As blood flows through arteries it pushes against the inside of the artery
walls. The more pressure the blood exerts on the artery walls, the higher the blood pressure will be. The size of small arteries also affects the blood pressure. When the muscular walls of arteries are relaxed, or dilated, the pressure of the blood flowing through them is lower than when the artery walls narrow, or constrict.
Blood pressure is highest when the heart beats to pump blood out into the arteries. Between beats, when the heart relaxes to refill with blood, the pressure drops to its lowest point. The blood pressure peak, when the heart pumps, is called systolic pressure. The blood pressure trough, when the heart is filling, is called diastolic pressure. When blood pressure is measured, the systolic pressure is stated first and the diastolic pressure second. Blood pressure is measured in millimeters of mercury (mm Hg). For example, if a person's systolic pressure is 120 and diastolic pressure is 80, it is written as 120/80 mm Hg. The American Heart Association considers systolic blood pressure less than 140 and diastolic blood pressure less than 90 normal for adults.