Diffusion of gases across the respiratory membrane

2021-05-17 12:11 AM

The respiratory membrane is a combination of respiratory bronchioles and alveolar ducts alveolar and capillary walls and their fused basement.


After the alveoli are ventilated, the next step in respiration is the exchange (diffusion) of O2 from the alveoli into the pulmonary blood and the diffusion of CO2 in the reverse direction. from the blood to the alveoli. Diffusion is simply the random back-and-forth diffusion of gas molecules across the respiratory membrane and adjacent fluids. However, in respiratory physiology, we are concerned not only with the basic mechanism of diffusion but also with the rate at which it occurs, which requires a great understanding of the physics of diffusion and gas exchange.

Let's first learn the structure of the respiratory unit (also known as the lobules), it is a small branch of bronchi around it, surrounded by alveoli. There are about 300 million alveoli in 2 lungs, each alveolus has an average diameter of about 0.2mm, the alveolar membrane is very thin. Surrounding the alveolar membrane, there are densely surrounded alveolar-capillary plexuses, so the alveolar gases are very close to the blood of the pulmonary capillaries.

Respiratory unit

A: Surface view of capillaries in the alveolar wall. B: Cross-section of alveolar walls and their vascular supply.

The respiratory membrane is a combination of respiratory bronchioles and alveolar ducts alveolar and capillary walls and their fused basement.

The structure of the respiratory membrane is drawn in the upper left part of the red blood cell. The respiratory membrane consists of 6 layers:

The first layer is the surfactant layer, this layer lining the inside of the alveoli has the effect of reducing the alveolar surface tension.

2nd layer: Is the alveolar epithelial cell layer, it is a thin layer.

The third layer is the basement membrane of epithelial cells.

Layer 4: A thin interstitial space between the alveolar epithelium and the capillary membrane.

5th layer: The basement membrane of the capillary, in this membrane there are many places that mix with the basement membrane of the capillary.

6th layer: Endothelial layer of the capillary membrane.

Although there are 6 layers, the thickness of the respiratory membrane is thin from 0.2 µm to 0.6 µm. For an adult male, the total area of ​​​​the respiratory membrane is about 70m2. The total amount of blood in the pulmonary capillaries ranges from 60ml to 140ml, we see that with a small volume of capillary blood, the capillary has a large total area, so it will be easy for the exchange of CO2 and O2.

The superstructure of the alveolar respiratory membrane is shown in cross-section.

The diameter of the pulmonary capillaries here is 5 µm and the average diameter of mature erythrocytes is 7.2 µm, so in order to have a gas exchange, red blood cells must pass through the capillaries, but because the red blood cell membrane is easily changed, it is easy to pass through Thus, the red blood cell membrane is close to the capillary membrane. CO2 and O2 diffuse back and forth from the alveoli with red blood cells and accelerate respiration.