Lucille is finding it difficult to play soccer after school. Her doctor thinks that her cells might not be getting enough oxygen. When Lucille talked about it with a friend, her friend said there might be a problem with Lucille's respiratory system or circulatory system. What do Lucille's cells need in order for her body to function properly? How might a problem with Lucille's respiratory system or circulatory system make it difficult for Lucille to play soccer?