Particles - molecules and ions - in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.
If there is a situation where particles of a substance are in a higher concentration, they will move from this region to where they are in a lower concentration. This is called diffusion.
It is important to remember that the particles:
Some substances move into and out of living cells by diffusion.
Carbon dioxide returns from the body in the blood and diffuses into the alveoli.
Oxygen diffuses into the blood from the alveoli and the blood carries it to all body cells.
Urea diffuses from the cells in the liver into the blood which carries it to the kidneys for excretion.