Hypertension is defined by an increase in systemic blood pressure above limits considered normal, currently set at 140 mmHg for systolic and 90 mmHg for diastolic pressure. Assuming central venous pressure to be near zero, mean arterial pressure is determined by the product of total peripheral resistance and cardiac output. In most cases of essential hypertension, as well as in animal models of hypertension, cardiac output and its main determinants, stroke volume and heart rate, are normal, whereas total peripheral resistance is increased. Total peripheral resistance is influenced by a number...