Absolute refractory period in heart:

Correct Answer: D

Rationale: the absolute refractory period (~0.25-0.3 s) lasts through systole (A), is longer than skeletal muscle (B, ~0.002 s), prevents restimulation (C), and matches the action potential (D).

Correct Answer: A

Rationale: Heart valves are made of dense connective tissue (A), providing strength and flexibility. Areolar tissue (B) is loose, hyaline cartilage (C) is in joints, and cardiac muscle (D) forms the heart wall.

Correct Answer: C

Rationale: High plasma potassium (C) (hyperkalemia) slows heart rate by depolarizing cells, reducing excitability. Norepinephrine (A), thyroid hormone (B), and calcium (D) increase rate.

Correct Answer: D

Rationale: Cardiac output = stroke volume × heart rate (D), e.g., 70 ml × 75 bpm = 5.25 L/min.

Correct Answer: B

Rationale: The atria (B) are the upper, receiving chambers. Ventricles (A) pump, auricles (C) extend atria, trabeculae (D) are in ventricles.