Gas Exchange: Diffusion & Partial Pressure Gradients

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  • 0:05 Gas Exchange Occurs in…
  • 1:40 Diffusion and Partial Pressure
  • 4:11 Composition of Alveolar Air
  • 5:04 Diffusion and Henry's Law
  • 7:06 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

If you've ever experienced shortness of breath on top of a mountain, this lesson is for you. Oxygen and carbon dioxide move into and out of our blood by diffusion. The rate of diffusion is determined by partial pressure gradients across the respiratory membrane in our lungs. Partial pressure is a function of both concentration and atmospheric pressure.

Gas Exchange Occurs in the Lungs

Have you ever wondered how oxygen gets from the air into our blood? Do you know how carbon dioxide gets out of our blood and enters our lungs? While ventilation ensures that air gets into and out of our lungs, it alone does not get oxygen into and carbon dioxide out of our blood. To answer our opening question, we need to look at how gas is exchanged in our lungs.

In this context, gas exchange simply refers to the movement of oxygen into the blood and carbon dioxide out of the blood. Oxygen and carbon dioxide move across the respiratory membrane, which includes the alveolus and pulmonary capillary. As you can see from the image below, oxygen moves out of the alveolus into the capillary, while carbon dioxide moves in the opposite direction - hence, the term gas exchange.

Gas exchange refers to oxygen moving into the capillary and carbon dioxide moving into the alveolus.
Gas Exchange

Gases are exchanged between the alveolar air and the blood by diffusion, the movement of molecules from an area of higher concentration to an area of lower concentration, where concentration refers to how much of one substance is present in a mixture of substances. The rate of diffusion is influenced by a variety of factors, including atmospheric pressure and the magnitude of the concentration gradient of the diffusing substance. In this lesson, we will discuss how atmospheric pressure and concentration gradients of oxygen and carbon dioxide influence diffusion and, thus, gas exchange.

Diffusion and Partial Pressure

It is a common misconception that there is less oxygen at high elevations.
Air Thin at High Altitudes

If you've ever been skiing or mountain climbing, you have probably experienced shortness of breath. You may have said or heard someone else say that the air is thin at high altitudes. What does it mean to say the air is thin? A common misconception is that less oxygen is available at high elevations. As it turns out, however, the concentration of oxygen remains pretty much the same regardless of the elevation. The concentration of oxygen is about 21% at sea level and on top of Mount Everest. Therefore, shortness of breath at high elevations is not due to less oxygen. That being the case, what causes shortness of breath at high elevations? To answer this question, we have to consider the effect of pressure on diffusion.

Atmospheric pressureDalton's lawpartial pressurePgas = Tp x [gas]PgasTp[gas]

At sea level, atmospheric pressure is 760 mmHg; therefore, the partial pressure of oxygen would be 160 mmHg, or 760 mmHg x 0.21. The base camp for Mount Everest is about 5000 meters above sea level, and the atmospheric pressure there is only about 400 mmHg. As the concentration of oxygen is still 21%, the partial pressure of oxygen is only 84 mmHg, or 400 mmHg x 0.21. So you see, at base camp, only 84 mmHg pressure pushes the oxygen into our blood, compared with 160 mmHg at sea level.

Composition of Alveolar Air

Once in the alveoli, the partial pressure of oxygen drops due to water vapor and carbon dioxide.
Drop in Partial Pressure

Now that we've discussed how concentration and pressure affect diffusion, let's consider what happens to the concentration of oxygen in our lungs. By the time the inspired air reaches the alveoli where gas exchange occurs, the concentration drops from about 21% to about 13%. At sea level, this reduces the partial pressure of oxygen from 160 mmHg to about 100 mmHg, as 13% of 760 mmHg is about 100 mmHg. What is responsible for this drop in partial pressure? The addition of water vapor and carbon dioxide to the inspired air decreases the percent composition of oxygen. As you can see in the picture above, the partial pressure of carbon dioxide in the alveolar air is about 40 mmHg.

Diffusion and Henry's Law

As we have stated, gas exchange occurs across a respiratory membrane. The respiratory membrane separates the alveolar air on one side from the blood on the other. Let's take a look at Henry's law. Henry's law tells us that pressure gradients affect the movement of gas into and out of a solution in a liquid.

For example, carbon dioxide is added to soda pop under high pressure. As long as the can is closed, the carbon dioxide stays in the pop, as the pressure in the pop is equal to the pressure in the can. When the can is opened, the pressure in the can decreases and carbon dioxide leaves the pop and enters the air. The same thing happens in our lungs as the gases move in and out of the blood from a high to a low partial pressure.

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