This physiological response has been mentioned so many times that I decided it needed its own specific post to plug into the Cold Water Swimming articles section.
What is peripheral vasoconstriction?
Following immersion in cold water, blood flow is reduced in human limbs and skin.
Why does peripheral vasoconstriction happen?
Peripheral vasoconstriction occurs to allow the body to retain core heat for longer as protection against hypothermia by allowing the skin to act as an insulating layer between core organs and the water.
It also allows more oxygen to be delivered to important and oxygen-sensitive organs such as heart and brain.
When does peripheral vasoconstriction happen?
Immediately on entering cold water, it’s initiated by cold receptors (aka thermoreceptors, which are different kinds of heat or cold detecting nerves) in the skin. The body doesn’t have to be fully immersed, just the lower legs being in cold water is sufficient.
Is any part of the body not affected by peripheral vasoconstriction?
The head is not effected. (Heat loss during cold water immersion is greatest in the head, neck, upper chest and groin).
At what water temperature does peripheral vasoconstriction happen?
Some sources seem to indicate that peripheral vasoconstriction occurs at temperatures of below the Lower Critical Temperature (LCT). Lower Critical Temperature is the temperature at which heat will leave the body regardless of peripheral vasoconstriction. LCT can be as high as 35º Celsius (95º Fahrenheit) though this is based on a non-swimming person. For swimmers the LCT is affected by many more factors such are body size, sub-cutaneous fat, metabolic rate, muscles work and wind. In thermo-neutral temperature water, which is water between 35º Celsius and body temperature, peripheral vasoconstriction alone is sufficient to retain core body heat.
It’s important to note that the difference between water and ambient air temperature plays an important part. For cold-acclimatized swimmers, LCT will be much cooler than 35º. Scientific studies of hypothermia in cold water use 18º Celsius (64º Fahrenheit) as the upper level of “cold” water.
What are the effects of peripheral vasoconstriction?
- The entire body surface will get cold and the skin will feel clammy, even when normal core temperature is retained. (I’ve measured my own skin temperature at 17º Celsius after a swim while I still felt fine and you can see in the image below that after a short swim the subject was barely over 17º Celsius).
- Fingers and toes will turn white.
- Blood pressure will rise.
- There will be a sudden desire to urinate (due to increased blood pressure). This is called cold water diuresis (aka “why do I have to pee after swimming?“)
- Longer term or more severe effects such as loss of motor control and shivering etc. are effects of hypothermia.
How long does peripheral vasoconstriction last?
Peripheral vasoconstriction is not a continuous process. One research paper indicates that cold water immersion can also cause a paradoxical response of an intermittent recurring peripheral vasodilation (“cold-induced vasodilation” aka CIVD), which opens up blood vessels and accelerates blood loss. This seems to alternate with peripheral vasoconstriction with the cycle repeating from three to five times an hour.
When does peripheral vasoconstriction stop?
One important factor with peripheral vasoconstriction is that it stops shortly after leaving water and the person regains a standing position. This results in the well-known effect to open water swimmers of Afterdrop. Afterdrop occurs because the cold blood in the periphery which was previously acting as barrier layer and protection quickly flows into the warmer core and causes internal organs to suddenly decrease in temperature.