Why do we have to eat at all? It is to feed the cells. If there were no cells, there would be no reason to even have a digestive system. Why do we even have lungs? It is to feed the cells. If there were no cells to feed there would no need for the lungs to exchange air. Why have a heart and blood vessels? Why have hormones? Much of their role has to do with controlling how much nutrition enters the cells at different times. Instead of looking at each of these systems separately, we can just look at those parts of all of your systems that are involved in controlling the quality and quantity of food being delivered to the cells, and we call that the Nutrition Delivery System.
High Intensity Versus Low Intensity Fuel
Sugar System: Whenever you have to perform precisely or powerfully, the involved cells rev up accordingly. The strong activity of the cell is represented by a thick wall on the picture.
A powerful acting cell requires much more sugar. The sugar can be burned rapidly to recharge the batteries, but at a cost of it being burned incompletely, which is also why higher intensity performance has to be short-lived. The sugar debris builds up.
Oxygen System: The oxygen system involves the complete digestion of the sugar. When the cell acts mildly like when you are relaxed or just walking, after a few minutes the cell starts eating more oxygen. It is the deficit of the oxygen and related nutrients that stimulates more flow. This system can run clean all day without the being clogged up as is the case in the sugar system.