Lucille was finding it difficult to play soccer after school. Her doctor tested her cells and found that they contained low levels of oxygen. When Lucille shared these results with a friend, her friend said there might be a problem with Lucille’s respiratory system. What do Lucille’s cells need in order for her body to function properly? How might a problem with Lucille’s respiratory system lead to low levels of oxygen in her cells? Why does having low levels of oxygen in her cells make it difficult for Lucille to play soccer?