What does diffusion do in the respiratory system and circulatory system

Diffusion in the Lungs trades out Carbon Dioxide (CO2) for Oxygen (O2) In the Circulatory system it trades Oxygen (O2) for Carbon Dioxide (CO2)

Explanation:

As you breathe the Lungs take in air, as they do that the air travels to the bronchi, from there it goes through a process where the veins which have carried de-oxygenated blood to be diffused and reloaded with Oxygen and trade off their CO2. Then the blood flows through arteries and delivers the needed O2 to cells throughout the body and the cycle repeats.