Humans share some adaptations with dolphins, whales and seals: the dive response, which minimises the body’s use of oxygen and protects it from hypoxia (low oxygen).

The dive response is the name given to a group of reflexes all mammals possess. It is the most important aspect of breath-hold, as it protects the human body from any adverse hypoxic effects. The dive response is an oxygen conservation mode – like fuel economy mode in a car – and its purpose is to optimally manage the body’s oxygen stores. It ensures constant oxygen supply to the most important parts of the body during a breath-hold – notably the heart and brain – and minimises the rate that oxygen is consumed in other unessential areas. Much like other aspects of human physiology, like strength, flexibility or cardiovascular efficiency, the dive response can be improved through specialised training. In elite divers, the body’s use of oxygen becomes highly efficient (similar changes, albeit smaller, are seen in synchronised swimmers). For instance, trained divers may hold their breath for three minutes with no effect on the oxygen levels in their blood; for a world record level diver, this may be true for a breath-hold of over five minutes. Of course, this is the result of years of slow improvement training under experienced instructors. It does not happen overnight!

The dive response is triggered by any (or all) of the following:

  • High levels of carbon dioxide: your body’s carbon dioxide levels begin to rise as soon as you start to hold your breath
  • Low levels of oxygen: change in blood oxygen levels stimulate the response
  • Facial or full body immersion in water: cold water strengthens the response

What does the dive response do?

The dive response is characterised by a number of adaptive mechanisms that are triggered during a breath-hold, which slow the rate of oxygen consumption in the body, protecting it from hypoxia. These include:

  • Bradycardia (slowing of the heartbeat), to less than 20 beats per minute;
  • A shift to anaerobic metabolism for all muscle activity, meaning that muscle use does not burn oxygen;
  • Splenic contraction – over half the oxygen in your body is located in the blood. The richly oxygenated blood stored in the spleen is released during breath-hold, which can boost the amount of oxygen-carrying red blood cells and haemoglobin by almost 10%; and
  • Blood shift, which includes
    • Peripheral vasoconstriction – constriction of the blood vessels in the arms and legs to minimise blood flow to extremities;
    • Increasing circulation to heart and brain, and decreasing blood flow to less important parts of the body, such as gastrointestinal and renal systems; and
    • In deep diving, increased blood flow to lungs allows the blood vessels to expand as the pressure increases and air spaces begin to shrink, protecting the lungs from the pressures at depth. This process reverses itself as the diver returns to the surface.