Diffusion is the movement of molecules from areas where they're highly concentrated to less-concentrated areas. Diffusion occurs throughout the human body and is crucial to many metabolic processes. The molecules that diffuse can be solids or gases, nutrients or wastes. Many organs, such as the lungs and small intestine, use diffusion to keep us alive and healthy.
Respiration In The Lungs
Respiration is an example of diffusing gas molecules that takes places in the lungs. The oxygen we inhale is exchanged for carbon dioxide in tiny air sacs in the lungs. Oxygen-depleted blood goes near the lungs, and carbon dioxide diffuses into the air sacs where it is eventually removed through exhalation. At the same time, oxygen diffuses from the lungs into the bloodstream. After it diffuses into the bloodstream, it is picked up by blood cells. These blood cells move the oxygen across the body where it diffuses into other cells requiring oxygen.
Neurons, or brain cells, diffuse ions across their membranes as part of a larger communication network in the nervous system. They send electrical signals by the diffusion of ions across their cell membranes. This releases an electric charge that flows from one cell to the next until it reaches the brain. The cell maintains a negative electric charge by controlling which ions go in and out of the membrane. Electric flow between neurons allows the brain to receive and process information about sensations we experience, such as physical pain and touching things in our environment.
The kidneys help maintain levels of molecules that the blood needs or doesn't need through diffusion with its membranes. For example, the kidney can move sodium into the blood when levels are low. They also take urea, a waste molecule, from the blood into their cells, which have a lower concentration of urea. The urea is eventually removed from the body by urination. Dialysis, which is the artificial regulation of kidneys with a machine, also relies on membranes to regulate the levels of ions and other compounds that pass through the kidneys.
Diffusion In The Intestine
Diffusion also happens in the intestine after the food we ingest has been completely broken down to extract nutrients. Specialized cells in the small intestine take up these nutrients. Sometimes the nutrients are too large to be absorbed directly by the cell membrane, so they have to be swallowed up by the whole cell. After these intestinal cells absorb the nutrients, they transfer them to blood cells in vessels on the other side on the membrane. Blood cells take up the nutrients and carry them to various parts of the body.