The Next Few Minutes

Your heart begins to pump blood faster and directs it to the muscles. Significantly less blood is directed into some parts of the body which are not needed at the moment, such as organs in the digestive system.

In order to better utilize glucose, muscles need increased oxygen levels which cause rapid and difficult breathing.

Running activates the largest muscle in the body -- gluteus maximus (your butt). You can activate the legs and abdomen muscles to help you in an upright posture and controlled movement. You begin to expend calories and deposited fats.

Spending glycogen and increased oxygen intake will raise body temperature. In order to cool down, the body begins to direct blood to your skin, causing redness. After that sweating starts up which will effectively cool the body.

The Next 10 Minutes

If you are in decent shape, your muscles and their supply of adenosine triphosphate are sufficient to continue to run. Your body can not effectively carry oxygen and consume fat and sugar.

If you are in poor physical condition, the supply of adenosine triphosphate is not sufficient to ensure the proper functioning of muscles. Muscles cannot get oxygen quickly enough, and the lactic acid begins to increasingly loose. Every minute will be more and more difficult.

After 30 Minutes

Finally it is over. Switch from running to walking. Now the energy starts to drop, and breathing gradually becomes normal.

You will probably feel great and have a great energy level. Your brain starts releasing dopamine. It is very likely that you will want to eat something sweet. Do not worry, because the glycogen consumed glucose molecules, thus the entered calories will not turn into fat, but in energy.