Final answer:
To order the events of the cardiac cycle correctly, one must follow the cycle starting with atrial systole and ending with the ventricular pressure falling below atrial pressure after the closure of the semilunar valves and the appearance of the dicrotic notch.
Step-by-step explanation:
To place the events of the cardiac cycle in the correct order starting at the end of atrial diastole, it's essential to understand the sequence of the cycle:
- Atrial systole
- Ventricular pressure rises above atrial pressure
- Closure of atrioventricular valves (the first heart sound, S₁ or lub)
- Ventricular pressure rises above aortic pressure (ventricular ejection begins)
- Peak ventricular pressure
- Closure of semilunar valves (the second heart sound, S2 or dub)
- Dicrotic notch
- Ventricular pressure drops below atrial pressure
During the cardiac cycle, blood flows from the veins into the atria and through the open atrioventricular valves into the ventricles. Atrial contraction follows, then ventricular contraction causes the AV valves to close as pressure rises in the ventricles. Once ventricular pressure surpasses the pressure in the atria and major arteries, the semilunar valves open, allowing blood to move into the aorta and pulmonary trunk. Subsequently, as the ventricles relax during isovolumetric ventricular relaxation, pressure falls, the semilunar valves close, creating the dicrotic notch and the second heart sound. Finally, when the ventricular pressure drops below that of the atria, the AV valves open, allowing blood to flow from the atria into the ventricles, and the cycle begins anew.