The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, encompassing the contraction (systole) and relaxation (diastole) of the heart chambers. It is a vital process that allows the heart to pump blood efficiently throughout the body, providing oxygen and nutrients to tissues while removing waste products.
The cardiac cycle begins with the initiation of an electrical impulse in the sinoatrial (SA) node, often referred to as the heart’s natural pacemaker. This impulse spreads across the atria, causing them to contract and pump blood into the ventricles. This phase, known as atrial systole, accounts for the first part of the cardiac cycle.
Following atrial contraction, the electrical signal travels to the atrioventricular (AV) node, where it is briefly delayed before being transmitted to the ventricles. This delay allows the ventricles to fill completely with blood from the atria before contracting. Once the signal reaches the ventricles, they contract forcefully during ventricular systole, pumping blood into the pulmonary artery and aorta.
After ventricular contraction, the heart enters the diastolic phase, characterized by relaxation and filling of the heart chambers. The atria and ventricles briefly relax, allowing blood to flow passively from the veins into the atria and then into the ventricles. This period of ventricular diastole allows the heart chambers to refill with blood in preparation for the next cardiac cycle.
The cardiac cycle is regulated by a complex interplay of electrical signals, mechanical forces, and neural and hormonal influences. Variations in heart rate, stroke volume, and cardiac output can occur in response to physiological demands such as exercise, stress, and changes in posture. Understanding the cardiac cycle is essential for diagnosing and treating cardiovascular disorders and optimizing heart function for overall health and well-being.