Which heart chamber receives venous blood returning to the heart via the superior and inferior vena cava?

Correct Answer: A

Rationale: The correct answer is A: Right atrium. The right atrium receives venous blood returning to the heart via the superior and inferior vena cava. Blood from the body enters the right atrium through the vena cavae, then flows into the right ventricle and eventually gets pumped to the lungs for oxygenation. Choices B, C, and D are incorrect because the left atrium receives oxygenated blood from the lungs, the right ventricle pumps blood to the lungs, and the left ventricle pumps oxygenated blood to the rest of the body.

Correct Answer: A

Rationale: The correct answer is A: SA node. The SA node is known as the natural pacemaker of the heart because it initiates the electrical impulses that regulate the heartbeat. It is located in the right atrium and generates the electrical signals that coordinate the heart's contractions. The other choices, B: AV node, C: Purkinje fibers, and D: Atria, do play roles in the heart's electrical conduction system, but they do not serve as the primary pacemaker. The AV node delays the electrical impulses, the Purkinje fibers distribute the impulses to the heart muscles, and the atria contract to pump blood into the ventricles. However, none of these structures serve as the primary pacemaker like the SA node does.

Correct Answer: A

Rationale: The correct answer is A: Excitability. Excitability refers to the ability of cardiac cells to respond to an impulse by generating an action potential and contracting. This is the fundamental property that allows the heart to beat. Contractility (B) is the ability of cardiac muscle to contract once stimulated, but it does not specifically refer to the response to an impulse. Rhythmicity (C) is the ability of cardiac cells to generate spontaneous electrical impulses, not directly related to responding to an impulse. Conductivity (D) is the ability of cardiac cells to transmit electrical impulses, not specifically related to the response of cells contracting.

Correct Answer: A

Rationale: Contractility is the correct answer as it specifically refers to the ability of the heart muscle to contract with a given force without changing its length. This term is used to describe changes in the inotropic state of the muscle, which affects the strength of contraction. Excitability, refractoriness, and automaticity do not directly relate to changes in muscle contraction force without altering fiber length. Excitability is the ability of the muscle to respond to stimuli, refractoriness is the recovery period after a muscle contraction, and automaticity is the ability of certain cells to generate spontaneous electrical activity.

Correct Answer: A

Rationale: The correct answer is A: Carotid body. The carotid body is the major chemoreceptor of the heart, detecting changes in oxygen levels in the blood. It sends signals via the glossopharyngeal nerve to regulate heart rate and blood pressure. The aortic body primarily monitors blood pressure, not chemoreception. The vagus nerve plays a role in regulating heart rate but is not a chemoreceptor. The SA node is the pacemaker of the heart, responsible for initiating the electrical signals for heart contraction, not chemoreception. Therefore, the carotid body is the correct choice for the major chemoreceptor of the heart.