Exercising in the heat is very stressful for your body. In warmer temperature environments, a significant amount of blood is redirected from the working muscles to the skin in order to cool the body. Cooling (which is very important to your organs as you don't want to overheat!) happens from sweating and evaporation. As you sweat, less blood is available to transport oxygen to the working muscles. This would be similar to what your body experiences when training at altitude. But in humid conditions, the air is full of water vapor and it can't hold any more water so sweat has a difficult time evaporating (which means it can't act as a cooling mechanism like it would in low humidity). As a result, you are more likely to experience dehydration. When you are dehydrated, you lose blood volume which causes your blood to get thicker. This makes it harder for your heart to supply blood to your muscles. As a result, your heart has to work even harder to pump that blood (resulting in...
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