The diving reflex links changes in heart rate and blood-vessel resistance to breath-holding and facial immersion.
What Happens During the Human Diving Reflex?
The human diving reflex is a set of responses that can be triggered when breath-holding is combined with facial immersion. These include a slower heart rate and increased resistance in peripheral blood vessels, which carry blood to areas away from the body’s centre.
In a study of 18 people, breath-holding together with face immersion reduced heart rate more than the separate effects of the two stimuli added together.
How Does the Reflex Conserve Oxygen?
The diving reflex can help conserve oxygen by slowing the heart and increasing resistance in peripheral blood vessels. The heart-rate reduction is driven by increased parasympathetic activity through cardiac vagal fibres, which carry signals that slow the heart.
Blood-vessel changes involve a different part of the nervous system. During simulated diving in 56 healthy volunteers, sympathetic nerve activity and vascular resistance increased, while heart rate fell later in the response.
The spleen may contribute too. It can store 200–250 millilitres of densely packed red blood cells. In a study of 18 people, breath-holding raised haemoglobin concentration by an average of 2.4 per cent.
How Have Researchers Measured the Response?
Experiments can compare breath-holding with and without face immersion, making it possible to test how much each part contributes. In one test, 15 participants cycled at 80 watts while holding their breath for 40 seconds, either in air or with their faces immersed in water at 10°C.
Oxygen uptake from the lungs was lower during breath-holding with cold-water face immersion than during breath-holding in air.
A separate study of 17 people found a greater heart-rate reduction when face immersion was added to breath-holding than with breath-holding alone. Together, these tests show how the response can be measured through changes in heart rate and oxygen use.
What Does the Reflex Achieve?
The human diving reflex can slow oxygen use during breath-holding, helping preserve oxygen for tissues that need it. The heart slows, and narrowing peripheral blood vessels changes how blood is distributed.
In the exercise experiment, the lung oxygen store was depleted more slowly during breath-holding with face immersion than during breath-holding in air. In a separate study of 18 participants, cerebral oxygen saturation was maintained during the tested breath-hold and rebreathing manoeuvres.
The response does not make oxygen or provide a fresh supply. Its likely function is to help protect oxygen-sensitive tissues, including the brain and heart, while the person is holding their breath.
Does the Diving Reflex Make People Safe Underwater?
The diving reflex does not guarantee a longer or safe breath-hold. In a study of 13 men without apnoea training, average maximum breath-hold time during face immersion was shorter in water at 0°C than in air.
In that study, face immersion in water at 10°C or colder could trigger a cold-shock-like response. This could shorten breath-holding and outweigh the oxygen-conserving effects of the diving response.
In a study of 65 healthy volunteers, heart-rate responses to cold-water face immersion varied between individuals, so the reflex cannot be treated as a dependable safeguard underwater.
