Which structure serves as the normal pacemaker of the heart?

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: 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.

Correct Answer: A

Rationale: Automaticity is the ability of cardiac cells to generate an electrical impulse spontaneously without external stimulation. It is a fundamental property of specialized cardiac cells like pacemaker cells. Contractility refers to the ability of muscle cells to contract in response to an electrical stimulus. Conductivity is the ability of cardiac cells to transmit electrical impulses. Refractoriness is the period during which cardiac cells are unable to respond to a new stimulus. Therefore, in this context, the correct answer is A: Automaticity.

Correct Answer: A

Rationale: The correct answer is A: Purkinje fibers. Purkinje fibers are specialized cardiac muscle fibers that distribute the electrical impulse from the AV node to the ventricles, causing them to contract. They are located in the ventricles and ensure synchronized and efficient contraction. The SA node (B) is the pacemaker of the heart, initiating the electrical impulse. The AV node (C) delays the impulse to allow for atrial contraction before ventricular contraction. The Bundle of His (D) is a pathway for the electrical impulse from the AV node to the bundle branches, which then lead to the Purkinje fibers for ventricular contraction.