What happens when ventricular pressure drops below the pressure in the aorta and pulmonary trunk during diastole?
The semilunar valves close, preventing backflow into the heart
The electrical impulse initiates ventricular depolarization
The atrioventricular valves close, preventing backflow into the atria
The ventricles contract, forcing blood into the great arteries
The Correct Answer is A
A. The semilunar valves close, preventing backflow into the heart: During ventricular diastole, the ventricles relax and ventricular pressure falls below the pressure in the aorta and pulmonary trunk. This pressure gradient causes the aortic and pulmonary semilunar valves to close, preventing blood from flowing backward into the ventricles.
B. The electrical impulse initiates ventricular depolarization: Ventricular depolarization occurs during the onset of systole, not diastole. It is triggered by the action potential conducted through the bundle branches and Purkinje fibers, leading to contraction. Depolarization does not directly cause semilunar valve closure.
C. The atrioventricular valves close, preventing backflow into the atria: The atrioventricular (tricuspid and mitral) valves close at the beginning of ventricular systole when ventricular pressure exceeds atrial pressure. This prevents regurgitation into the atria and generates the first heart sound (S1), distinct from the semilunar valve closure occurring later in diastole.
D. The ventricles contract, forcing blood into the great arteries: Ventricular contraction occurs during systole when ventricular pressure rises above aortic and pulmonary pressures to open the semilunar valves. Closure of these valves happens during diastole when the ventricles relax, not during active ejection.
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Related Questions
Correct Answer is B
Explanation
Correct answer: False
The atrioventricular (AV) node, located in the lower portion of the interatrial septum near the tricuspid valve, functions as a critical electrical relay between the atria and ventricles. While the SA node sets the normal heart rhythm, the AV node limits the number of impulses transmitted to the ventricles, providing a protective delay that allows adequate ventricular filling. Under extreme SA node stimulation, impulses exceeding 220 per minute can overwhelm the AV node, resulting in ineffective ventricular contractions, reduced cardiac output, and compromised heart function. The AV node’s intrinsic conduction limits are essential for maintaining coordinated and efficient cardiac performance.
Correct Answer is {"dropdown-group-1":"C"}
Explanation
A. SA node: The sinoatrial node is the primary pacemaker of the heart. It is richly innervated by both sympathetic and parasympathetic fibers, allowing autonomic modulation of heart rate. Its sympathetic and parasympathetic balance is critical for initiating and regulating cardiac rhythm rather than forceful contraction.
B. Atria: The atrial myocardium receives autonomic innervation from both sympathetic and parasympathetic fibers. Parasympathetic input via the vagus nerve predominates in controlling heart rate and atrial conduction, whereas sympathetic input modulates contractility modestly. Atrial innervation is less dominated by sympathetic fibers compared with the ventricles.
C. Ventricles: Ventricular myocardium is more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation increases ventricular contractility (positive inotropy) and conduction velocity, which enhances stroke volume and cardiac output during stress or exercise. Parasympathetic influence on ventricles is minimal.
D. AV node: The atrioventricular node is innervated by both sympathetic and parasympathetic fibers. Parasympathetic input slows conduction through the AV node, prolonging the PR interval, while sympathetic stimulation increases conduction velocity. However, its innervation is not predominantly sympathetic compared with ventricular myocardium.
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