Final answer:
The phase in which the cardiac muscle relaxes and the heart chambers fill with blood is known as diastole. Diastole is part of the cardiac cycle, consisting of diastole and systole. Precise regulation of these phases ensures the heart efficiently pumps blood.
Step-by-step explanation:
The phase during which the cardiac muscle is relaxed and the chambers of the heart fill with blood is called diastole. In contrast to diastole, systole is the phase where the heart contracts and pumps blood into circulation. The entire process, including both systole and diastole, is termed the cardiac cycle. It is crucial for these phases to be precisely regulated and coordinated to ensure efficient blood circulation throughout the body.
In ventricular diastole, the ventricular muscle relaxes, leading to a pressure drop in the ventricles, which in turn allows blood to flow back into the heart, but the semilunar valves close to prevent backflow. Initially, during this phase, known as the isovolumic ventricular relaxation phase, there is no change in blood volume in the ventricles, as the atrioventricular valves also remain closed. However, as diastole progresses, the pressure in the ventricles falls below that of the atria, causing the atrioventricular valves to open and blood to flow freely into the ventricles. This second phase, called the late ventricular diastole, completes the cardiac cycle.