6 Orthostatic Hypotension

Orthostatic hypotension (postural hypotension) is a sudden decrease in blood pressure that occurs when a person quickly assumes a standing position and blood pools down in the lower extremity under the influence of gravity (ASIDE: Orthostatic means “upright posture,” and hypotension means low blood pressure.) The gravitational redistribution of blood volume reduces blood flow to the heart causing the amount of blood pumped by the heart to decrease, a drop in arterial blood pressure, less flow to the brain, and potential dizziness. The baroreceptor firing rate falls and triggers the sequence of events in the baroreceptor reflex . The increased sympathetic outflow and reduced parasympathetic outflow speed the heart rate and constrict the vessels. This “autonomic reflex” boosts the blood pressure and baroreceptor spike activity in a manner that ultimately restores blood pressure to its normal resting set point. It is involuntary and quick. You are not aware that it is happening except that you may realize that it is good advice to the elderly to support themselves for a few seconds after getting out of bed or after standing up from a crouching position–like after retrieving objects off a bottom shelf.

Conclusion: The autonomic nervous system controls many involuntary actions including regulatory, homeostasis, and housekeeping functions of the body. The sympathetic and parasympathetic branches using NE and ACh, respectively, usually exert opposite physiological effects: flight or fight versus rest and digestion. They can be understood from the different G proteins that are engaged. Many patients arriving in the emergency room need immediate pharmacological intervention of their autonomic function.