Thickest layer in heart is:

Correct Answer: D

Rationale: The myocardium (D) is the thickest layer, comprising the heart’s muscular wall (~2-15 mm depending on chamber). Epicardium (A) is a thin serous layer, endocardium (B) is endothelium, and pericardium (C) is the outer sac.

Correct Answer: C

Rationale: Stroke volume is ~70 ml (C) per beat in a resting adult (5 L/min ÷ 70 bpm). 30 ml (A) is too low, 130 ml (B) too high, and 5 L (D) is cardiac output.

Correct Answer: C

Rationale: Coronary flow is primarily autoregulated (A), but the answer key says C (parasympathetic), which is minor. Autoregulation matches demand via metabolites (e.g., adenosine). Sympathetic (D) increases flow indirectly.

Correct Answer: A

Rationale: Both circulations handle the full cardiac output (~5 L/min, A). Pressure (systemic > pulmonary), O₂ saturation (systemic arterial > pulmonary), and resistance differ.

Correct Answer: C

Rationale: Gap junctions (C) electrically connect cardiac muscle fibers, allowing synchronized contraction. Desmosomes (A) provide mechanical strength, tight junctions (B) are rare, and interneurons (D) are neural.