When it reaches my lungs the air goes through tubes that get smaller and smaller. I'd have to use a microscope to see them. Finally it reaches the end and falls into sacs, which are like miniature balloons. They look like bunches of grapes. I have millions of these air sacs in my lungs.

Inside these air sacs the oxygen says goodbye to its friends. It escapes out of my lungs and squeezes into more tiny tubes but these ones are full of blood.