Part A The beginning of the QRS complex of the electrocardiogram (ECG) immediately precedes which of the following event View Available Hint(s) closing of the atrioventricular valves blood ejection from the ventricles atrial contraction closing of the semilunar valves

The **beginning of the QRS complex** on an electrocardiogram (ECG) represents **ventricular depolarization**, which triggers the **ventricular contraction** phase of the cardiac cycle. This contraction leads to the **ejection of blood from the ventricles** into the aorta and pulmonary artery. **Correct Answer:** - **Blood ejection from the ventricles** **Explanation:** - **Closing of the atrioventricular valves:** This event occurs shortly after ventricular contraction begins, preventing backflow into the atria, and is associated with the first heart sound (S1). However, it is not **immediately** preceded by the QRS complex. - **Blood ejection from the ventricles:** This is directly initiated by the ventricular contraction that follows ventricular depolarization (QRS complex), making it the event **immediately** preceded by the QRS complex. - **Atrial contraction:** This occurs before the QRS complex during the P wave and is not related to the events immediately following the QRS complex. - **Closing of the semilunar valves:** This happens after the ventricles begin to relax (repolarization), which is later in the cardiac cycle and not immediately after the QRS complex.