Oxygen - It enters our blood through the respiratory system when we breathe, Carbon dioxide - It leaves our body when we breathe out, Respiratory system - It is made up of the nostrils, the trachea, the bronchi and the lungs, Nostrils - Holes in our nose, Trachea - Tubes that divides into two smaller tubes called bronchi, Lungs - Two sacks protected by the ribs. Inside of them oxygen passes from the air into the blood and carbon dioxide passes from the blood into the air,

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