Final answer:
The diastolic blood pressure is the level of pressure present during the relaxation of the heart, specifically during the ventricular diastole phase. This pressure represents the minimum pressure in the arteries when the heart is at rest between beats.
Step-by-step explanation:
The diastolic blood pressure is the level of pressure present during the relaxation of the heart. This phase occurs during ventricular diastole, when the ventricular muscle relaxes and the pressure on the remaining blood within the ventricles falls. The diastolic pressure represents the minimal value corresponding to the pressure remaining during this relaxation phase. When the pressure in the ventricles drops below that in the pulmonary trunk and aorta, the semilunar valves close to prevent backflow into the ventricles, resulting in the isovolumic ventricular relaxation phase where no blood enters or leaves the ventricles, thus maintaining the same blood volume.
Therefore, the correct answer to the original question is c) Relaxation of the heart. Diastolic pressure is an essential indicator of cardiovascular health and typically ranges from 60 to 110 mm Hg, with a desirable diastolic pressure being less than 80 mm Hg. It is measured during the diastole phase of the cardiac cycle, which follows the systole (contraction) phase.