The Cardiac Cycle
Every heartbeat follows the same three-stage pattern, taking about 0.8 seconds in total. Atrial systole kicks things off (0.1 seconds) - atria contract, pushing blood through atrioventricular valves into the relaxed ventricles below.
Next comes ventricular systole (0.2 seconds) - the main event! Ventricles contract powerfully, forcing blood up through semi-lunar valves into the pulmonary artery and aorta. The atrioventricular valves slam shut to prevent backflow.
Finally, diastole (0.5 seconds) - everything relaxes. Ventricles expand, pressure drops, and semi-lunar valves close. Meanwhile, blood flows back into the atria from the vena cavae and pulmonary veins, ready for the next cycle.
The sinoatrial node (SAN) in your right atrium acts as the natural pacemaker, firing electrical signals that spread across both atria. The atrioventricular node (AVN) creates a crucial delay, ensuring atria finish emptying before ventricles start contracting.
Exam Tip: The delay at the AVN is vital - without it, atria and ventricles would contract simultaneously and blood wouldn't flow properly!