Oxygen is in the air and your body needs oxygen to carry out Respiration, so your chest expands to fill the lung cavities. Oxygen flows into your lungs, and at the alveoli gas exchange occurs. Oxygen flows from high concentration to low concentration. (Diffusion) Since your blood in your body is oxygen deficient, diffusion occurs and oxygen flows into your blood. Blood obtains oxygen through a protein called hemoglobin. CO2 is also transported along with CO2 and since their is more CO2 in your body than the environment it to diffuses. And your blood is not oxygen-rich and travels throughout your body to deliver the oxygen. :) And repeats, that's a simplified version if it, but I hope this helps.
