Alveoli is the functional unit of the lungs. It is a grape like structure adhered by networks of tiny blood capillaries.There are millions of alveoli in our lungs. The wall of the alveoli and the capillaries together form Alveolar-capillary membrane which plays important role in the process of diffusion.
Diffusion is the process by which the molecules from the higher concentration pass through the semipermeable membrane to the lower concentration. If you understand this process then it will be easy for you to understand the gas exchange mechanism between the lungs and the blood.
At the end of the inspiration, your alveoli contain oxygen which is normally higher than the oxygen(partial pressure) in the blood capillaries that surround the alveoli. So, the oxygen pass through the Alveolar-capillary membrane and enter into the blood by the process of diffusion. Similarly, the Co2 concentration(partial pressure) is higher in the blood capillaries(de-oxygenated blood) than the Co2 within the alveoli. So, as the result of diffusion, the Co2 from the blood enter into the alveoli and released to the environment when the person expires the air out.
Hope, you have got the message.
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