Re: I'm reasonably fit, so why do I go bright red when a do strenuous exercise?

Why does my face get red and blotchy when I exercise?

There are a number of reasons why this happens, and none of them have to 
do with overall physical fitness. The best reason that I can think of it 
temperature regulation. 

When you exercise, you build up a lot of excess body heat from muscular 
activity. One of the ways the body cools itself is to change the course of 
blood flow, from the body core where the heat is building up to the 
surface where it can be dissipated into the environment. The skin all over 
your body will show a deepened redness due to this process, but the parts 
of the body with the greatest amounts of blood vessels just below this 
skin will show the most color change. The face has a lot of small blood 
vessels just below the surface, and so it will show a greater color change 
than other parts of the body. This phenomenon is also more generally 
noticeable in fairer skinned people, the lighter your skin color the 
greater the color change due to exercise.