Coronary blood flow is usually predominantly controlled by:

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: 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.

Correct Answer: D

Rationale: Cardiac reserve (D) is the difference between maximum and resting cardiac output, reflecting heart capacity.

Correct Answer: B

Rationale: The mitral valve (B) opens into the left ventricle from the left atrium during diastole. Aortic (A) is outflow, tricuspid (C) and pulmonary (D) are right-sided.

Correct Answer: B

Rationale: Vasodilation (B) relaxes vascular smooth muscle, reducing resistance and increasing flow. Vasoconstriction (A) increases resistance, others (C, D) are unrelated.