The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms that help to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure reflexively causes the hea rate to decrease and also causes blood pressure to decrease. Decreased blood pressure decreases baroreflex activation and causes hea rate to increase and to restore blood pressure levels. The baroreflex can begin to act in less than the duration of a cardiac cycle (fractions of a second) and thus baroreflex adjustments are key factors in dealing with postural hypotension, the tendency for blood pressure to decrease on standing due to gravity. The baroreceptors are stretch-sensitive mechanoreceptors. At low pressures, baroreceptors become inactive. When blood pressure rises, the carotid and aoic sinuses are distended fuher, resulting in increased stretch and, therefore, a greater degree of activation of the baroreceptors. At normal resting blood pressures, many baroreceptors are actively repoing blood pressure information and the baroreflex is actively modulating autonomic activity. Active baroreceptors fire action potentials ("spikes") more frequently. The greater the stretch the more rapidly baroreceptors fire action potentials. Many individual baroreceptors are inactive at normal resting pressures and only become activated when their stretch or pressure threshold is exceeded. Ref: guyton and hall textbook of medical physiology 12 edition page number:216,217,218