The pressure exerted by the flow of blood through the major arteries of the body. This pressure is greatest during the contraction of the ventricles of the heart (systolic pressure; see systole), which forces blood into the arterial system. Pressure falls to its lowest level when the heart is filling with blood (diastolic pressure; see diastole). Blood pressure is measured in millimetres of mercury using an instrument called a sphygmomanometer. Normal blood pressure for a young average adult human is in the region of 120/80?mmHg (the higher number is the systolic blood pressure; the lower number the diastolic blood pressure), but individual variations are common. Abnormally high blood pressure (hypertension) may be associated with disease or it may occur without an apparent cause. Mean arterial pressure (MAP)—the mean of systolic and diastolic pressures—is determined by cardiac output (CO) and total peripheral resistance (TPR) according to the equation MAP = CO × TPR. It is regulated by various neural and hormonal mechanisms, both local and systemic. CO depends on stroke volume and heart rate, which are governed chiefly by the autonomic nervous system, and hormones such as adrenaline. TPR is determined by the resistance of the arteries and arterioles feeding the capillary beds (see microcirculation). Contraction and relaxation of smooth muscle in the walls of these vessels alters their internal diameter, and hence resistance to blood flow. These changes may occur in response to local requirements for oxygen or delivery/removal of metabolites, or more general changes. For instance, in the alarm response noradrenaline binds to receptors in vessels in the gut and other ‘non-essential’ tissues, causing vessels to constrict. Another hormone, angiotensin, also causes constriction of arterioles in peripheral tissues and is important in raising blood pressure.