Explain the diffusion of gases in the alveoli, and alveoli adaptions

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Pulmunary alveoli  is a structure with hollow cavity that faciliates exchange of gases, particularly in mammalian lungs. Blood exchange occurs through diffusion. Diffusion is a process by which molecules move passively (that is, without energy input). It is dependent primarily on three things - surface area, distance (of membrane), and concentration gradient. In our lungs, this happen in the alveoli. Carbon dioxide-rich blood arrives in our lungs into alveolar blood vessles. At this point, diffusion takes over. The blood releases carbon dioxide, which we do not need, and absorbs oxygen from the alveoli. The exchange occurs through diffusion through the alveolar membrane. After this, the blood again circulates the body (now oxygen rich) while the carbon dioxide is released when we exhale. 

Diffusion in the alveoli is efficient because of some adaptations of the structure.

1. There are a lot of alveoli present thus increasing surface area for gas exchange. Remember that this is one of the factors that affect diffusion. More surface area means that diffusion can more easily occur as more atoms/molecules can pass through at a given time. 

2. Alveoli are only one-cell thick. This means that they are very thing. Another factor for diffusion is distance travelled. Having only a thickness of a single cell decreases this distance and promotes easier diffusion.

3. A requirement for diffusion is that the gas (in this case oxygen) should first be dissolved. Hence, the alveoli are also moist to facilitate oxygen dissolution, and ultimately, diffusion through the membrane to be absorbed by the blood.

4. Lastly, alveoli have a good supply of blood. This is practically to facilitate distribution of oxygen and elimination of carbon dioxide. More blood coming near these structures mean that the carbon dioxide-oxygen exchange occurs quickly.

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