Well, somehow my cells know to generate more ATP molecules because the skeletal muscle and the cardiac muscle will need those ATP molecules to basically increase the rate of contraction. So for instance, the heart will be able to pump more blood and the rate at which the heart actually pumps the blood to the tissues will increase as a result of those ATP molecules. Now, the question is how do the cells know to basically begin carrying out these processes? Well, the short answer is the cells actually respond to changes in the chemicals in the molecules that exist on the outside of the cell. And these chemicals, these molecules can actually influence the processes that the cells carry out. And these processes ultimately lead to physiological responses such as running away from that hungry bear.