The opening between the two atria in the fetus is the

Correct Answer: C

Rationale: The foramen ovale (C) is the fetal atrial opening, shunting blood from right to left atrium to bypass lungs. Ductus venosus (A) shunts liver, ductus arteriosus (B) lungs, circle of Willis (D) is cerebral.

Correct Answer: C

Rationale: Blood from the right ventricle exits via the pulmonary valve (C) to the pulmonary artery. Tricuspid (A) is inflow, bicuspid (B) is left-sided, and aortic arch (D) is systemic.

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: B

Rationale: Arterioles (B) regulate flow via resistance (smooth muscle). Arteries (A) conduct, capillaries (C) exchange, venules (D) collect.

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.