What is the original source of heat while exercing?

The organ responsible for the elevated temperature in your body would be the heart. When you exercise your heart pumps harder to get blood to the muscle groups you are using. Thus supplying them with oxygen and nutrients.

This movement of blood within your body will cause your body to heat up. This then signals your skin to produce sweat in order to cool itself off.

This does not mean that if you are riding a bike only your legs will heat up and sweat. You're body's initial reaction will be to try to cool off your head - in order to protect your brain from overheating. That is why you will first start perspiring around your head and neck.

(posted after tottie3) - Your metabolism, energy is what causes the heat. You're body's natural response to this heat is to try to cool it down. Your metabolism is what causes your body to sweat, but the sweat is what actually cools the body down. (I was a biology major in college)

A lot of heat is generated by your muscles during intense exercise, but they aren't the only ones - your entire body temperature rises.

Your skin is the most important organ in the regulation of body temp. I

It is your muscles working, and using energy that produce heat. The muscles you are working as well as your heart and lungs because they work harder, which this too produces heat.

your energy,metabolism,not your sweaat that is like your coolant