The document summarizes key aspects of breathing and respiration in the human body. It explains that (1) cells require oxygen to function, so we must breathe to supply oxygen through the respiratory system. It then (2) outlines the cycle of breathing where air enters the nose and trachea, oxygen diffuses into the blood in the lungs, and carbon dioxide diffuses out. Finally, it (3) describes the major functions and structures of the respiratory system including the lungs, alveoli, and transportation of gases in the bloodstream.