What is orthostatic hypotension and its vascular basis?

• A. A rise in blood pressure on standing due to excessive vasoconstriction
• B. No change in BP with posture
• C. A drop in blood pressure on standing due to inadequate vasoconstriction and venous return
• D. Chronic high blood pressure due to dehydration

Answer: (C)

Orthostatic hypotension is a drop in blood pressure that happens when you stand up. The vascular basis is that gravity pulls blood into the veins of the legs and abdomen, and the body's normal response is to constrict those veins and raise peripheral resistance to keep blood returning to the heart. If the venous vessels don’t constrict enough or the autonomic reflexes fail to compensate, venous return decreases, preload and cardiac output fall, and arterial pressure drops. This is what produces dizziness or lightheadedness upon standing. The other scenarios describe an increase in BP with standing, no change, or a chronic high blood pressure related to dehydration, which do not reflect the vascular mechanism of orthostatic hypotension.