Do you know how diffusion of gasses occurs between the alveoli and capillary walls? In the previous lesson we discussed how air reaches our lungs (click here to check out the previous notes). In this lesson, we will learn about how exchange of gases takes place in the lungs.
What is gas exchange?
Organisms obtain oxygen for aerobic respiration through the process of gas exchange. Gas exchange is the exchange of gases between the environment and an organism. In order to understand gas exchange in humans it is important to know:-
Q: How do unicellular organisms exchange gases?
Unicellular organisms ( organisms made of 1 cell ) have a large surface area to volume ratio, hence they do not need any special system for gas exchange because their body is in direct contact with the environment. Oxygen and carbon dioxide can be easily exchanged by diffusion through the cell surface membrane. Animals like humans have a small surface area to volume ratio, and their surfaces, unlike the unicellular organisms are thickened for protection that is why exchange of gases do not take place through the skin of humans but through a special organ like lungs. The absorption of oxygen and removal of carbon dioxide in humans occur in the alveoli or known as air sacs.
Process of gas exchange:-
From the heart, deoxygenated blood is sent to the lungs for exchange of gases (carbon dioxide and oxygen). The air we breathe in contains oxygen. Alveoli is surrounded by blood capillaries that maintain the concentration of gases for gas exchange. Capillaries bring the deoxygenated blood. Oxygen in the alveolar air, dissolves in the thin film of moisture, surrounding the walls of the alveoli and then diffuses inside the capillaries. There oxygen combines with hemoglobin, in the blood, to form oxyhemoglobin. Whenever the blood passes by any oxygen deprived tissues, oxyhemoglobin is converted back to oxygen and hemoglobin and oxygen is supplied to the required tissues.
In the same way, CO2 diffuses out of the capillaries in lungs. It is produced in the tissue cells due to aerobic respiration. Carbon dioxide diffuses in red blood cells and reacts with water to make carbonic acid. It is then converted into hydrogen carbonate ions in RBCs, carrying dissolved CO2. When capillaries reach lungs, they are converted back into carbonic acid and then back into CO2 diffusing out of the capillaries, into the alveoli and then into the environment.
In the next lesson, we will go discuss the mechanism of breathing in human.
*All pictures are taken from google*
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