Oh no! Bob the oxygen molecule is being sucked into someone's nose!
It looks like Bob is in the trachea. The trachea is a tube that air flows through. There's a flap called the epiglottis that closes down over the trachea to make sure that food doesn't get into it.
Bob is going to enter the lungs now. The lungs are an organ in your chest that is the most important part of the respiratory system. Oxygen is absorbed by the blood in the lungs.
Bob is now in the bronchus. Bronchi are small passages in your lungs that carry air.
Bob is going into the bronchioles. They are branches of bronchi that air passes through to get to the alveoli.
Bob is in the alveoli now. They are tiny sacs where the exchange of oxygen moves into the bloodstream. From the alveoli, Bob will move to the capillaries.
Bob is now in the capillaries. The capillaries are the smallest blood vessel that allow the exchange of oxygen and carbon dioxide.
Bob's now in a finger. Bob's job is to give the cells in this finger energy to function. The cells will use the energy from Bob and after that they'll send carbon dioxide back to the lungs.
Bobby the carbon dioxide molecule is going through the bloodstream to get to the capillaries. At the capillaries, Bobby is going to be transfered back to the lungs.
Bobby has finally reached the lungs! From the lungs, he is now going to be set free and go out of the body!
Bobby is free! :)