4. Physical principles of gas exchange. O2 and CO2
Molecules move randomly & rapidly in relation to each other
Net diffusion is from [high] to [low]
Partial pr. of the gas is proportional to [gas]
 nitrogen  79% 600 mmHg
 Oxygen  21% 160 mmHg
According to Henry's law the partial pr. of a gas in sln. depend on:
1- concentration
2- solubility coefficient
P gas = _concentration of dissolved gas______
solubility coefficient
gas
Sol.
Co. O2
0.024
CO2
0.57 CO
0.018

5. Solubility coefficient :
Molecules dissolved in water if they are attracted to water more can dissolved without build up excess partial pressure within the solution as CO2

6. Physical Principles of Gas Exchange
Diffusion in response to concentration gradient
Pressure proportional to concentration
Gas contributes to total pressure in direct proportion to concentration
CO2 20 times as soluble as O2
Diffusion depends on partial pressure of gas
Air is humidified yielding a vapor pressure of 47 mmHg.

7. Determinants of Diffusion
Ficks Law
Diffusion = (P1-P2 ) * Area * Solubility
Distance * MW
Pressure Gradient
Area
Distance
Solubility and MW are fixed

8. Determinants of Diffusion
Diffusion coefficient proportional to Solubility
Different gases at the same partial pressure
Will diffuse proportional to their diffusion
coefficient MW

9. Composition of Alveolar Air
Pn2 = ( ) * 0.79 = 713 * 0.79 = 563
Questions:
What is the effect of humidification on the partial
pressures?
Explain the expired air partial pressures?
Calculate Po2 in alveoli

10. Diffusion between gas phase & dissolved phase
Net diffusion is determined by gradient
Vapor pr. of H2O is the partial pressure that water excretes to escape through the surface
at normal body temperature 47 mmHg
the greater the temperature the greater kinetic activity higher PH2O
Temerature
PH2O
0 ̊C
5 mmHg
100 ̊C
760 mmHg

13. VA VT F I E A VD
Expired air has alveolar and dead space air

14. Po2 IN THE ALVEOLI PAlvO2= PIO2 - (PCO2/R) PO2 = 149 - (40/0.8) = 99
R is respiratory exchange ratio ~0.8
Remember in a normal person alveolar PO2 = arterial PO2, and
alveolar PCO2 = arterial PCO2 .

15. Pco2 IN THE ALVEOLI
PCO2 = CO2 production * K
Alveolar Ventilation
K is constant
If ventilation is doubled then Pco2 is ½
If ventilation is halved then Pco2 is doubled

16. Question
A person is breathing from a gas tank containing 45% oxygen. What is the alveolar PO2? A. 149 mmHg B. 250 mmHg C. 270 mmHg D. 320 mmHg E. 340 mmHg

17. Answer
760 – 47 = * 0.45 = 321 mmHg = inspired PO2 Alveolar PO2 = (40/0.8) = = 271 mmHg

18. Question
An alveoli that has normal ventilation and no blood flow (V/Q=0) has an alveolar PO2 of A. 40 mmHg B. 100 mmHg C. 149 mmHg D. 159 mmHg
O2=?

19. V/Q = 0 V/Q = normal V/Q =  O2 = 150 CO2 = 0 O2 = 150 CO2 = 0 O2 = 40

20. Ventilation/perfusion
Physiologic shunt
Va/Q < normal
low ventilation
Physiologic dead space
Va/Q > normal
wasted ventilation
Abnormalities
Upper lung Va/Q 3 x normal
Lower lung Va/Q .5 x normal

21. Diffusion rate (D) proportional to P x AxS d x MW
S : diffusion coefficient of gas.
√MW
A-cross- sectional area
S-solubility of the gas
d-distance
P- pressure gradient
Gas
Diffusion co. O2 1
CO2
20.3 CO
0.81 N
0.53

22. Most gasses are lipid soluble so the diffusion in tissue is similar to diff. in water because these gases can pass easily through the cell membrane.
Rate at which alveolar air is renewed by atmospheric air
FRC 2.3 L only 350 ml of new air each breath. One seventh of the total, so many breaths are required to exchange most of the alveolar air.
Half of gas will be removed in 17sec.
Why this graduate clearance:
- to prevent sudden change in [gas] in the blood. - to make respiratory control mechanism much more stable.

25. Concentration of gasses in alveoli
O2 is supplied by inspiration and removed by diffusion
PO2 is controlled by:
a- rate of diffusion into blood (250ml/min)
b- rate of O2 entry by ventilation
normally Po2= 104 mmHg in alveoli  if alv. Ventilation 4.2L/min

27. Concentration of gasses in alveoli
CO2 in alveoli depends on:
a- rate of CO2 excretion
b- ventilation rate
If vent. Rate= 4.2 l/min, and rate of excretion = 200ml/min PCO2 40 mmHg
Expired air= alveolar air+ air in dead space

31. Diffusion through respiratory membrane
300 million alveoli, each alveolus with the diameter of 0.2 mm.
Respiratory membrane:
1-fluid layer with surfactant
2-epithelium of alveoli
3-basement membrane of epithelium
4-interstitial space
5-capillary basement membrane
6-endothelial cells of the capillary

32. Diffusion through respiratory membrane
Respiratory membrane specifications:
– 0.6 μm
2- 70m2 surface area
3- total volume of blood ml
4- capillary diameter is 5 μm so RBCs
squeeze through

33. Diffusion through respiratory membrane
Diffusion rate depends on:
1- thickness
2- surface area
3- Pgas gradient
4-Diffusion coefficient
Diffusion capacity: the volume of a gas that will diffuse through the res. membrane each minute for a partial pressure difference of 1mmHg

36. Diffusion through respiratory membrane
O2 21 ml/min/mmHg .
11 mmHg Mean O2 part. pr. In all lungs
230 ml/min “at rest” .
65 ml/min/mmHg “exercise”
CO2  ml/min/mmHg “at rest”.
 ml/min/mmHg “exercise”
average of P CO2 gradient is 1mmHg