The Mammalian Dive Response
You don’t need to be a whale or an elite freediver to comfortably dive to depths you probably consider beyond your reach.
All mammals, including you, have an innate ability to dive. As a human being and mammal you are equipped with some amazing ‘tools’ to dive beneath the surface.
When your face is immersed in water the Dive Response is activated autonomically. Then what happens?
One of the most primal but oh so important responses is apnea. Your usual reflex to breathe stops. This I think is very important for a novice freediver to realise. When diving you won’t feel the sudden need to breathe under water. This of course up to a certain point when you will reach your CO2 treshold. Then you will receive a signal from your body urging you to breathe again. But more of that in another post.
What else happens in your body when you dive on a breath-hold?
Short-term (they will end once you finish your dive) physiological reactions during a breath-hold dive include a slower heart rate (bradycardia) and blood flow, greater contractility of your heart, ejection of extra blood cells from your spleen (spleen contraction), centralisation of blood flow (peripheral vasoconstriction) and opening of capillaries (central vasodilation). Other effects include hormonal regulation, muscle fatigue and immersion diuresis (yes, we all pee in our suits). And I am sure this is not all since we are complex organisms that are not yet completely understood.
The parasympathetic response of bradycardia is counteracted by the the sympathetic nervous system to prevent a dangerous drop in arterial blood pressure. This is done through peripheral vasoconstriction and redistribution of blood flow. Less blood flow will be available to your extremities and non-essential muscles. More blood flow will be available to the central nervous system and heart.
Why does this happen?
Basically it is a survival mechanism to save oxygen.
Simply said, during your dive this mechanism reduces your oxygen consumption by regulating heart rate and metabolism and by elevating your oxygen carrying capacity. Elevated levels of hematocrit (the percentage of red cells in your blood), hemoglobin (an oxygen-binding protein in your blood), and myoglobin (oxygen-binding protein in muscles) make sure you are capable of transporting more oxygen in your blood to places where it is needed.
The result is that during a breath-hold dive you have more oxygen available while using less. This is why it actually feels easier holding your breath while diving on a breath-hold compared to holding your breath on the surface.
What I find fascinating is that your body is so smart at adapting to its environment without you consciously making any decisions.
Yes, by training you will eventually be able to increase your breath-hold and the dive response apparently becomes stronger when you dive more frequently. But this capacity to dive is within all of us, already from the moment we are born.
(Update 2024: Not all the above mentioned responses of the body are solely happening when diving. Therefore in some definitions of the dive response for example the spleen contraction is not included. Since the spleen contraction can also occur under other circumstances such as exercising). That being said it is still an important part of the dive response influencing your capacity to hold your breath.