A patient's blood pressure is significantly low due to hypovolemia. What mechanism primarily acts to compensate for this condition?

• A. Decreased cardiac output
• B. Increased systemic vascular resistance
• C. Increased heart rate
• D. Decreased venous return

Answer: (B)

In the context of hypovolemia, the body responds with several compensatory mechanisms to help maintain blood pressure and ensure adequate perfusion to vital organs. When blood volume is low, one of the primary compensatory responses is an increase in systemic vascular resistance.

As blood volume decreases, the baroreceptors in the carotid sinus and aortic arch detect the drop in blood pressure. This triggers the sympathetic nervous system to activate, leading to vasoconstriction in peripheral blood vessels. The constriction of these vessels increases systemic vascular resistance, which helps to raise blood pressure by maintaining or increasing the pressure even with a reduced volume of circulating blood.

Increased systemic vascular resistance allows the remaining blood volume to be more effective in maintaining blood pressure, ensuring that adequate blood flow reaches essential organs despite the overall decrease in blood volume. Thus, the correct response to hypovolemia in this question is the increase in systemic vascular resistance as the main compensatory mechanism.