What are the two main phases of the cardiac cycle?

The correct answer identifies systole and diastole as the two main phases of the cardiac cycle. Systole refers to the phase of the heartbeat when the heart muscle contracts and pumps blood from the chambers into the arteries. This contraction is critical for maintaining blood circulation throughout the body, as it creates the pressure necessary to drive blood forward.

Diastole, on the other hand, is the phase when the heart muscle relaxes and the chambers fill with blood. During diastole, the heart undergoes a crucial process of receiving returning blood from the body and lungs, allowing for proper filling and preparation for the next contraction.

These two phases are integral to understanding cardiac function and physiology, illustrating how the heart alternates between contraction and relaxation to efficiently circulate blood and maintain homeostasis. The other options present components or specific aspects of the cardiac cycle but do not encompass the two primary phases that define the cycle itself.