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 an elevated heart rate). Additionally, when fluid levels drop, you have a harder time controlling body temperature which causes the core temperature to rise even faster.
Although you are losing sodium when you sweat, you do not lose sodium without losing water. Dehydration comes from fluid and sodium loss - not directly from sodium loss. You must drink enough of a well-formulated sport drink (each hour) to properly keep yourself hydrated during your training session.
The bottom line is that the harder you work in the heat, the more heat is generated from the working muscles. Blood in your body competes for the working muscles (energy), the skin (to cool you) and the gut (aid in absorption and digestion of calories). The greater the metabolic heat production, the more energy is needed to cool the body (top priority to prevent you from overheating) - which means more stress on the cardiovascular system. Heat stress also causes an increase in fluid, electrolyte and glycogen loss (which is why pacing is critical) but the heat impacts the ability to effectively digest and absorb calories and fluids. Thus the greater risk of GI issues in higher heat environments.
Even if you do an excellent job hydrating during your workout (aka consuming a well-formulated sport drink) you will still experience some (or a lot) dehydration. As a result, your blood volume (volume of blood, specifically plasma and red blood cells) and total body water decreased. Conversely, sodium content in blood increases.
Although you may crave plain water, drinking plain water after a sweaty workout session may quench your thirst but it's not hydrating you. Instead, you are diluting your blood before blood volume has returned to normal values. The end result is that you will urinate a great amount of diluted urine (to bring the concentration of blood to a normal level) but you are not rehydrating yourself.
To help optimize rehydration after a hot and sweaty workout, consider the following DIY rehydration beverages.
