Which statement best describes the primary physiologic response to the mammalian diving reflex?

• A. Increased heart rate
• B. Peripheral vasodilation
• C. Bradycardia with peripheral vasoconstriction
• D. Increased metabolic rate

Answer: (C)

The primary physiologic response to the mammalian diving reflex is a slowing of the heart coupled with constriction of peripheral blood vessels. When the face is immersed in cold water, signals trigger the vagal (parasympathetic) system to slow the heart rate, while the sympathetic system constricts vessels in the limbs and other nonessential areas. This combination reduces oxygen use and redirects blood toward vital organs like the brain and heart, helping preserve oxygen during submersion and apnea.

This explains why the other patterns don’t fit: an increased heart rate would use more oxygen, peripheral vasodilation would waste blood flow from core organs, and an increased metabolic rate would raise oxygen demand—all opposite to the diving reflex’s oxygen-conserving goals.