The sympathetic nervous system and baroreflexes in hypertension and hypotension.

Blood pressure and blood volume are closely regulated by the interrelated actions of the sympathetic nervous system (SNS) and the renin-angiotensin-aldosterone system (RAAS). Reflex vasoconstriction caused by parallel SNS and RAAS activation is modulated by two interactive negative feedback systems called baroreflex. The aortic-carotid baroreflex systems respond to momentary changes in systolic blood pressure, adjusting the degree of SNS-dependent peripheral vasoconstriction and cardiac output to allow maintenance of a relatively constant perfusion pressure. Cardiopulmonary baroreflexes respond to momentary changes in cardiac filling, adjusting the degree of peripheral venoconstriction and venous return to maintain cardiac preload and stroke volume. Under normal conditions, each baroreflex system exhibits a degree of tonic negative feedback so that it can alter SNS output immediately, providing counterregulatory increases or decreases in pressure or volume to maintain homeostasis. The SNS is inappropriately active in obesity and hypertension and plays a causal or permissive role in all forms of chronic hypertension. If the negative feedback control exerted by the baroreflexes over the SNS and renin-angiotensin-aldosterone system (RAAS) were perfect, chronic hypertension would not occur. Activity of the baroreflexes, however, is chronically altered by maladaptive changes such as cardiac and vascular fibrosis and hypertrophy. Long-term increases in SNS and RAAS activity also exert ongoing deleterious effects on the heart and vasculature by directly facilitating further cardiac hypertrophy and arterial stiffening. These effects appear to contribute to a vicious cycle of chronic hypertension and target organ damage. Other syndromes of abnormal blood pressure (BP) control, including orthostatic hypotension and baroreflex failure are examples of abnormal baroreflex activity and SNS control.