Which heart chamber receives oxygenated blood from the four pulmonary veins and serves as a reservoir during ventricular systole?

Correct Answer: B

Rationale: The correct answer is B: Left atrium. The left atrium receives oxygenated blood from the pulmonary veins, acting as a reservoir during ventricular systole. This blood is then pumped into the left ventricle for distribution to the body. The right atrium receives deoxygenated blood from the body via the vena cavae. The right ventricle pumps this blood to the lungs for oxygenation. The left ventricle is responsible for pumping oxygenated blood to the rest of the body. Therefore, the left atrium is the correct choice based on its role in receiving oxygenated blood from the pulmonary veins.

Correct Answer: A

Rationale: The impulse from the SA node is delayed at the AV node to allow time for the atria to contract before the ventricles. The AV node acts as a gatekeeper, slowing down the electrical signal before transmitting it to the Bundle of His, which then conducts the signal to the ventricles. This delay ensures proper coordination between atrial and ventricular contractions. Choices C and D are incorrect as the SA node does not delay its own impulse, and the aorta is not involved in the conduction system.

Correct Answer: A

Rationale: The correct answer is A: Refractoriness. Refractoriness refers to the period during which cardiac cells are unable to respond to a new stimulus while still contracting from a previous stimulus. This is essential for preventing premature re-stimulation and ensuring proper cardiac function. B: Excitability refers to the ability of cardiac cells to respond to a stimulus, which is the opposite of what is described in the question. C: Contractility is the ability of cardiac cells to contract in response to an electrical stimulus, not related to the inability to respond to a new stimulus during contraction. D: Automaticity is the ability of cardiac cells to spontaneously generate electrical impulses, unrelated to the inability to respond to a new stimulus during contraction.

Correct Answer: A

Rationale: The correct answer is A: Afterload. Afterload refers to the amount of tension the ventricle must develop during contraction to eject blood into the aorta. It is the resistance the heart must overcome to eject blood. Preload (B) is the stretch on the heart muscle before contraction. Automaticity (C) refers to the heart's ability to generate electrical impulses. Ejection fraction (D) is the percentage of blood pumped out of the ventricle with each heartbeat.

Correct Answer: A

Rationale: The correct answer is A: Sympathetic nervous system. The sympathetic nervous system releases norepinephrine, which acts as a central vasodilator by dilating blood vessels in the heart and skeletal muscles. It also acts as a peripheral vasoconstrictor by constricting blood vessels in other areas to redirect blood flow to vital organs during fight-or-flight responses. B: Parasympathetic nervous system is incorrect as it primarily promotes rest and digestion, not vasodilation or vasoconstriction. C: Norepinephrine is incorrect as it acts as a peripheral vasoconstrictor, not a central vasodilator. D: Acetylcholine is incorrect as it primarily functions in the parasympathetic nervous system and is not associated with central vasodilation or peripheral vasoconstriction.