1. Diffusion of Gases Review of the Physics and Physiology
RET 1613C
Mini Lecture 1
Dr. J. B. Elsberry
Special thanks to: Barbara L. Kenney, RRT
LSU, USC and UEL websites

2. Review of Important Gas Laws
Ideal Gas Law PV = nRT
Charles’ Law V1 /T1 = V2 /T2
Gay-Lussac’s P1 /T1 = P2 /T2
Boyle’s Law P1 V1 = P2 V2
*Examples:
Ventilation
Body plethysmograpy.. remember this from last semester
A gaseous cylinder
A Hot Air Balloon

3. Barometric Pressure
Gases in the atmosphere exert a force on the earth’s surface called the Barometric pressure
Pb at sea level is 760 mmHg (torr)
Pb decreases with an increase in altitude
Pb increases with a decrease in altitude

4. Gas Diffusion Diffusion Gradients random molecular activity
net movement of gas particles
from an area of high concentration to an area of lower concentration
Diffusion Gradients
the individual gas partial pressure differences across a membrane

5. How do Fish “breathe” ?

6. Human (Mammalian) External Respiration

7. Dalton’s Law of Partial Pressure
In a mixture of gases, the total pressure is equal to the sum of the partial pressure of each separate gas
In the Alveolus:
PO2 + PH2O + PCO2 + PN2 = PB
e.g.: add 5 mmHg of another gas to the alveolus and other gases are displaced….
If 10 molecules of a gas is in a container - total eesure is

8. Atmospheric Gases by apx. %
Nitrogen (N2) 78%
Oxygen (O2) 21%
Trace Gases 1%
( argon, CO2, radon, CO etc.)
density of gases change with changes in Barometric Pressure
Individual Gas Percentages stay the same, regardless of elevation

9. Partial Pressure
Partial Pressure is determined by the Barometric Pressure times the % of the gas in the atmosphere
O2 = 21% or .21 at sea level
760 x .21 = 159; so the partial pressure of Oxygen in this room is 159 mmHg
This is referred to as PIO2

10. This is what happens to Barometric Pressure as you ascend in the atmosphere…..

11. Internal Respiration: Gas exchange at the Cellular Level
External Respiration: Ventilation and Gaseous Diffusion within the Lung
Internal Respiration: Gas exchange at the Cellular Level

12. What about Water?

13. Water Vapor Pressure Water can exist as a gas, liquid or solid
When in the molecular form it exerts a partial pressure just as oxygen does

14. Saturated Alveolar gas PIO2 & PAO2
Gas in the alveoli is assumed to be fully saturated when at body temperature 37o C
A water vapor pressure of 47 mmHg is exerted in the human lung
H2O mass, fully humidified is 44mg/L

15. The Partial Pressure of Oxygen (PAO2)in the Alveoli is 100 mmHg
O2 must mix or compete with carbon dioxide and water vapor
by the time the O2 reaches the alveolus it is diluted by the CO2 and H2O
FIO2 = Fraction of Inspired Oxygen %
PIO2 = PP of Inspired Oxygen
PAO2 = PP of Alveolar Oxygen
PaO2 = PP of Arterial Oxygen

16. The Balance between CO2 Excreted and O2 Uptake
Every Cell in the body “bleeds” CO2 as part of Cellular Metabolism
Every Cell in the body requires O2 for aerobic metabolism O2
CO2

17. Factors that Affect the Diffusion Rate:
R = Respiratory Exchange Ratio VCO2 = 200 ml/min of CO2 removal VO2 = 250 ml/min of O2 uptake R = 200/250 = 0.8
Factors that Affect the Diffusion Rate:
membrane surface area
diffusivity of the gas or diffusion coefficient
molecular weight and solubility
membrane thickness
pressure gradients

18. Alveolar Capillary Membrane
alveolar fluid
alveolar epithelium
basal alveolar membrane
interstitial space
basal capillary membrane
capillary epithelium
Red Blood Cell

19. Dynamics of Gas Exchange in the Respiratory Exchange Zone:

20. The Oxygen Cascade

21. Alveolar Air Equation PAO2 = (Pb - H2O)FiO2 - PaCO2(1.25).8
In Fort Myers Pb is 760 mmHg & room air is 21% oxygen
PAO2 = ( ) (1.25)
(713) 99

22. A-aDo2 = Difference between Alveolar and Arterial Oxygen Tensions
Figure PAO2; then subtract the PaO2
Calculates the gradient b/w the alveoli and the blood to help identify problems associated with the A-C capillary membrane
PAO2 = 99 PaO2 = 95 difference is 5 mm Hg
normal gradient is 5 to 15 mmHg

23. Diffusion Rate Fick’s Law = A x D x (P1 - P2) T
A= membrane surface area
D= diffusion coefficient
molecular & solubility
inversely proportional to weight; > weight the < rate
directly proportional to solubility; > solubility the > rate
T = membrane thickness
P1 - P2 = pressure gradient

25. Law of Mass Action
Increasing surface area, diffusivity or partial pressure increases the diffusion rate
Increasing membrane thickness decreases diffusion rate
Graham’s Law - gas diffusion rate is inversely proportional to the square root of its gram molecular weight
Oxygen is lighter (smaller molecular mass); thereby it diffuses 1.7 x faster than CO2

26. Gases dissolve due to Driving Pressures
Henry’s Law - amount of gas dissolving in a liquid is directly proportional to the partial pressure of that gas; however,
CO2 is 24 x more soluble than O2
Combined Henry’s and Graham’s
CO2 diffuses about 20 x faster than O2

27. Capillary Transit Time
At resting Cardiac Output, a RBC is exposed to the alveoli for .75 seconds
Normal gas equilibrium is met after .25 seconds
during exercise, transit time is shortened
doesn’t affect healthy individuals
with vascular compromise, gases don’t reach an equilibrium so diffusion rate is decreased

28. Diffusion during Exercise
Diffusion at Rest
Diffusion during Exercise

29. Hypobaric Conditions Hypoxia 2o Hypobaric exposure:
Altitude and PO2
Hypoxia 2o Hypobaric exposure:

30. Dramatic Movie Clip Airport 1970

31. Perfusion and Diffusion Limitations
Oxygen diffusion is normally perfusion limited:
dependent on the amt and speed of blood flow
increases in blood flow decrease transit time
decreased transit time can prevent an equilibrium of gases thus
thus decreasing diffusion
Diffusion limited impairments occur when:
A/C capillary membrane is thickened
preventing an equilibrium of gases regardless of transit time
thus decreasing diffusion rate

32. Summary of Factors Determining Diffusion Rates
Reduced Diffusion Surface Area
Emphysema - destruction of alveoli
Tumors (distortion of lung parenchyma)
Pulmonary embolus
V/Q mismatch (COPD/Atelectasis/Pneumonia)
Increased Membrane Thickness
Fibrotic thickening of alveoli, capillary or interstitial space
Interstitial edema fluid
Dilated, engorged capillaries (CHF)

33. Summary of Factors Determining Diffusion Rates (cont.)
Blood Flow
exercise
low pulmonary capillary volume
low cardiac output (heart failure & blood loss)
Decreased Uptake by RBC
Anemia & Hb pathologies
High carbon monoxide levels (smokers)
Diffusivity
predetermined by gas molecule physics
Pressure Gradient
as the pressure gradient ↓
the diffusion rate↓

34. Remember Diffusion occurs at many levels
Preview to Next Week:

35. Let’s break… O 2 Transport Discussion is next

36. O2 transport can take 2 distinct pathways:
RBCTransport
Dissolved in Plasma

37. Whole Blood Transport components

38. Oxygen Transport due to Diffusion
Dissolved in Plasma
Henry’s Law - the amount of oxygen dissolved in plasma is directly proportional to the partial pressure or PO2 that it is exposed to.
As PO2 increases - dissolved O2 increases
.003 ml/dl of dissolved oxygen per 1 mmHg of PO2
At a PO2 of 100 the blood has .3 ml/dl of dissolved oxygen (100 x .003 = .3 ml/dl)
ml/dl can also be expressed as vols %

39. Henry’s law regulates dissolved gases

40. How Much O2 can be transported by the Plasma?
Let’s do the math…

41. Red Blood Cell— Gas Transport Workhorse

42. The O2 Journey at a Glance

43. How Much O2 can be transported by the Hemoglobin?
Let’s do the math…

44. Next Week Top Ten Review O2 Transport Mechanisms
Hypoxemia & Hypoxia Types
Clinical Implications of Hypoxia