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In the lungs, the bronchi branch out into smaller tubes known as bronchioles. The air in the bronchiole then pass into million of tiny air sacs called alveoli which are present at the end. In the alveoli diffusion of gases takes place. There is more oxygen present in the alveoli than in the blood of the capillaries. Therefore, the oxygen move from the air to the blood through diffusion. Since there is more carbon dioxide in the blood than in the air, Carbon dioxide diffuses from the blood to the air in the alveoli. It is sent out of the body when the air is exhaled.

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The main respiratory surface in humans is the alveoli,[5] which are small air sacs branching off from the bronchioles in the lungs. They are one cell thick and provide a moist and extremely large surface area for gas exchange to occur. Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli. They are also extremely thin, so the total distance gases must diffuse across is only around 2 cells thick. An adult male has about 300 million alveoli, each ranging in diameter from 75 to 300 µm. Inhaled oxygen is able to diffuse into the capillaries from the alveoli, while CO
from the blood diffuses in the opposite direction into the alveoli. The waste CO
can then be exhaled out of the body. Continuous blood flow in the capillaries and constant breathing maintain a steep concentration gradien
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