I find that a bath after hard training helps me recover. But a hot bath makes my heart race. Why?
Physiological Homeostasis. This effect would also apply to stepping into a bath filled with ice water (but most of the actions and effects would then be in the opposite direction).
Lots of things tend to happen when you slide into that piping hot tub. For our purposes, the relevant facts are:
- Core temperature rises and the body has to do something about that. It expands blood vessels and pushes more blood close to the skin to dissipate heat. Your heart rate goes up in order to pump more blood to get rid of the extra heat.
- Peripheral vasodilatation reduces the pressure of the vascular system because the “pipes” are increasing in diameter, blood pressure drops. The heart attempts to pump more blood in order to maintain an adequate blood pressure.
- Pain stimulates the sympathetic nervous system. Scalding hotness causes most people at least some discomfort. Pain signals the hypothalamus and pituitary to release adrenocorticotropin hormone (ACTH) which stimulates the adrenal glands to release adrenalin with subsequent elevation of your pulse rate.