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Particles in liquids and gases are in constant motion. Because of this, particles of a substance will spread evenly throughout a liquid or gas and this process is called diffusion.
Remember that diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration.
Diffusion is an essential process in the human body. Many substances move into and out of living cells by diffusion.
In the lungs, oxygen that has been breathed in through the nose travels down into the lungs where it needs to get into the blood to circulate round the body. The way the oxygen gets into the blood is by diffusion. There is a high concentration of oxygen in the lungs and low concentration in the blood, so the oxygen diffuses down the concentration gradient into the blood and away.
Similarly, the body needs to get rid of the waste carbon dioxide it has in the blood, so the carbon dioxide moves from an area of high concentration in the blood to an area of lower concentration in the lungs, where it is then breathed out through your mouth.
Let's take a look at some more examples of diffusion in action in the body in the following activity.
