When the face is submerged in cold water, sensory receptors in the face send signals to the brain. This activates the parasympathetic nervous system, leading to bradycardia, a slowing of the heart rate. Concurrently, peripheral vasoconstriction reduces blood flow to extremities, conserving oxygen for vital organs. Blood shift, in deeper dives, helps protect the lungs from collapse by redistributing blood to the thoracic cavity.
