Oxygen Diffusion Lexicon

Previously I wrote posts about how we breathe and why we breath. This lexicon will show how oxygen moves from the atmosphere, through the lungs and arteries, and through the tissues.

We get the following information from page 129-133 of Egan's Fundamentals of Respiratory Care (6th Edition):

1. Gaseous Diffusion: This is the process molecules move from an area of high concentration to an area of low concentration. For Diffusion to occur a difference must occur between partial pressures. For example, p1 must be higher than p2 (this will make more sense as you read on).

2. Kinetic Energy: This is the driving force behind diffusion. For this reason, diffusion is diffusion is most rapid among gases, as compared to liquids and solids.

3. Graham's Law: This law basically states that lighter gases diffuse rapidly, and heavier gases diffuse more slowly. Because diffusion is based on kinetic energy, anything that increases the activity of molecules (like heat) increases the rate of diffusion.

4. Gas Pressure: All gases exert pressure, whether free in the atmosphere (air), enclosed in a container (oxygen tank), or dissolved in a liquid (blood). Pressure is based on kinetic energy (which is random) and gravity. Since graviity increases the density of gas, the pressure exerted on a gas is highest near the surface of the earth and becomes less the higher up you go.

5. Tension: The pressure of a gas is referrred to as a tension.

6. Atmospheric Pressure: This is how we measure the pressure exerted by a molecule (such as air). Since there are many layers of molecules at the surface of the earth, these molecules exert a pressure on the earth. This is measured by a barometer. Since there are fewer molecules the higher you go, the atmospheric pressure is lower the higher you go. You'll see the significance of this on how we oxygenate as we continue.

7. Dalton's law: The pressure exerted by a mixture of gases must equal the sum of the kinetic activity of all the component gases. The Pressure of air is the sum of Nitrogen and oxygen.

8. Fraction Concentration: The percentage of a gas in a mixure. In air, oxygen is 21% and nitrogen is 79%, or states as a fraction of inspired oxygen, oxygen = 0.21 and nitrogen 0.79. Usually this is listed as: FiO2 = 0.21, N2 = 0.79 and CO2 = zero.

9. Partial Pressure: The pressure exerted by a single gas in a mixture.

10. Dalton's law: The pressure exerted by a mixure of gases must equal the sum of the kinetic activity of all the component gases.

11. Partial Pressure Formula:

• Partial Pressure = Fractional Concentration * total pressure
• PO2 = 0.21 * 760 = 160 mm Hg
• PN2 = 0.79 * 760 = 600 mm Hg
• Pressure of Air = 760 mm Hg

12. What if the barometric pressure were to change? Assume we go up, and the atmospheric pressure decreases. Say we go up to 30,000 feet in an airplane. The barometric pressure outside the airplane is 226 mm Hg. If the cabin were not pressurize, would you be able to oxygenate?

• Partial Pressure = Fractional Concentration * total pressure
• PO2 = 0.21 * 226 = 47 mmHg

At a PO2 of 47 you would not be able to breathe. You would go unconscious. This shows why people become dyspneic as they climb a mountain. So what happens if we add an oxygen mask at 0.70 FiO2?

• PO2 = 0.70 * 226 = 158 mmHg

Now we are getting enough oxygen to oxygenate. Still, why is 158 PO2 enough and a PO2 of 48 not enough. Let us see why.

We get the following from pages 266-269 of Egan's Fundamentals of Respiratory Care (6th Edition):

13. Diffusion gradient: This is the difference between the partial pressure of a gas at location 1 (P1) and partial pressure of a gas at location 2 (P2).

Diffusion gradient for oxygen at the alveolar capillary membrane: 100-40=60

(If the inspired oxygen is only 46, the pressure gradient would be too low to work.)

Diffusion gradient for carbon dioxide at the alveolar capillary membrane: 46-40 =6

14. Diffusion of Oxygen: See #1 above, which explains that for diffusion to occur P2 must be less than P1. Thus, partial pressure of oxygen is like this from air to capillary level:

• At sea level: PO2 = 160 mm Hg
• Alveolar level: PO2 = 100 mm Hg
• Capillary level: PO2 = 40 mm Hg
• Gradient = 60

So you can see that as you inhale, it's easy for oxygen to move into the lungs and then to the capillaries because the PO2 is lower as we move into the body.

Once oxygen enters the capillary, the PO2 of the capillary is similar to the PO2 at the alveolar level, so diffusion to the tissues is easy:

• Capillary PO2 now 100
• Cellular PO2 is 40
• Room air PO2 is 0
• Gradient again is 60

Hypoxemia: This is lack of oxygen in the blood.

• Alveolar PO2 of 100 is normal
• Alveolar PO2 of 60 is hypoxemia
• Alveolar PO2 of 40 is critical
• Alveolar PO2 of 21 is death

So if you are inhaling a PO2 of 47, there isn't much of a pressure change for the oxygen to travel to the lungs, and by the time it gets there it won't be enough oxygen (it will definitely be less than 40) to support life. You will lose consciouslness.

However, as you increase your FiO2 to 70% at 30,000 feet, you will be able to support life. Yet as you go even higher, that added FiO2 will do you less good.

So you can see why people need to take care as they proceed to higher altitudes. You can also see that patients who have compromised lungs (such as COPD patients) also need to take care. Consider the COPD patient with a normal PO2 of 50 at 30,000 feet on room air.

If the normal for the COPD patient is 50 breathing 0.21 FiO2 when the atmospheric PO2 is 160, then if the inspired PO2 is only 47, the PO2 of the COPD patient will be something like 23 if that. This will cause hypoxia and death.

However, so long as the cabin is pressurized in an airplane, no person should be exposed to a problem. Yet at one moves to higher altitudes on earth, as one moves up a mountain for instance, you can see how a COPD patient might become dyspneic before a person with normal lungs.
Diffusion of carbon dioxide (CO2): CO2 diffuses from the capillaries to the atmosphere by the opposite means. The partial pressure of capillary CO2 is greater than atmospheric CO2:

• Capillary CO2: pCO2 = 46


• Alveolar CO2: pCO2 = 40
• Air CO2: PCO2 = 0
• Gradient = 6

Once CO2 leaves capillary, the capillary CO2 is now similar to what it is at alveolar level, so diffusion of CO2 from tissues to lungs is easy:

• Cellular PCO2 = 46
• Capillary PCO2 = 40
• Gradient = 6 again

Diffusion capacity: Oxygen diffuses much slower than carbon dioxide, and that is why oxygen needs a normal diffusion gradient of 60 and carbon dioxide needs only a gradient of 6. Disease processes that effect diffusion of gases will have more an effect on oxygen than carbon dioxide.

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