Remember oxygen wants to go to all parts of our body. In my blood the oxygen catches a ride with a bus called a red blood cell.One teaspoon of blood would have about 25 million of these buses. It is usually pretty easy to get a ride. It catches a ride with a red blood cell just like I catch a bus to go to school or to go downtown.

The buses deliver their passengers, oxygen, to all the cells in our body. Before oxygen can have a seat on the bus, however, another part of the air has to get off. Carbon dioxide, or CO2, is made in my cells. Big names for such little things. Too much carbon dioxide can be poisonous so I have to get rid of it. I just send it to my lungs. It leaves the bus as the blood goes through the lungs and jumps into an air sac. Now I breathe it out. That's a relief.