1 Section II Respiratory Gases Exchange
Dec 22, 2015
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• Partial pressure– The pressure exerted by each type of gas in a
mixture
• Diffusion of gases through liquids– Concentration of a gas in a liquid is determined
by its partial pressure and its solubility
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Partial Pressures of GasesBasic Composition of Air• 79% Nitrogen• 21 % Oxygen• ~ 0% Carbon Dioxide In a mixture of gases, each gas exerts a partial pressure proportional to its mole fraction.
Total Pressure = sum of the partial pressures of each gas
Pgas = Pb x Fgas
PN = 760 x 0.79 = 600.4 mm HgP02 = 760 x 0.21 = 159.6 mm Hg
Total Pressure (at sea level) Pbarometric = 760 mm Hg
PPbb
760 mm760 mmHg Hg
PPbb
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Consider a container of fluid in a vacuum
Partial Pressure of Gases in Fluids
Each gas has a specific solubilityO2 Solubility coefficient = 0.003 ml/100 ml BloodC02 = 0.06 ml/100 ml Blood (x 20 of 02)
Gases dissolve in fluids by moving down aPartial Pressure gradient rather than a concentration gradient
That is opened to the air
Molecules of gas begin to enter the fluid
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Partial Pressure of Gases in Fluids
After a short time, the number of molecules the number of molecules
ENTERING = LEAVING
At equilibrium, if the gas phase has a PO2 = 100 mm Hg, the liquid phase also has a PO2 = 100 mm Hg
An easy way to talk about gases in fluids.
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• Transport of gases between the alveoli and
(pulmonary) capillaries and eventually from
the capillaries to the tissues
• diffusion dependent on perfusion and the
partial pressure (pp) exerted by each gas
• gases diffuse from area of conc. (pp) to
conc. (pp)
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concentration pp of gas diffusion
CO2 more soluble than O2, therefore it diffuses faster
Diffusion
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Diffusion: Blood Transit time in the AlveolusDiffusion: Blood Transit time in the Alveolus
AlveolusAlveolus
Blood capillaryBlood capillary
Time for exchangeTime for exchangePO2PO2
Time0 0.75 sec
40
100
Saturated very quickly
Reserve diffusive Capacity of the lung
45
mm Hg
PCO2PCO2
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Oxygen and Carbon Dioxide Diffusion Gradients
• Oxygen– Moves from alveoli into
blood. – Blood is almost completely
saturated with oxygen when it leaves the capillary
– P02 in blood decreases because of mixing with deoxygenated blood
– Oxygen moves from tissue capillaries into the tissues
• Carbon dioxide– Moves from tissues into
tissue capillaries– Moves from pulmonary
capillaries into the alveoli
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Diffusion Gradients of Respiratory Gases at Sea Level
Total 100.00 760.0 760 760 0
H2O 0.00 0.0 47 47 0
O2 20.93 159.1 105 40 65
CO2 0.03 0.2 40 46 6
N2 79.04 600.7 569 573 0
Partial pressure (mmHg)
% in Dry Alveolar Venous DiffusionGas dry air air air blood gradient
NB. CO2 is ~20x more soluble than O2 in blood => large amounts move into & out of the blood down a relatively small diffusion gradient.
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Oxygen Content in Alveolus Gas
(measured during exhalation))
Oxygen Content in
arterial blood
(equivalent to that leaving
lungs)
What is an A - a gradient ?
The DIFFERENCE between::
In a healthy person, what would you expect the A - a to be?No difference, greater than 0, or less than 0
Normal: A – a, up to ~ 10 mm Hg, varies with age
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Factors contributing to A - a GradientFactors contributing to A - a Gradient
1. Blood Shunts
2. Matching
1. Blood Shunts
2. Matching
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Alveolar SPACE
arterial vessel
SIMPLE CONCEPT OF A SHUNT
BLOOD FLOWBLOOD FLOW
COCO22 OO22
No Gas Exchange = SHUNT
AIR FLOW
Blood
MixingLowered O2/l00 ml
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Total Perfusion, Q
Total VentilationTotal Ventilation
NEXT NEW CONCEPT
Matching What? BloodBlood to to Air FlowAir Flow
ExchangeOxygen
If the volumes used for exchange are aligned – We might consider the system to be
“ideally matched”
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Arterial Perfusion (Qc)
Slide or Misalign the distribution volumes
Alveolar Ventilation (VAlveolar Ventilation (VAA))
ExchangeOxygenOxygen
Dead Air Space (Airways)
Shunt (Qs)(Bronchial Artery)
Some Volumes are wasted, Matching Ratio = VA/Qc = 0.8
Normal Case; Small Shunt, low volume Dead Space
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Matching ventilation & perfusion
Ventilation and perfusion (blood flow) are both better at the bottom (base) of the lung than that at the top (apex).
But the change in blood flow is more steep than in ventilation.
Therefore the ventilation/perfusion ratio rises sharply from the base to the apex.
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Matching ventilation & perfusion (cont)
Result: V/Q is greater or less than 0.8 in different regions
If V/Q <0.8 = shunt like, If V/Q > 0.8 little benefit, Increases A - a gradient
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Alveolar VentilationVA
Arterial Perfusion Q
ExchangeOxygen
Dead Air Space
Shunt
= Lung Disease with a Large A – a gradient
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1. The Properties of the Gas1) Molecular weight. Diffusion rate is inversely proportional
to the square root of the molecular weight2) Temperature3) Solubility in waterEach gas has a specific solubility
O2 Solubility coefficient = 0.003 ml 02/100 ml BloodC02 = 0.06 ml/100 ml Blood (x 20 of 02)
PO2PO2
Time0 0.75 sec
40
100
Saturated very quickly
Reserve diffusive Capacity of the lung
45
mm HG
PCO2PCO2
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2. Partial Pressure of the Gases
1) Alveoli ventilation
2) Blood perfusion in the lung capillary
3) Speed of the chemical reaction
The slow speed of the chemical reaction HCO3- + H+
----- H2CO3 ---H2O + CO2 reduces the CO2 exchange in the lung.
So, during the gas exchange in the external respiration, the exchange of CO2 is a little lower than that of O2.
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3. Properties of the Lung
1) Area of the respiratory membrane
2) Distance of the diffusion
3) VA/Qc
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V Pulmonary Diffusion CapacityConcept:
The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood
defined as the volume of a gas that diffuses through the membrane each minute for a pressure of 1 mmHg.
DL = V/(PA – PC)
V is a gas that diffuses through the membrane each minute,
PA is the average partial pressure of a gas in the air of alveoli,
PC is the average partial pressure of a gas in the blood of pulmonary capillary.
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Factors Affecting the DL
1. Body posture
2. Body height and weight
3. Exercise
4. Pulmonary diseases
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VI Internal Respiration
• All cells require oxygen for metabolism
• All cells require means to remove carbon
dioxide
• Gas exchange at cellular level
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Concept: Gas exchange between the capillary and the
tissues throughout the body
Process:
Factors affecting the internal respiration:
1. Distance between the cells and the capillary
2. Rate of metabolic rate
3. Speed of the blood flow in capillary