Answer :

When You Inhale Oxygen come in it goes through trachea bronchi and lungs and blood carry oxygen to the body cells to remove carbon dioxide
1. Inhale Oxygen
2. Goes to lung (bronchi - alveoli)
3. Exhale Carbon Dioxide

How to know that blood is with O2 or CO2:

Vein = Blood + CO2 = Blue
Artery = Blood + O2 = Red