mm/s) to allow exchange.

Correct Answer: C

Rationale: Venous return relies on muscle pump (A), respiratory pump (B), and blood volume (D). Speed of flow (C) is an outcome, not a driver of return.