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.
The uptake of oxygen and the release of carbon dioxide by the blood of the alveolar capillaries can be explained by diffusion, i.e., the gases pass from the regions of high pressure to those of low pressure. The pressure of the gas refers to the partial pressure that the gas exerts in a mixture of gases. If the atmospheric pressure at the sea level is 760mm of mercury, the partial pressure of oxygen will be 21% of 760 mm, or 159 mm. The partial pressure of oxygen in the alveolar air is about 100mm of mercury (since oxygen constitutes only 13% of alveolar air) compared with that of 159mm of mercury for the oxygen in the atmosphere. Accordingly, the oxygen diffuses from the alveolar air into the blood. By the time the blood leaves the capillaries its oxygen pressure has been raised to about 100 mm of Hg, the same as alveolar oxygen.
In a similar manner, the carbon dioxide in the blood of the lung capillaries has a higher concentration than it has in the lung alveoli. This accounts for the diffusion of carbon dioxide out of the blood into the alveoli. by the time the blood leaves the lungs, its carbon dioxide pressure has been lowered to approximately 40mm. Thus, in the diffusion of both these gases there is a marked diffusion gradient that determines the direction of their flow.