The diving reflex (submersion reflex) in humans produces a characteristic cardiovascular response pattern. The correct description of this integrated autonomic response is:

• A Tachycardia and peripheral vasodilation to increase oxygen delivery to vital organs
• B Generalized sympathetic activation causing both tachycardia and vasoconstriction
• C Bradycardia via enhanced vagal tone combined with peripheral vasoconstriction via sympathetic activation, preserving perfusion to brain and heart ✓
• D Selective vagal withdrawal from skeletal muscle vessels causing shunting to the core

Explanation

The diving reflex is a powerful O2-conserving autonomic response triggered by facial immersion in cold water (via trigeminal nerve afferents). It produces: (1) profound bradycardia mediated by increased vagal (parasympathetic) tone to the sinoatrial node — heart rate may drop 10-25% in adults; (2) simultaneous intense peripheral vasoconstriction via sympathetic noradrenergic activation — arterioles in splanchnic bed, muscle, and skin vasoconstrict. This combination maintains blood pressure while reducing cardiac work. Blood is redistributed to the brain and heart (which lack the same degree of vasoconstriction). The diving reflex is stronger in neonates and explains survival after prolonged submersion in cold water.

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

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