The Mammalian Diving Reflex and Freediving (Apnea)

Calf Profile
by wildestanimal / Getty Images

All mammals have an innate reflex known as the mammalian diving reflex, which serves to preferentially divert oxygen supplies to the essential organs of the brain and heart when the mammal is immersed in water. The reflex is very strong in water mammals, such as whales and dolphins, and it is the physiological adaptation that lets them dive to great depths between surface breaths.

Many other animals have this reflex as well, including humans.

An associated reflex is apnea--the instinct to hold the breath when immersed in water. The mammalian diving response, combined with apnea, is what makes human free diving possible. Further, humans also have a natural instinct to swim. 

You can see evidence of these reflexes in very children who haven't learned to fear the water.  A newborn baby placed in the water will reflexively hold his breath (the diving reflex) and swim (the swimming reflex). Fear of water usually comes later in a child's development. 

The diving reflex is part your nature as a human being. If you are just learning to free dive, you can relax!. You already have the tools you need to survive underwater. 

How the Mammalian Diving Reflex Is Triggered

Interestingly, studies show that holding one's breath (apnea) in a dry environment does not result in the same physiological reactions as the wet apnea that occurs upon submersion.

 Submersion in water is necessary to trigger the mammalian diving reflex. In humans, there are specific nerve receptors in the face that initiate the response to hold one's breath, and which also begins the reflex that diverts oxygen to the heart and brain. Specifically, it is the chilling of the face that causes the reflexive apnea and begins the diving reflex.

This perhaps explains why being suddenly splashed in the face, or getting a blast of cold air, can cause us to suddenly catch our breath.  

All this is great news for free divers, as the mammalian dive reflex helps them effortlessly hold the breath and to dive deep. 

The Physiological Reactions

 Once a diver is submerged in water, two cardiovascular reactions occur.

1. Vasoconstriction
The term vasoconstriction refers to the narrowing of blood vessels to reduce blood flow. Vasoconstriction occurs when the muscles in a freediver's blood vessel walls contract. 

Vasoconstriction is helpful to free divers because it reduces the amount of blood that flows to peripheral organs, which do not need a high level of oxygen to function, while conserving blood and oxygen for the body's vital organs, such as the heart, lungs, and brain, which need high levels of oxygen. Aquatic Mammals, humans, and diving birds all experience vasoconstriction when submerged, but not when they hold their breaths above water.

2. Heart Rate Reduction
The second physiological reaction that occurs during the mammalian diving reflex is the reduction of the freediver's heart rate (known as bradycardia). Interestingly, a diver does not need to be completely submerged to trigger this response.

Simply wetting the face is enough to drop a diver's heart rate.

For the average human, exposure of the face to water will trigger a 10 to 30% reduction in heart rate. Individuals such as free divers who have trained to augment their mammalian diving reflex may experience a heart rate reduction of up to 50%.

The intensity of the reaction is also related to temperature. The colder the water, the greater the heart rate reduction.

Heart rate reduction may sound frightening, but it is actually beneficial for free divers. It is a natural adaptation of the human body to conserve oxygen, which allows free divers to make longer dives. Studies conducted on free diver Umberto Pelizzari showed that his heart rate drops to 30 beats/minute during static apnea.

Conclusion

Aquatic mammals and humans are born with the necessary adaptations to spend long periods of time underwater.

The mammalian diving reflex is a natural physiological reaction that occurs when a human, mammal or diving bird is submerged in water, and it includes vasoconstriction and heart rate reduction. These reactions help to reduce a diver's consumption of oxygen while continuing to provide sufficient quantities of oxygen to his vital organs.

On deeper dives where there is increased water pressure, free divers experience additional physiological reactions, including Blood Shift and the Spleen Effect.