Blood pressure is the force of blood pushing against artery walls and is measured in millimeters of mercury (mmHg). It is defined by two numbers - the systolic pressure when the heart beats and the diastolic pressure between beats. Adults can have normal, prehypertensive, or hypertensive blood pressure levels depending on whether their systolic and diastolic pressures are below, somewhat above, or above certain thresholds. Since high blood pressure often has no symptoms, it is important for people to get tested using a sphygmomanometer and stethoscope. Maintaining a healthy diet, exercise, and weight can help prevent hypertension.