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How is the diffusion of gases between the alveoli and the alveolar blood vessels facilitated?

Alveoli are small sacs at the end of the terminal branches of the lung (the brochioles). It is at the alveoli where gas exchange takes place - oxygen moves into the blood and carbon dioxide is removed. The blood then travels to the left side of the heart via the pulmonary vein, and from there it is distributed to the body to supply oxygen to the tissues. It is therefore crucial that gas exchange at the alveoli is effective. This is achieved in three key ways: 1) Both the alveolar walls and the walls of the pulmonary (lung) blood vessles are just one cell thick, and these cells are very thin. This decreases the distance across which gases must diffuse, facilitating gas diffusion. 2) Movement of air in and out of the alveoli during breathing, and movement of blood through the vessels surrounding them, removes substances being removed and replaces those being supplied in both the alveolar space and the alveolar vessels. This maintains a high concentration gradient - the concentration of oxygen remains high in the alveolar space and low in the blood, and vice versa for carbon dioxide. This allows for fast movement of these gasses from an area of high concentration to low concentration. 3) The lining of the alveoli is moist - this allows the gases to dissolve in the fluid lining the alveolar space and diffuse across the alveolar and vessels wall into the blood.

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