In response to hemorrhage, how do vascular mechanisms help preserve blood pressure?

• A. Local tissue hypoxia causes vasodilation to maintain MAP.
• B. Parasympathetic activation increases heart rate to maintain MAP.
• C. Baroreceptor reflex causes vasodilation to maintain MAP.
• D. Sympathetic activation causes vasoconstriction and increases heart rate/contractility to maintain MAP.

Answer: (D)

When blood loss occurs, arterial pressure tends to drop. The body’s immediate response comes from the baroreceptor reflex, which detects the fall in stretch of sensors in the carotid sinuses and aorta and sends signals that boost sympathetic activity. This sympathetic surge constricts vascular smooth muscle, especially in arterioles, raising peripheral resistance, and also constricts veins to push more blood back toward the heart (increasing venous return and preload). At the same time, sympathetic nervous system activation increases heart rate and strengthens heart contractions, boosting cardiac output. The combination of higher total peripheral resistance and greater cardiac output helps restore mean arterial pressure and maintain blood flow to critical organs like the brain and heart.

Local tissue hypoxia would actually promote vasodilation to improve blood flow to the affected tissue, which would lower MAP, not help maintain it. Parasympathetic activation would slow the heart and reduce contractility, which would not support blood pressure during hemorrhage. The baroreceptor reflex does not produce vasodilation in this context; it drives vasoconstriction to raise MAP.