Remember that ATP is constantly being both produced from ADP and phosphate and also consumed to regenerate ADP and phosphate. When ATP is consumed, the ADP produced is stored in dense bodies inside blood platelets. When the cell undergoes cellular respiration to regenerate ATP, it can use the supply of ADP in the platelets for raw material. This is also similar with the phosphate group. The phosphate group is highly polar and can be dissolved and stored in water for use later when it is needed. So this really is a perfect cycle where the products from one part of the cycle are reused as starting materials of the other part of the cycle.