Describe the mechanical events of the cardiac cycle. (use terms like atrial systole, atrial diastole, ventricular diastole, ventricular systole, isovolumic ventricular contraction, isovolumic ventricular relaxation, ventricular ejection, period of rapid filling)
The correct answer and explanation is :
The cardiac cycle refers to the sequence of mechanical events that occur during one heartbeat, ensuring the effective pumping of blood through the heart and to the body. It involves alternating periods of contraction (systole) and relaxation (diastole) in the heart chambers.
- Atrial Systole: The cycle begins with the atria contracting (atrial systole), pushing blood into the ventricles. This phase contributes to the final 20% of the ventricles’ blood volume. The atrioventricular (AV) valves (mitral and tricuspid) are open, allowing blood to flow into the ventricles.
- Atrial Diastole: After atrial contraction, the atria relax (atrial diastole), and the chambers begin filling passively with blood from the veins, mainly during ventricular systole. The AV valves remain open as the atria continue to receive blood.
- Ventricular Diastole: During ventricular diastole, the ventricles relax and fill with blood. Blood flows from the atria through the open AV valves into the ventricles. This phase is divided into two sub-phases:
- Period of Rapid Filling: Occurs when the ventricles are initially relaxed and blood rapidly fills the ventricles from the atria.
- Diastasis: The slower phase of ventricular filling.
- Isovolumic Ventricular Contraction: The ventricles begin to contract (ventricular systole), but the pressure is not yet high enough to open the semilunar valves. Both the AV and semilunar valves are closed during this phase, so the volume of blood in the ventricles remains constant, and the pressure rises.
- Ventricular Ejection: Once the pressure in the ventricles exceeds that in the aorta and pulmonary trunk, the semilunar valves open, and blood is ejected into the aorta and pulmonary artery. This is the phase of ventricular ejection.
- Isovolumic Ventricular Relaxation: After ventricular ejection, the ventricles relax, causing a decrease in pressure. The semilunar valves close, and the AV valves are still shut. During this phase, the volume of blood in the ventricles remains constant as the pressure drops.
The cycle repeats with each heartbeat, ensuring continuous blood circulation.