Following moderate hemorrhage, sympathetic stimulation raises mean circulatory filling pressure even though total blood volume is reduced. Which vascular segment accounts for most of this compensatory mobilization?

• A Veins, because they hold about two thirds of blood volume and constrict strongly via alpha-1 receptors ✓
• B Arterioles, because they generate the greatest resistance change
• C Capillaries, because precapillary sphincter closure traps blood centrally
• D Large elastic arteries, because their recoil expels stored blood

Explanation

The systemic veins are capacitance vessels containing roughly 60 to 65 percent of total blood volume at low transmural pressures. Norepinephrine acting on vascular alpha-1 receptors constricts them, shifting unstressed volume toward the heart and raising mean circulatory filling pressure and venous return without requiring added fluid. Arterioles are resistance vessels and their constriction mainly defends arterial pressure, not venous return. Precapillary sphincters control local flow, and elastic recoil of large arteries contributes little acutely.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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