Olly and Olivia are two oxygen molecules that were enjoying a nice day floating along with the breeze when they saw a big crowd of people all cheering, so they went to check it out. 

They floated down towards the crowd where they saw people running a race. They stayed to watch, but a big gust of wind blew Olly and Olivia to the runners, and it went dark. 

The wind had taken Olly and Olivia through the runner’s nasal cavities where they got covered in mucus, dust and germs. They kept on going through the pharynx, which is known as the back of the mouth.

As they pass the pharynx, they start to go down a tube leading to the lungs called the trachea. Olly looks over while they are going through the back of the mouth, and he sees a flap that looks like a trap door. Olivia sees it too and she tells him that its called the epiglottis, and it's closed so they don’t go into the stomach instead.

Olly and Olivia keep going down the trachea and pass the larynx, which is also called the voice box. The more air that comes through the larynx, the louder the voice can be.

The pathway suddenly split into 2 pathways, one for each lung. These tubes are called the bronchi (one is a bronchus). Olly yelled goodbye to Olivia as the were separated, going down different bronchi.

They kept travelling and realised that these tubes kept splitting into even tinier tubes, kind of like an upside down tree. These smaller tubes are called bronchioles, and Olly and Olivia found it hard to squeeze through these narrow pathways because the runner has asthma.

When Olly and Olivia get to the end of the bronchioles, there are lots of little sacs that look like bunches of grapes, these are called the alveoli. Because the alveoli have thin walls, it makes it easy for Olly and Olivia to diffuse through them, into the tiny capillaries surrounding them. When this happens, its called gas exchange.

After the gas exchange in the alveoli, Olly and Olivia meet up again in the bloodstream and grab onto a special part of the blood cells called haemoglobin. This is so they can travel around and give all the energy they have to the cells in the runner's body. Before they can do this though, they have to go into the heart. They can hear it beating really fast as they go along with the blood.