Blood pressure is the measurement of the force of blood against the walls of the arteries as the heart pumps blood through the body.Systolic blood pressure (the top number of the fraction) is the pressure in the arteries during systole (i.e., when the ventricles are contracting and causing the ejection of blood into the aorta and pulmonary arteries).Diastolic blood pressure (the bottom number of the fraction) is the resting pressure in the arteries during diastole (i.e., the phase between each contraction of the heart 20 when the ventricles are filling with blood).It is reported as millimeters of mercury (mmHg).This pressure inside the arteries changes when the heart is contracting compared to when the heart is resting and filling with blood.