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Diffusion is the spread of molecules from regions of high to regions of low concentration. Molecules that are concentrated will randomly move about(diffuse) and eventually, after colliding, they get distributed uniformly, reaching an equilibrium. In terms of gas exchange, the respiratory gases oxygen and carbon dioxide are exchanged at the thin, moist respiratory surface called alveoli, air sacs located in the lungs. These are surrounded by blood capillaries, thin-walled vessels which allow diffusion across their surface. When oxygen is transported into the respiratory system when one inhales, the high concentration of oxygen in the alveoli, diffuses through these thin- walled air sacs and enters the blood capillaries. There, oxygen is absorbed by hemoglobin-bearing red blood cells and carried throughout the body. There is a gradient between the high concentration of oxygen in the alveoli and a lower concentration of oxygen in the circulating blood. This causes the movement of oxygen into the bloodstream, from the external environment. When cells produce the waste product carbon dioxide, it is transported by the blood back to the alveoli. Because there is a higher concentration of carbon dioxide in the blood capillaries, relative to the alveoli, carbon dioxide will then diffuse from the bloodstream, to the alveoli and one can then exhale it to the environment. Therefore, respiratory gases rely on diffusion through thin, moist surfaces between the external environment and the internal body.
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