Partial Pressures: Dalton's, Henry's, & Fick's Laws
To understand gas exchange, we need to understand how the partial pressures of oxygen and carbon dioxide change as we move through the respiratory tract.
First, recall that the air we breathe is a mixed gas – it comprises nitrogen, oxygen, carbon dioxide, and water vapor.
Each of these gases exerts a different force on the surfaces it encounters (i.e., our respiratory tract); partial pressure is the pressure of a single gas in the mixture.
For example, atmospheric pressure at sea level is 760 mmHg – this is the total pressure of all gases in the atmosphere; the partial pressure of oxygen in the atmosphere is only a fraction of this, 160 mmHg.
Let's see how we use Dalton's and Henry's Laws to home in on specific gases in respiration.
Dalton's Law indicates that the partial pressure of a gas is the pressure that gas would exert if it occupied the total volume of a mixture.
In other words, the total pressure of a mixture of gas types is equal to the sum of the partial pressures of each gas present.
We show this with a series of containers:
First, draw a container of solution with two types of molecules; we'll call them "a" and "b."
The total pressure of the gases in this mixture equals 7 mmHg.
Then, show that this is equal to the partial pressure of "a," which happens to be 4 mmHg, plus the partial pressure of "b," which happens to be 3 mmHg.
The partial pressure of a gas is not the same as its concentration, but the two are related:
Henry's Law states that the concentration of a dissolved gas is equal to its partial pressure multiplied by a solubility coefficient.
Fick's law states that the diffusion rate is determined by: the diffusion coefficient (D), the partial pressure gradient between two points (P1-P2), the surface area (A), and, the thickness of the barrier (T).
Thus, we can use the following diffusion rate equation:
The rate of gas diffusion = (D
((P1 - P2)) A)/T
The diffusion coefficient multiplied by the partial pressure gradient multiplied by the surface area, all divided by the thickness of the barrier.
With this understanding, we can predict that respiratory diseases will negatively affect diffusion rates.
Emphysema is characterized by alveolar destruction, and, therefore, decreased surface area available for diffusion.
At the other extreme,
fibrotic diseases thicken the alveolar wall, and, therefore, increase the barrier to diffusion.
