Here’s the story.
Function of alveoli
The function of the alveoli is to get oxygen into the blood stream for transport to the tissues, and to remove carbon dioxide from the blood stream.
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Structure of alveoli
In the lungs, air is diverted into smaller and smaller microscopic branches called respiratory bronchioles, which connect to the alveolar ducts.
At the end of each duct are approximately 100 alveolar sacs, each containing 20 to 30 alveoli that are 200 to 300 µm in diameter.
Each alveolar membrane is one cell thick and is in direct contact with capillaries that are also one cell thick.
There are about 600 million alveoli in the lungs, with a total surface area of about ##”75 m”^2##.
The large alveolar surface area, combined with the thin membranes, allows gases to diffuse easily across the alveolar walls.
The diameter of the capillaries surrounding the alveoli is so small that only one red blood cell can pass through at a time.
This slows down the blood cells, so that gases have more time to diffuse through the capillaries.
It also forces the red blood cells close to the walls of the capillaries, decreasing the diffusion distance.
The mechanism of gas exchange
Oxygen from the inhaled air diffuses through the walls of the alveoli and the capillaries into the red blood cells, which carry it through the blood to the body tissues.
Carbon dioxide produced by the body’s tissues returns to the alveoli via the blood.
It then diffuses across the capillary and respiratory membranes into the air space to be removed by expiration.