The respiratory membrane includes millions of alveoli with a surface area as large as a tennis court. This large respiratory surface area, combined with other factors, makes for efficient gas exchange to meet our metabolic needs.
Gas Exchange: The Respiratory Surface
Oxygen and carbon dioxide move across the respiratory membrane
You may appreciate the fact that we breathe to get air into and out of our lungs. Have you ever wondered what happens inside the lungs? As we look closely, we can see that gas exchange occurs in the lungs between the air and our blood. Now, gas exchange in the lungs refers to the movement of oxygen into the blood and carbon dioxide out of the blood. Oxygen and carbon dioxide move across what we call the respiratory surface, and that consists of an alveolar epithelial cell and a pulmonary capillary endothelial cell. Now, this respiratory surface is also referred to as the respiratory membrane. Note that the air is on one side of the membrane and blood is on the other side of the membrane. As you can see from the image above, oxygen moves out of the alveolar air into the blood, while carbon dioxide moves in the opposite direction. Therefore, we use the term gas exchange, as oxygen is exchanged for carbon dioxide across the respiratory surface.
Gas Exchange Is Efficient
Our bodies require a lot of oxygen to help our cells make the energy they need to do work. Likewise, we need to get rid of the carbon dioxide that's made by our cells when they work. Therefore, gas exchange needs to be efficient to get enough oxygen into our body - and ultimately to our cells - and to get the carbon dioxide out of the body. So, let's discuss some of the factors that help gas exchange.
Gas Exchange Is Fast
First of all, gas exchange is extremely fast. The respiratory membrane is very thin, as it consists of only two cells - that is, the alveolar epithelial cell and the pulmonary capillary cell. The distance for gas exchange is only about a half a micrometer. That's smaller than the width of even your thinnest hair! This short distance allows gas exchange to occur very quickly. Furthermore, gases move across the respiratory membrane by diffusion, which is the movement of a substance from a high to a low concentration. Diffusion is extremely fast for short distances. If you put a drop of food coloring in a glass of water, for example, you can see just how fast diffusion occurs.
High Surface Area
The large amount of alveoli on the lung surface makes gas exchange efficient
Additionally, gas exchange is efficient because of the large surface area available for gas exchange. Each of our lungs contains hundreds of millions of these tiny alveoli. Gas exchange occurs across each of the alveoli. This large number of alveoli greatly increases the surface area for gas exchange. Now, the total surface area of alveoli is about 140 square meters or 1,500 square feet. Wow! That's about the size of a tennis court. So, if you wonder why we have alveoli, well, now you know.
Gases Are Lipid Soluble
Carbon dioxide and oxygen are lipid-soluble
Did you know that our cell membranes, including those that make up the respiratory surface, are composed of fat-like, or lipid, molecules? Now, that may seem gross, but it is nonetheless true and very important. You see, oxygen and carbon dioxide are both lipid-soluble molecules, and as such, they can slide right across the cell membrane. Now, if oxygen and carbon dioxide were not lipid-soluble, they would bounce right off the cell membrane, and they could not be exchanged.
In summary, gas exchange is the movement of oxygen into the blood and carbon dioxide out of the blood across the respiratory membrane in the lungs. This respiratory surface, also known as the respiratory membrane, includes the alveolar epithelial cells as well as the pulmonary capillary endothelial cells. Gas exchange occurs across this membrane and is diffusion of oxygen into and carbon dioxide out of the blood. Gas exchange is efficient due to a thin respiratory membrane, the high speed of diffusion, high surface area and lipid solubility of carbon dioxide and oxygen.
At the end of this lesson, you'll be able to:
- Explain how gas exchange occurs
- List the factors that make gas exchange efficient