Oli the Oxygen molecule had had a long day in the atmosphere with his friend, the Nitrogen molecule. But then, whilst floating downwards towards a human, fell under his nose during the human's aerobic respiring (a form of respiration using oxygen) and was sucked up. Oli had a journey down the larynx and trachea and into the right main bronchus. Immediately after, he entered a narrower tube like the bronchus called the bronchioles and into a small air sac.
Oli had arrived in an alveoli! Other oxygen molecules (like Oli) ended up with him, which starts diffusion. You see, when Oli was inhaled, other oxygen molecules came with him, which increases the amount of oxygen molecules inside the alveoli to a concentration of a greater amount than what's in the deoxygenated blood.
Oli started to diffuse through the lining of the alveoli and into the bloodstream of the capillary, a branching of blood vessels in between the vein and the artery. Oli makes friends with the red blood cells when he's there.
Whilst the human respires, carbon dioxide is produced with Oli and a carbon molecule. This makes the concentration of carbon dioxide in the blood to increase, becoming greater than the amount of carbon dioxide in the lungs. Now Oli and his new friend Carbon start to diffuse.
Carbon dioxide (Oli and the carbon molecule) inside the blood starts to diffuse back through the lining of the alveoli and into the alveoli. Now Oli, in the form of carbon dioxide, will be exhaled.
Oli will now exit the body through exhalation, the same way it entered. So Oli comes out the alveoli, through the bronchioles and right main bronchus and up the trachea (also known as the windpipe). Finally, he will pass the larynx and out the nose/mouth! Oli is now a carbon dioxide molecule and is very happy! But Oli is now on a flower and will be used for a plant's respiration..... And the story continues!!