Which structure in the heart initiates action potentials that stimulate contraction of the heart at constant rate of about 100 beats per minute?

Correct Answer: D

Rationale: The SA node (D) initiates action potentials at ~100 bpm, though vagal tone slows it to 60-80 bpm in vivo. AV node (B) is slower (~40-60 bpm), nerves (A) and center (C) modulate.

Correct Answer: C

Rationale: The parietal pericardium (C) is the tough outer layer of the pericardial sac. Epicardium (A) and visceral pericardium (B) are on the heart, myocardium (D) is muscle.

Correct Answer: B

Rationale: Cardiac output (B) is the total blood volume ejected from a ventricle per minute (stroke volume × heart rate). Stroke volume (A) is per beat, heart rate (C) is beats, afterload (D) is resistance.

Correct Answer: D

Rationale: RBCs primarily transport oxygen and CO2 (D) via hemoglobin. Nutrients (A), cytokines (B), and proliferation (C) are not their roles.

Correct Answer: B

Rationale: Albumin (B) is ~55% of plasma proteins (assuming I is plasma). Globulins (A) ~35%, fibrinogen (C) ~7%, thrombin (D) is an enzyme.