GAS EXCHANGE IN HUMANS
In humans the gas exchange organ system is the respiratory or breathing system. The main features are shown in this diagram.
The actual respiratory surface is on the alveoli inside the lungs. An average adult has about 600 million alveoli, giving a total surface area of about 100m", so the area is huge. The walls of the alveoli are composed of a single layer of flattened epithelial cells, as are the walls of the capillaries, so gases need to diffuse through just two thin cells. Water diffuses from the alveoli cells into the alveoli so that they are constantly moist. Oxygen dissolves in this water before diffusing through the cells into the blood, where it is taken up by haemoglobin in the red blood cells. The water also contains a soapy surfactant which reduces its surface tension and stops the alveoli collapsing. The alveoli also contain phagocyte cells to kill any bacteria that have not been trapped by the mucus.
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