With extreme stimulation from the SA node, the AV node can transmit impulses at rates higher than 220 per minute without affecting heart function.

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.

A. P: The P wave represents atrial depolarization. It reflects the electrical activity as the impulse spreads from the sinoatrial (SA) node across the atrial myocardium, causing atrial contraction. Because the atria are smaller than the ventricles, the electrical signal is smaller in amplitude.

B. QRS: The QRS complex represents ventricular depolarization. It corresponds to the rapid spread of electrical impulses through the ventricles via the bundle branches and Purkinje fibers, leading to ventricular contraction. Since the ventricles have significantly greater muscle mass than the atria, this depolarization produces a larger and stronger electrical signal on the ECG.

C. T: The T wave represents ventricular repolarization, the process by which ventricular muscle cells return to their resting membrane potential after contraction. It does not represent depolarization.

D. PR: The PR interval reflects the time required for the electrical impulse to travel from the atria through the atrioventricular (AV) node to the ventricles. It represents conduction delay, not ventricular depolarization itself.