Diffusion

 Diffusion is the movement of atoms, molecules, ions or particles from an area of higher concentration to an area of lower concentration down the concentration gradient.

This movement does not stop until the concentration of particles is uniform throughout the system. Diffusion is a passive process that does not require energy, particles only move from one area to another due to collisions amongst them and their concentration gradient.

Oxygen and carbondioxide exchange in the lungs in the respiratory system takes place by Diffusion.  Oxygen from the air which is high concentration diffuses across the alveoli membrane in the lungs then also into the bloodstream where it can be transported to cells throughout the body which is a low concentration area.

Carbondioxide on the other hand diffuses from the bloodstream into the alveoli where it is in high concentration so that it can be exhaled out into the atmosphere where it is in low concentration.