What is the mammalian dive reflex?
The mammalian diving response, or "dive reflex," is the way our body automatically prioritizes blood delivery to the brain and heart when we hold our breath underwater. When your head is underwater, you are using up the oxygen in your blood and not replacing it by breathing, a condition known as hypoxia. The brain and the heart are exceptionally sensitive to hypoxia, whereas other parts of our body, such as our arms and our legs, can tolerate hypoxia much more easily.
Ingrid Eftedal, Ph.D.: Diving is associated with environmental factors that affect the cardiovascular system, and as long as the total amount of physiological stress is limited and the diver is reasonably fit, no data indicate that diving is harmful to the cardiovascular system. In addition to the health-promoting effects of exercise, one may speculate that diving could have additional benefits for the heart. It has been known for some time that breathing high amounts of oxygen activates an important type of proteins known as molecular chaperones (for historical reasons they are sometimes called heat shock proteins). Some of these molecular chaperones directly protect the heart from injury; extra oxygen is sometimes administered to patients before heart surgery for this reason.
Dawn Kernagis, Ph.D.: Scuba diving can be a fantastic way to burn calories, with a recreational dive burning an estimated 300 to 600 calories per hour depending on temperature and activity level. Although most recreational diving can be very relaxing while still involving increased activity and low levels of joint stress, which is beneficial to individual health, some forms of diving can be strenuous and could put an individual with predisposing conditions at potential risk of injury or incident. It's important to remember that physical fitness is a major factor in determining potential risk with any form of increased physical activity, so risk associated with increased activity will largely depend on a diver's overall physical condition.