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In massage school we are being taught hydrotherapy -- applying cold and heat to specific areas.

It says that when applying cold, first the body goes through vasoconstriction, and then later vasodialation. That makes sense -- preserve heat through the former, but if the cold is applied for too long the body still has to keep the cells alive, thus the latter.

What doesn't make sense is this passage, in relation to applying heat:

Law of Action and Reaction: Initially blood will rise to the surface, applied for too long the blood will recede to the interior causing inner congestion to be greater than before

Is that true?

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Local warming of a patch of skin will cause vasodilation. However, after prolonged warming there is a 'die away' phenomenon, is which blood vessels slowly constrict back to their normal diameter.

Johnson JM, Kellogg DL Jr. Local thermal control of the human cutaneous circulation. J Appl Physiol (1985). 2010;109(4):1229–1238. doi:10.1152/japplphysiol.00407.2010
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2963328/

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You are right, it doesn’t make sense. No matter what, the inner organs have to function at some level, producing heat which the body has to transfer to the surroundings, otherwise the inner temperature will rise. The main physiological mechanisms for losing heat are to increase the blood flow to the skin (vasodilation) and sweating (evaporation). And the body will try to use these mechanisms, no matter how futile it may be in a high-temperature/high-humidity environment.

Actually, neither does the suggested responses in low temperature make much sense. Again, the body has to preserve its inner temperature, this time by reducing the blood/heat-transfer to the skin (vasoconstriction), shivering and raising the feathers or fur. Maintaining the core temperature is paramount, and eventually, the skin and extremities will be sacrificed if needed. Numerous examples can be found!

There is however a small amount of truth in the hypothermic response: people adapted to cold environments can control bloodstream to different extremities (fingers or feet), avoiding frost-bites or for dumping excess body heat after heavy exercise, avoiding to sweat all over the body (Ref.: Schmidt-Nielsen, Animal physiology adaptation and environment, Cambridge University Press 1990).

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    $\begingroup$ Welcome to Biology.SE! We prefer answers that are supported by references. Can you edit your answer to include references? $\endgroup$ – De Novo Mar 14 at 17:32

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