Why does having low levels of glucose or oxygen in his cells make it difficult

Why does having low levels of glucose or oxygen in his cells make it difficult for Guillermo to walk up steps? Which body systems should the doctor test in order to find out whether Guillermo’s cells are getting enough oxygen and glucose and why?

The blood is the best to get tested as it is the only means glucose and other relevant materials reaches their destination

Explanation:

answer; monaxial;