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The principle of diffusion consists in the movement of substances through a membrane from a zone with high concentration, to a zone with low concentration, until the concentration is equal on both sides of the membrane.
An example of diffusion is the process that happens when we breathe. Two substances in the body that will move based on the principles of diffusion are Carbon Dioxide and Oxygen. Carbon Dioxide will move from the bloodstream to the alveoli, and Oxygen will move from the alveoli to the bloodstream.