Diffusion is the process by which atoms or molecules move from an area of high concentration to low concentration. The rate of diffusion is effected by a number of factors which include temperature, concentration and molecular mass. Diffusion is an important process within the human body and is essential to the transport of molecules within a number of organs including the lungs, kidneys, stomach and eyes.

The lungs possess millions of tiny air sacs that are called alveoli, each of which are in close contact with capillaries. As air is breathed in the alveoli inflate and oxygen diffuses across the alveoli wall and into the capillaries. At the same time, carbon dioxide, which is a waste product from respiration, diffuses from the capillary and into the alveoli. As the person exhales, the alveoli deflate and the carbon dioxide is breathed out of the lungs.

The kidneys remove waste products and help regulate the concentrations of ions and other small molecules. Kidneys consist of millions of small tubular structures called nephrons, which terminate on a semi-permeable walled structure called the glomerulus. Blood that contains waste is guided through a knot of blood vessels that are surrounded by a glomerulus. Small molecules such as water, sodium, and potassium glucose can pass through the glomerulus and into the nephron. The collective name for the material passing into the nephron is filtrate. Whilst filtrate contains a large amount of waste products it also contains molecules such as glucose that can be reused by the body. The tubule of the nephron is surrounded by capillaries that have a low concentration of useful molecules. Diffusion allows these molecules to reenter the bloodstream. The remaining waste molecules within the tubule is converted into urea.

The small intestine is part of the digestive tract and is responsible for the digestion of food and absorption of nutrients. The lining of the small intestine is covered by epithelial cells with tiny hair-like follicles known as micro-villi. Lipids can diffuse directly into the epithelial cells lining the small intestine where they are then processed by organelles. Other molecules such as amino acids are transferred into the epithelial cells with a process known as facilitated diffusion. In this process special transfer proteins within the membranes of epithelial cells help to remove the molecules from the small intestine.

The cornea in the eye does not have any blood vessels supplying oxygen to its cells. This makes the eye unusual in that it instead obtains the required oxygen by diffusion from the atmosphere. Oxygen first dissolves within the tears of the eye and then diffuses into the cornea. Similarly, carbon dioxide waste diffuses out of the cornea and into the atmosphere.