There's a trick to describing how two systems work together, and it's always the same: name the thing they exchange. Systems link up because one hands something over to another. Find the hand-over and you've found the link.
The Hand-over Rule
Every link between two body systems is a delivery of something. So to describe any link, say three things in order: who hands it over, what gets handed over, and who receives it. The four hand-overs you need are short: respiratory → blood: oxygen (the lungs load oxygen in); digestive → blood: glucose (the gut loads the fuel in); circulatory → cells: oxygen + glucose (the blood carries both and delivers them); and cells → blood → out: carbon dioxide and wastes (cells hand their rubbish back, and the blood drops it at the lungs and kidneys).
The Three-step Method for Any Question
One — name the two systems. Which two are we linking? Respiratory and circulatory? Digestive and circulatory?
Two — name the thing they exchange. This is the heart of it: oxygen, glucose, carbon dioxide or waste. Pin down the single thing being handed over.
Three — say which way it flows, and why the receiver needs it. “Respiratory loads oxygen into the blood; circulatory carries it to the cells, which need it to release energy from glucose in respiration.” Finish on what it's for — that last clause is where the marks live.
A Worked One, Slowly
Explain how the respiratory and circulatory systems work together to keep a muscle cell supplied. Name the two systems: respiratory and circulatory. Name the exchange: oxygen. Now the flow with a reason: the respiratory system breathes air into the lungs and loads oxygen into the blood; the circulatory system then pumps that blood to the muscle cell, which needs the oxygen to release energy from glucose in cellular respiration. Two systems, one exchange, the flow and its purpose. That structure earns full marks every time.