Anhvanyds – Respiratory – Part 4: GAS EXCHANGE


Translation of pages 156-158, English in Medicine


As discussed earlier, the main role of the respiratory system is to supply O2 to the body and expel waste gases such as CO2. Gas exchange takes place between the alveoli and the capillaries in the lungs. To understand how gas exchange occurs, first look at the basic anatomy of the alveoli and capillaries in the lungs.

The alveoli are composed of elastin fibers, like those found in your skin that help keep your girdle tight. As you inhale, each alveoli expands and when you exhale it returns to its original size. Each alveolus is surrounded by capillaries. The walls of both alveoli and capillaries are very thin so that O2 and CO2 can pass easily between the alveoli and capillaries.

How do O2 and CO2 move between the alveoli and capillaries? O2-poor blood is carried from the heart to the lungs by the pulmonary artery. The pulmonary capillaries, the terminal branches of the pulmonary artery, surround each alveoli. When the capillaries carry O2 to the alveoli, CO2 passes through the walls of the alveoli easily. It travels to the center of each alveoli and is then exhaled out of the lungs. Likewise, when you breathe in, O2 moves down the airways down your lungs, into the alveoli, and then through both the walls of the alveoli and the pulmonary capillaries into the pulmonary capillaries. Once in the capillaries, O2 enters the red blood cells and then attaches to the hemoglobin molecules. The blood now has a high concentration of O2 and moves back to the heart through the pulmonary veins. Gas exchange takes place at the same time. In other words, O2 is entering the capillaries at the same time that CO2 is leaving the capillaries.

Gas exchange: O2 and CO2

The process that allows O2 and CO2 to move in and out of the blood stream is called diffusion. Diffusion is the actual movement of molecules from an area of ​​higher molecular concentration to an area of ​​lower concentration. For example, when capillaries carry CO2 to the alveoli, there is more O2 in the capillaries than in the alveoli. Therefore, CO2 is more likely to move into the alveoli than out of them. Likewise, when O2 molecules enter the alveoli, they are more likely to move into the capillaries where there is less O2 than into the alveoli where there is more O2.

You may temporarily think that the molecules move to the gaps because it is empty. However, that is not the case. Diffusion is simply the random movement of molecules. Molecules move all the time and they move to empty areas because they are not blocked by other molecules. Diffusion means that most of the molecules move in the direction to more empty regions, but not all of them. This is an example of diffusion.

Imagine that the two bins in Figure 10.7 are separated by a membrane, like the outer membrane of a cell, and that this membrane has holes large enough for blue molecules to pass through. Now imagine all the blue molecules moving regularly inside their crates. The molecules on the left frequently collide with other molecules. Since there are more molecules, some of them move randomly across the membrane to the other side. However, the molecules on the right have plenty of room to move. It would be less likely (but still possible) that 1 of them would migrate transmembrane to the left.

In the end, the two sides will have the same number of molecules. They will still move across the membrane but will not be diffuse in any one direction. Again, diffusion occurs only when the two regions have different numbers of molecules.


Now imagine that the blue molecules represent the O2 molecules. The barrel on the left represents the alveoli filled with O2. The barrel on the right represents the pulmonary capillaries with very little O2. So in which direction will the diffusion of O2 occur? The answer is on the pulmonary capillary side. (medical english drduy)

Finally, for gas exchange to take place, the alveoli must be filled with air. If the alveoli collapse, air will not be able to enter them and there will be very little O2 to exchange. Because there is water in the air you breathe, the water ends up in the alveoli. If there is only water lining the alveoli, the water molecules will attract other water molecules and pull the alveoli closer. They will collapse and you may die. so the alveoli are kept filled with air by a molecule called surfactant. Surfactant is a detergent-like molecule that helps prevent alveoli from collapsing by disturbing the water molecules lining the alveoli. Surfactant molecules keep water molecules away from other water molecules and thus keep the alveoli taut. Surfactant is produced by large, round cells in the walls of the alveoli.

gas exchange translation


When babies are born prematurely (born before their due date), they are not yet able to make surfactant. As a result, their lungs will tend to collapse. These premature babies are placed on a ventilator, a machine that helps fill the alveoli with air. Some hospitals also use a nasal spray, which actually helps spray surfactant into the lungs. Eventually, as children get a little older, they develop the ability to make their own surfactant and can then give up the CPR.

Translated by: Giang Dao Truc Quynh

See more:

>>Respiratory – Part 5: How to maintain a healthy respiratory system

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>>Medical English – Medical English – DR.DUY

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