Final answer:
The diastolic number is the lower value in a blood pressure reading, representing arterial pressure during the heart's relaxation phase (diastole), with a healthy value generally being less than 80 mm Hg.
Step-by-step explanation:
The diastolic number in a blood pressure reading represents the lower number recorded and refers to the arterial pressure when the heart's ventricles are relaxed, which is known as diastole. When systemic arterial blood pressure is measured, it is written as a ratio of systolic pressure over diastolic pressure, with the systolic being the higher value occurring during ventricular contraction, and the diastolic being the lower value during relaxation. A healthy or optimal diastolic pressure is typically considered to be less than 80 mm Hg.
The diastolic pressure is crucial because it reflects the minimum pressure in the arteries, which in turn indicates the resistance to blood flow and the overall health of the blood vessels. Factors such as hormones, stress, exercise, eating, as well as the body's regulatory mechanisms involving vasoconstriction and vasodilation affect blood pressure readings.