How does oxygen get into the bloodstream?
Inside the air sacs, oxygen moves across paper-thin walls to tiny blood vessels called capillaries and into your blood.
A protein called haemoglobin in the red blood cells then carries the oxygen around your body. At the same time, carbon dioxide that is dissolved in the blood comes out of the capillaries back into the air sacs, ready to be breathed out.
Blood with fresh oxygen is carried from your lungs to the left side of your heart, which pumps blood around your body through the arteries.
Blood without oxygen returns through the veins, to the right side of your heart. From there it is pumped to your lungs so that you can breathe out the carbon dioxide and breathe in more oxygen.