First of all the term, Mammalian Dive Reflex is a little bit misleading term since not only mammals have it. So, let’s call it Dive Reflex or Dive Response or just DR 😉
Doesn’t matter, you are complete beginners or you already Freediving Instructor, Dive Reflex is your best friend!
A bit of history. Many years ago one French doctor made a statement that man could not dive deeper than 50 meters because the thoracic cavity is going to be crushed (some sources say 30-40 meters). Why? Because every 10 meters pressure increasing with 1 atm and when you are 50 meters surrounding pressure already 6 atm. And it is huge. But back to those time, no one was even trying to do it (ok, there a couple of exceptions). But in 1961 Enzo Maiorca dived to this depth and survive! Why? Because of the blood shift! And blood shift is a part of DR!
DR is activated when our face is cooled (by cold water for example) or when we hold our breath. When we do both – even better!
This reflex helps us to hold our breath longer and dive deeper! How? By:
- Apnea
- Bradycardia
- Peripheral vasoconstriction and blood shift
- Spleen contraction
1. DR is responsible for spontaneous activation of Apnea. If we place infant underwater (don’t ask me why) their windpipe would spontaneously close (by vocal cords) and this prevents water from entering the lungs. This reflex quite strong upon 6 months and then start to disappear. My assumption – around this age baby start learning how to crawl and probably decide that Dive Reflex is not that important for them! Does it happen because of our genetic memories of our aquatic past or because nine months before birth our natural environment is liquid? Who knows 😉
2. DR causing bradycardia – slowing your heart rate (HR). Quite common is 10-30 % 
reduction of HR for Freedivers (up to 50% or more in highly trained athletes). There are stories with even more impressive results, but let’s skip them now. Sounds impressive? How about this – laboratory rats have 80% decreases in HR while submerged underwater!!
Bradycardia is usually followed by tachycardia (increase in HR) after breath hold is over.
Why bradycardia is important for Freediver? Well, it is a protective mechanism of our body, it decreases O2 consumption, which means we can hold our breath hold longer without risk of losing the conscious! It also compensates the result of peripheral vasoconstriction effect (which cause increased blood pressure)
3. Next benefit of DR is a peripheral vasoconstriction and blood shift
Back to 1974 study showed that during dives to 40-60 meters, the amount of blood in the thorax (chest cavity) increased more than twice! And this reflex was called (pretty obviously) blood shift.
Peripheral vasoconstriction (PV) is a narrowing of the blood vessels to reduce blood flow to non-vital organs (such as skin or inactive muscles, for example) ensuring that oxygen-sensitive organs like the brain or heart receive enough O2 for normal function. In another word PV is a redistribution of blood to vital organs from peripheral organs. PV also induces anaerobic metabolism, with an increase in lactic acid as a by-product. Interesting that the release of lactic acid into the bloodstream doesn’t occur (or at least significantly reduced) until Freediver resurface (at least this is what experiments on laboratory rats show).
For all of the above, you can say that blood shift (BS) happens (blood moves from non-vital organs to vital organs) when PV happens, but quite common Freedivers are using the term BS when describing the movement of the blood to the chest cavity to protect it from increasing pressure while diving deep.
Due to PV certain amount of blood pushed to the lungs, the capillaries in the lungs receive a greater blood flow and increase in size, compensating for space lost in the lungs due to increasing of ambient pressure. The lungs become filled up with the blood, which is reabsorbed when Freediver ascending.
IMPORTANT! Blood shift not pushing the blood into alveoli! It pushes it into capillaries around alveoli!
Why PV is very important for Freedivers? Well, it helps to move O2 from organs which can survive longer without it, to organs which are in constant demand of O2. So, it helps us to hold our breath longer and dive deeper (by moving blood to the chest cavity).
4. And the last but not least benefit of DR is the spleen contraction. Spleen in the human body has two main functions – mechanical filtration of red blood cells (RBC) and as a part of the immune system. We are interested in the first function. About 240 ml of RBC’s can be held in the spleen and released when needed (due to hypoxia for example). When the contraction of the spleen happens oxygen-rich RBC’s gradually start their journey to circulatory system increasing O2 carry capacity of our blood (and helping us to hold our breath longer).
Interesting that spleen not recovering fast, even after an hour it is only partially recovered (however there are studies which show that spleen can be fully recovered in size in less than 20 minutes).
5. This is not a benefit, but still part of DR. Immersion diuresis. Yes, this is an explanation 
why while Freediving you want to pee much often! As you know part of DR is PV and it causes increased blood flow to the torso area and increased blood pressure as a result. Our body detects it and releases a specific hormone responsible for liquid regulations, which increase urine production. Don’t be embarrassed because of it! But make sure that this reflex doesn’t make you dyhadrated (drink enough before and after Freediving session).
6. Another side effect of DR is faster muscles fatigue. And again you can blame PV. When PV happens and blood moves away from your muscles, they start to work in an anaerobic way and produce more lactic. And even after you finish apnea, the effect does not disappear quickly (depends how long and intense your apnea was). Do you need proof? Try to do DYN bi fins 100 meters and 100 meters surface swim (with the same fins) and compare how do you feel.
If you have any question about Freediving, let me know in comments below!