06 lecture 5 gases laws and alveolar equations
TRANSCRIPT
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GASES LAWSGASES LAWS
and and
ALVEOLAR EQUATIONALVEOLAR EQUATION
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DefinitionsDefinitionsF – fraction of gas in a given medium
– I: Inspiratory air– A: alveolar space
P– partial pressure of a gas in a given medium– I: Inspired air – A: alveolar space – a: pulmonary capillary
V – rate of production/movement of gas– VA: alveolar ventilation– VCO2: rate of production/elimination of CO2
– VO2: rate of production/elimination of O2
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Boyle’s Law (P1Boyle’s Law (P1XXV1=P2V1=P2XXV2)V2)
As the size of closed container decreases, pressure inside is increased
The molecules have less wall area to strike so the pressure on each inch of area increases.
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Charles lawCharles law
At constant temperature
2
1
2
1
TT
VV
* If temp rises gas volume increases too
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For Ideal gasFor Ideal gas
TRnVP
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Dalton’s LawDalton’s Law Each gas in a mixture of gases exerts its own
pressure– as if all other gases were not present– partial pressures denoted as p
Total pressure is sum of all partial pressures– Fir inspired air at atmospheric pressure (760
mm Hg) = pO2 + pCO2 + pN2 + pH2O– to determine partial pressure of O2-- multiply
760 by % of air that is O2 (21%) = 160 mm Hg
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Dalton’s lawDalton’s law
In dry gas
In air saturated with water vapor
xbarx FPP
xOHbarx FPPP )(2
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Dalton’s Law of Partial Dalton’s Law of Partial PressurePressure
Total Pressure = Sum of individual gas pressures.
Partial Pressure = Pressure that each gas would exert if it were alone.
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What is Composition of Air?What is Composition of Air?
Oxygen – 21% or 159.6 torr
Nitrogen – 78% or 592.8 torr
Carbon dioxide - .004% or 3 torr
Water vapor – 1.25% or 8.75 torr
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Alveolar GasesAlveolar Gases
Nitrogen- 74.9% or 569 torr
Oxygen- 13.7% or 104 torr
Carbon dioxide- 5.2% or 40 torr
Water vapor- 6.2% or 47 torr
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Gas ExchangeGas Exchange
Earth’s atmosphere is about 78% Nitrogen and
about 21% O2
What happens to the air when we inhale?
GAS INHALED EXHALED
O2 20.71% 14.6%
CO2 .004% 4.0%
H2O 1.25% 5.9%
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Gas Exchange in Lungs & Gas Exchange in Lungs & TissuesTissues
Gases in Atmosphere[%]
– N2 78.6
– O2 20.9
– CO2 0.04
– H2O 0.46
Gases in Alveoli[%]
– N2 74.9
– O2 13.7
– CO2 5.2
– H2O 6.2
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Henry’s LawHenry’s Law
Quantity of a gas x that will dissolve in a liquid
Vx depends upon the solubility coefficient of the
gas in liquid and to it’s partial pressure in the
liquid-gas interface
xxx PV
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Oxygen partial pressure at Oxygen partial pressure at sea levelsea level
mmHgPPFP OHatmIOIO 150)47760(21.0)(222
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Production and elimination of CO2Production and elimination of CO2
AOHatm
aCOA
OHatm
ACOAACOCO V
PP
PV
PP
PVFV
2
2
2
2
22
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Elimination of O2Elimination of O2
AAOAIOO VFVFV 222
AAOAIOOHatmO VPVPPPV 2222
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The alveolar gas equationThe alveolar gas equation
OHatmA
OIOAO PP
V
VPP
2
2
22
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Alveolar gas equationAlveolar gas equation
OHatmA
OIOAO PP
V
VPP
2
2
22
AOHatm
aCOA
OHatm
ACOAACOCO V
PP
PV
PP
PVFV
2
2
2
2
22
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Alveolar gas equationAlveolar gas equation
22
2
22 aCOCO
OIOAO P
V
VPP
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Alveolar gas equationAlveolar gas equation
22
2
22 aCOCO
OIOAO P
V
VPP
2
2
O
CO
V
VR
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R
PPP
aCOIOAO
2
22
Alveolar gas equationAlveolar gas equation
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R - Respiratory coefficientR - Respiratory coefficient
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Property of Property of Gases/AtmosphereGases/Atmosphere
Increasing altitude - reduced air pressure - reduced O2 levels– e.g. at 3,000 m 563 mm Hg, O2 14.5 %
– (in water descend of 10 m increased pressure by 760 mm Hg)
Water solubility of Gases (relative)– CO2:O2:N2 ca. 20:1:0 (overall decrease with rising
temperature)
– hyperbaric O2-chamber: treatment of Gangrene, asphyxiation, circulatory shock, etc.
O2 toxicity >2.5 - 3 x above normal
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(A-a) O2 difference(A-a) O2 difference Clinical significance –
– Bed-side measure of gas (oxygen) transfer
Can be calculated easily from the alveolar gas
equation and from blood gases analysis
Normal values < 20 mm Hg
Different forms of Hypoxia:
– With normal (A-a) O2 difference
– With increased (A-a) O2 difference
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Calculation of (A-a) O2 Calculation of (A-a) O2 differencedifference
(A-a) O 2 Difference = PAO2 – PaO2
PAO2 = PIO2 – PaCO2/R
PIO2 = (Pb – PH2O) x FIO2
PAO2 = (Pb – PH2O) x FIO2 – PaCO2/R
PaO2 & PaCO2 – Measured in ABG