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Part 3
Respiratory Gases Exchange
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I Physical Principles of Gas Exchange
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• Partial pressure– The pressure exerted by each type of gas
in a mixture
• Concentration of a gas in a liquid– determined by its partial pressure and its
solubility coefficient
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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
PN2 = 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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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
Diffusion: Blood Transit Time in the Alveolus
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II Gas exchange in the lung and in the tissue
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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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PO2 and PCO2 in Blood
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III. A-a gradient, the efficiency of the gas exchange in alveoli
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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. Blood Mixing
3. Matching
1. Blood Shunts
2. Blood Mixing
3. 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).
the change in blood flow is more steep than in ventilation.
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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IV Factors Affecting the Gas Diffusion in the Lung
1) Area of the respiratory membrane
2) Distance of the diffusion
3) VA/Q
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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
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EXTERNAL AND INTERNAL RESPIRATION
HEART
TISSUECELL
O2 + FOOD
CO2 + H2O+ ATP
LUNGS
ATMOSPHERE
PULMONARYCIRULATION
SYSTEMICCIRCULATION