So what is the Dive reflex?
The mammalian diving reflex is a reflex in mammals which optimizes respiration to allow staying underwater for extended periods of time. In free diving we learn how to trigger this reflex. It is triggered firstly when our faces touch water and later on during the dive.
Immediately upon facial contact with cold water, the human heart rate slows down ten to twenty-five percent. Slowing the heart rate lessens the need for bloodstream oxygen, leaving more to be used by other organs.
Next, blood gets restricted to the essential areas. When under high pressure induced by deep diving, capillaries in the extremities start closing off, stopping blood circulation to those areas.Toes and fingers close off first, then hands and feet, and ultimately arms and legs stop allowing blood circulation, leaving more blood for use by the heart and brain. Human musculature accounts for only 12% of the body’s total oxygen storage, and the body’s muscles tend to suffer cramping during this phase. Aquatic mammals have as much as 25 to 30% of their oxygen storage in muscle, and thus they can keep working long after capillary blood supply is stopped.
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..
Last is the blood shift that occurs only during very deep dives. When this happens, organ and circulatory walls allow plasma/water to pass freely throughout the thoracic cavity, so its pressure stays constant and the organs aren’t crushed. In this stage, the lungs’ alveoli fill up with blood plasma, which is reabsorbed when we leave the pressurized environment.
In short, AMAZING, we are equipped ! As you become a more experienced diver you will experience the dive reflex for yourself. I personally like to do a static dive as soon as Im in the water so as to trigger the response. All that means is that as im holding my breath safely, I witness the changes happening in my body, after some time contractions start and that’s when I feel the reflex kicking in and my heartbeat dropping. Like my teacher says, the dive starts when the contractions start. So learning to deal with your contractions is important. Another common effect of the dive reflex is involuntary urinating, it happens often to me and a few other freedivers I know, in this case always have re-hydration with you. There might also be unique ways in which you experience the dive reflex, everyone is different, there are no carbon copies!
When the contractions kick in, the dive reflex gets stronger, so learn to live with them, contractions are our friends! It is important to be with a freediving instructor before you attempt anything and never dive alone. Before a dive always tell your buddy what you would like to attempt. Be safe and enjoy.