What is the Mammalian diving reflex?
The mammalian diving reflex is a set of physiological responses that occur in mammals, including humans, when they are submerged in water. This reflex is more pronounced in marine mammals, such as seals and whales, but it is also observed in humans, especially in infants. The primary purpose of the diving reflex is to conserve oxygen and prioritize the oxygen supply to vital organs when submerged.
How do I trigger the mammalian diving reflex (response)?
Water on your face and immersion coupled with apnea (holding your breath) triggers the mammalian diving refl ex. (Many physiologists call it the “mammalian diving response” which is probably the more accurate term, but people say “reflex” more commonly.)
This response is common in all mammals to a larger or lesser degree. It is very strong in aquatic mammals, and much less so in humans. Colder temperatures seem to make the response faster and stronger. Some freedivers try to help prompt this response with relaxed
no-mask snorkel breathing, eyes closed, for several minutes after entering the water. Once triggered, your heart rate slows, which helps conserve oxygen. With training,
the heart rate may slow as much as 50 percent. Blood vessels in your extremities constrict. This reduces blood fl ow to them and causes blood to pool in the core.
- This conserves oxygen for your vital organs.
- Blood vessels around the lungs enlarge, which helps offset the loss of gas volume, especially in deeper freedives.
- To some extent, this blood shift occurs with immersion even when not holding your breath.
Key Triggers of the Mammalian Diving Reflex
- Cold Water Exposure:
The most significant trigger for the diving reflex is cold water. Cold temperatures stimulate specialized thermoreceptors in the skin and mucous membranes (especially in the face and nose) when the body is immersed in water. These thermoreceptors send signals to the brain, activating the dive reflex. The colder the water, the more intense the response. It is important to note that the reflex is much stronger when the water temperature is below 21°C (69.8°F).
When the face is exposed to cold water, particularly in the region around the nose and mouth, the body reacts by slowing down its physiological functions to conserve oxygen. This process is believed to be an evolutionary adaptation that improves survival during underwater activity. - Facial Immersion:
Immersion of the face, particularly the nasal cavity, plays a key role in triggering the reflex. Research suggests that contact with water around the face stimulates sensory nerves, specifically the trigeminal nerve (which is responsible for sensations in the face). These signals travel to the brain, activating the autonomic nervous system and initiating the physiological changes associated with the diving reflex.
Interestingly, it's not the full submersion of the body that triggers the reflex—it's the facial contact with cold water that acts as the primary cue. This is why activities like "face dunking" or splashing cold water on your face can activate the reflex, even if you're not completely submerged. - Breath-holding (Apnea):
Breath-holding, or apnea, is another critical element of the diving reflex. When you hold your breath, it further enhances the effect of the reflex by increasing the oxygen-conserving mechanisms in the body. The body's instinct is to hold its breath to prevent water from entering the lungs, which also contributes to conserving oxygen during the reflex.
How the Mammalian Diving Reflex Helps Survival
When triggered, the Mammalian Diving Reflex produces several physiological changes that work together to conserve oxygen and maximize survival during submersion. Here's how the body reacts:
- Bradycardia (Slowed Heart Rate):
One of the first responses to the diving reflex is bradycardia, which refers to a slowing of the heart rate. In some cases, the heart rate can drop significantly, sometimes by as much as 50% or more of its normal rate. This reduction in heart rate reduces the amount of oxygen the body requires and minimizes the risk of hypoxia (lack of oxygen).
The slowing of the heart is controlled by the vagus nerve, which is part of the parasympathetic nervous system. This is a protective response, ensuring that the oxygenated blood is conserved for vital organs such as the brain and heart. - Peripheral Vasoconstriction:
Another important response triggered by the diving reflex is peripheral vasoconstriction, which refers to the narrowing of blood vessels in the extremities. This process helps to redirect blood flow from the arms, legs, and skin towards the core of the body, particularly the brain and heart.
By shunting blood to the vital organs, the body ensures that the most critical areas have a sufficient supply of oxygen during prolonged submersion. The extremities, which are not essential for survival, receive less oxygenated blood, which helps to conserve the body's resources. - Blood Shifting to Core Organs:
As part of the vasoconstriction, the blood flow is prioritized to the brain, heart, and lungs. This ensures that these key organs receive the oxygen they need to continue functioning while other tissues, which are less critical for immediate survival, receive less oxygen.
This is particularly important in cold-water diving, where oxygen levels are critical to maintaining consciousness and awareness. The brain, being highly sensitive to oxygen deprivation, benefits from this process, which helps avoid unconsciousness during deep dives or submersion. - Increased Oxygen Storage in the Blood:
The human body is equipped with oxygen-carrying molecules such as hemoglobin in the blood and myoglobin in muscles. Under the influence of the diving reflex, the body's ability to store and use oxygen efficiently is heightened, allowing it to stay submerged for longer periods.
The Mammalian Diving Reflex is a vital survival mechanism, particularly for marine mammals such as seals, dolphins, and whales, which often dive deep into cold waters for long periods. This reflex allows them to conserve oxygen and minimize the risk of drowning while they hunt, explore, or travel underwater. Similarly, humans benefit from this reflex, especially in emergencies involving submersion, as it helps to delay the onset of hypoxia and increase the chances of survival.
For freedivers and swimmers, understanding and training the Mammalian Diving Reflex is essential. Freedivers, in particular, rely on the reflex to maximize their breath-hold times, allowing them to dive deeper and longer while keeping their bodies oxygenated. Training techniques like "cold-water immersion" or controlled breathing exercises can help enhance the response, making it easier for individuals to adapt to extended periods underwater.
* Medical studies have found that the spleen releases red blood cells,
increasing breathhold capacity