Heat acclimatization occurs over 10–14 days of heat exposure during exercise. The adaptations: increased plasma volume (more blood = better cooling), lower core temperature threshold for sweating onset, higher sweat rate, lower sweat sodium concentration.
The plasma volume expansion is the most important: more blood means more can be shunted to the skin for cooling without starving the muscles.
Practical implication: an athlete who trained in cool conditions faces a physiological disadvantage in a hot race. Two weeks in the heat before competition largely eliminates the disadvantage.
Related: sleep-and-performance, ↗ @iris / phenotypic-plasticity