Humans and other mammals have a diving response consisting of a set of reflexes that are activated when our face is cooled (such as by the water during a dive) or if we hold our breath. The diving reflex is a clever physiological mechanism enabling the body to tolerate a low level of oxygen.
Three changes occur in the body:
- Bradycardia, a slowing of the heart rate. The human heart rate slows down 10 – 30% and up to 50% or more in trained individuals.
- Peripheral vasoconstriction (a narrowing of blood vessels to reduce blood flow by muscle contraction in the blood vessel’s wall), causes reduced blood flow to the limbs ensuring that oxygen sensitive organs like the brain and heart receive oxygen.
- During deep dives a blood shift occurs allowing blood plasma and water to pass through organs and circulatory walls to the chest cavity to protect the organs from the increase in pressure. The lungs gradually fill up with blood plasma, which is reabsorbed when pressure drops.
A slowing of the heart rate occurs relatively quickly upon facial contact with cold water. The trigeminal facial nerves (5th cranial nerve) transmit the information to the brain which innervates the vagus nerve (10th cranial nerve) causing bradycardia and peripheral vasoconstriction. The colder the water, the faster the reaction. Temperatures above 21° C (70° F) do not elicit a response. Reduced blood flow to the limbs occurs more gradually. The reflex is preventive because it is initiated before the level of oxygen becomes critically low. In addition the large amount of blood that accumulates in the blood vessels of the lungs acts as a protective measure, because fluids - as opposed to tissue and bones - cannot be compressed. The blood thus prevents the lungs from collapsing under the high pressure of the deep.
When vasoconstriction shunts blood away from arms and legs, the amount of blood we have available is concentrated in a "small" circulatory system between the lungs, heart and brain. These are the most oxygen sensitive organs of the body and the blood shunting is thus a perfect survival mechanism to a low oxygen level. In addition the decrease in heart rate also helps lower oxygen consumption, since the heart muscle is working at a lower intensity.
Another action of the diving response can be observed in infants when they are under water. The windpipe by the vocal chords spontaneously closes to prevent water from entering the lungs. This reflex is initiated as soon as there is contact with water. However, it disappears when the child reaches the age of roughly six months.
Recent investigations have shown that the spleen, which contains red blood cells, also plays a significant role during dives and breath holds. Following a number of dives, the spleen contracts and releases a large quantity of red blood cells to the circulatory system. Spleen contraction occurs much slower than the other diving reflexes. The release of more red blood cells allows more oxygen to be stored in the blood. Finally, the additional amount of blood cells allows the body to regain its normal balance faster after a prolonged breath hold. Popularly speaking, the spleen acts as a kind of “turbo” - during and after a long dive.
Diving mammals such as whales and seals naturally have a well-developed diving reflex to allow them to forage below the surface for extended periods of time. There are several reasons why the sperm whale, seals and elephant seals are excellent breath holders and can dive for more than an hour. Firstly, these animals have quite a lot of blood and a high concentration of blood cells which bind oxygen in the so-called hemoglobin protein. In addition, they have a higher concentration of an oxygen-binding molecule called myoglobin in their muscles. Have you ever seen whale or seal meat, and wondered why it is so dark? Myoglobin is the answer. A high content of iron in myoglobin colors the meat brown.
The diving mammals are also able to cool their brain down, which helps them during prolonged dives. Several studies show that seals can lower their body and brain temperature with up to 3 degrees Celsius, and thereby lower their metabolism and oxygen consumption dramatically. As opposed to humans, seals may completely shut off blood supply to the limbs and thus direct oxygenated blood to the lungs, heart and brain.
How can I use the diving reflex?
If you are going to perform e.g. at a competition or at a business event, activating the diving reflex can help you relax. The diving reflex is activated by breath holds and by facial contact with cold water. If you cover your face, especially the forehead and the area around the nose (area of the trigeminal nerve) with a cold wet towel, the diving reflex will be activated. Because the diving reflex innervates the vagus nerve your pulse will drop and your body will relax. In this way you can quickly calm your nerves before a performance.