MEDICINE
Blood pressure is determined both by the amount of blood your heart pumps and the amount of resistance to blood flow in your arteries. The more blood your heart pumps and the narrower your arteries, the higher your blood pressure. A blood pressure reading is given in millimeters of mercury (mm Hg). It has two numbers.
Top number (systolic pressure). The first, or upper, number measures the pressure in your arteries when your heart beats.
Bottom number (diastolic pressure). The second, or lower, number measures the pressure in your arteries between beats.