What physiological response does the body exhibit in cool water during apnea?

During apnea in cool water, the body exhibits bradycardia as a physiological response. This response is part of the mammalian dive reflex, which is triggered when a person is submerged in water, particularly cold water. The dive reflex helps to conserve oxygen and protect vital organs during prolonged periods of breath-holding.

Bradycardia is characterized by a slowing of the heart rate. In colder water, the body automatically decreases the heart rate to reduce the oxygen consumption of the heart, allowing more oxygen to be directed toward essential organs like the brain and heart itself. This mechanism is crucial for survival in aquatic environments where access to oxygen is limited during apnea.

The other options do not correspond to the body's primary response in this context. Increased heart rate would increase oxygen demand, hyperventilation would ultimately consume more oxygen, and muscle spasms could signify distress or a lack of oxygen, which is contrary to what the dive reflex aims to achieve.