When you excercise in the heat, however, a competition for blood occurs between the exercising muscles (which need blood for the oxygen and for removal of waste) and the skin, as the body tries to cool down.

The result of this is that in hotter temperatures you are unable to exercise for as long as you would be able to in cooler temperatures, as the muscles become starved of oxygen, and waste such as lactic acid builds up in the tissues. In the heat our blood also becomes thicker, as moisture is lost through the sweat glands.

As the blood gets thicker, the flow through the blood vessels becomes more turbulent, and this may activate the clotting system. Small clots can form, blocking blood vessels and leading to heart attacks and strokes.