Physical principles of gas exchange. O2 and CO2€¦ · Physical principles of gas exchange. O2 and...
Transcript of Physical principles of gas exchange. O2 and CO2€¦ · Physical principles of gas exchange. O2 and...
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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
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• 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
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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.
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Determinants of Diffusion
Ficks Law Diffusion = (P1-P2 ) * Area * Solubility
Distance * MW
• Pressure Gradient
• Area
• Distance
• Solubility and MW are fixed
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Determinants of Diffusion
Diffusion coefficient proportional to Solubility
Different gases at the same partial pressure Will diffuse proportional to their diffusion coefficient
MW
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Composition of Alveolar Air
Pn2 = (760 - 47) * 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
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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
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VA
VT
F I F E
F A
VD
Expired air has alveolar and dead space air
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Po2 IN THE ALVEOLI
PAlvO2= PIO
2 - (PCO
2/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 .
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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
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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
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Answer
760 – 47 = 713
713 * 0.45 = 321 mmHg = inspired PO2
Alveolar PO2 = 321 - (40/0.8) = 321 - 50 =
271 mmHg
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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=?
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O2 = 40
CO2 = 45
O2 = 40
CO2 = 45
O2 = 100
CO2 = 40
O2 = 40
CO2 = 45 O2 = 100
CO2 = 40
O2 = 150
CO2 = 0
O2 = 40
CO2 = 45
O2 = 150
CO2 = 0 O2 = 150
CO2 = 0
V/Q = 0 V/Q = normal V/Q =
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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
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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
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• 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.
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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
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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
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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
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Diffusion through respiratory membrane
• Respiratory membrane specifications:
1- 0.2 – 0.6 μm
2- 70m2 surface area
3- total volume of blood 60-140ml
4- capillary diameter is 5 μm so RBCs
squeeze through
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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
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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 400 - 450 ml/min/mmHg “at rest”.
1200-1500 ml/min/mmHg “exercise”
average of P CO2 gradient is 1mmHg