Each system hands something over to keep the cells supplied. Pick the thing exchanged for every link, then check whether your supply chain holds together.
Respiratory system
→ loads →
Digestive system
→ loads →
Circulatory system
→ carries →
Cells (in respiration)
→ hand back →
Remember the hand-over rule: name who hands what to whom. Respiratory loads oxygen, digestive loads glucose, circulatory carries both, cells hand back the rubbish.