Ollie was an oxygen molecule who lived with his friends in Airland.
One day, he was off to the park when suddenly he was sucked up the nasal passage of a giant monster!
Ollie was afraid. It was dark and he had the feeling he was not alone. He tried to hold on but he kept being dragged, down the trachea and round a sharp turn into the bronchi of the left lung.
The tubes were getting smaller and darker, until ‘KLUNK’, he landed in the alveolar space. It was light now and the alveolus was almost see-through, as its wall was very thin. All he could see everywhere were blue and red capillaries surrounding the alveolus.
“I’M TOO SQUASHED!”, complained a little old oxygen lady. Now, Ollie could see that there were hundreds of oxygens from Airland in the alveolar space with him. Ollie found a friendly-looking oxygen molecule and asked where they were going. The kind molecule told him that they were going to diffuse across the thin alveoli wall and capillary wall, to bind to haemoglobin.
Once across, Ollie was whipped away quickly towards the pulmonary vein, so that the steep concentration gradient could be maintained, between the alveolus and the blood stream. As he was carried away, he noticed a line of carbon dioxide molecules waiting to diffuse across the capillary endothelium, to be breathed out.