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Mechanical ventilation:gas
exchange
Hosam m. Atef;MD
SUEZ CANAL UNIVERSITY
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Concentration and Partial Pressure of
Respired Gases
• Partial pressure = Percentage of
concentration of specific gas × Total
pressure of a gas
• Dalton’s law
– Total pressure = Sum of partial pressure of all
gases in a mixture
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Ambient Air
• O2 = 20.93% = ~ 159 mm Hg PO2
• CO2 = 0.03% = ~ 0.23 mm Hg PCO2
• N2 = 79.04% = ~ 600 mm Hg PN2
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Tracheal Air
• Water vapor reduces the PO2 in the
trachea about 10 mm Hg to 149 mm Hg.
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Alveolar Air
• Alveolar air is altered by entry of CO2.
• Average alveolar PO2 = 103 mm Hg
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Movement of Gas in Air and Fluids
• Henry’s law
– Gases diffuse from high pressure to low
pressure.
• Diffusion rate depends upon
– Pressure differential
– Solubility of the gas in the fluid
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Pressure Differential
• The difference in the pressure of specific
gases from the capillary blood to the
alveoli dictates the direction of diffusion.
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Solubility
• CO2 is about 25 times more soluble than O2.
• CO2 and O2are both more soluble than N2.
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Gas Exchange
• Exchange of gases between lungs and blood
and gas movement at the tissue level
progress passively by diffusion, depending
on their pressure gradients.
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Gas Exchange in the Lungs
• PO2 in alveoli ~ 100 mm Hg
• PO2 in pulmonary capillaries ~ 40 mm Hg
• Result: O2 moves into pulmonary capillaries
• PCO2 in pulmonary capillaries ~ 46 mm Hg
• Average arterial blood gases equal
– PO2 100 mm Hg
– PCO2 40 mm Hg
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Pulmonary Disease
• Gas transfer capacity may be impaired by
– Thickening of membrane
– Reduction in surface area
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Gas Transfer in Tissues
• Pressure gradients cause diffusion of O2 into
and CO2 out of tissues.
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Transport of O2 in the Blood
• Two mechanisms exist for O2
transport
– Dissolved in plasma
– Combined with hemoglobin
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Oxygen in Physical Solution
• For each 1 mm Hg increase, 0.003 mL O2
dissolves into plasma.
• This results in ~ 3 mL of O2/liter blood.
• With 5 L total blood volume = 15 mL dissolved
O2
• Dissolved O2 establishes the PO2 of the blood.
– Regulates breathing
– Determines loading of hemoglobin
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Oxygen Combined with Hemoglobin
• Each of four iron atoms associated with
hemoglobin combines with one O2
molecule.
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Oxygen-Carrying Capacity of Hb
• Each gram of Hb combines with 1.34 mL
O2.
• With normal Hb levels, each dL of blood
contains about 20 mL O2.
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Anemia Affects Oxygen Transport
• Volume percent (vol%) refers to the
milliliters of oxygen extracted from a
100-mL sample of whole blood.
• Human blood carries O2 at 14 vol%.
• Iron deficiency anemia reduces O2
carrying capacity considerably.
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PO2 and Hb Saturation
• Oxyhemoglobin dissociation curve
illustrates the saturation of Hb with
oxygen at various PO2 values
• Percent saturation = 12 vol% / 20 vol% ×
100 = 60%
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PO2 in the Lungs
• Hb ~ 98% saturated under normal
conditions
• Increased PO2 doesn’t increase saturation.
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PO2 in Tissues
• At rest
– PO2 = 40 mm Hg
– Venous blood carries ~ 70% of the O2 content
of arterial blood.
– Venous blood carries 15 mL O2 per dL blood.
– Tissues have extracted 5 mL O2 per dL blood.
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Arteriovenous O2 Difference
• The a- O2 difference shows the amount of O2
extracted by tissues.
• During exercise a- O2 difference increases up to 3
times the resting value.
v
v
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Bohr Effect
• Conditions creating the Bohr effect
– Increased PCO2
– Increased temperature
– Increased 2,3-DPG
– Decreased pH
• Shift to the right of the oxyhemoglobin curve
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RBC 2,3-DPG
• 2,3-DPG is a byproduct of glycolysis/
• RBCs contain no mitochondria.
– Rely on glycolysis
• 2,3-DPG increases with intense exercise
and may increase due to training.
• Helps deliver O to tissues
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Myoglobin, The Muscle’s O2 Store
• Myoglobin is an iron-containing globular
protein in skeletal and cardiac muscle.
• Stores O2 intramuscularly
• Myoglobin contains only 1 iron atom.
• O2 is released at low PO2.
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CO2 Transport
• Three mechanisms
– Bound to Hb
– Dissolved in plasma
– Plasma bicarbonate
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CO2 in Physical Solution
• ~ 5% CO2 is transported as dissolved CO2.
• The dissolved CO2 establishes the PCO2of
the blood.
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CO2 Transport as Bicarbonate
• CO2 in solution combines with water to
form carbonic acid.
• Carbonic anhydrase
– Zinc-containing enzyme within red blood cell
• Carbonic acid ionizes into hydrogen ions
and bicarbonate ions.
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CO2 Transport as Carbamino
Compounds
• CO2 reacts directly with amino acid mq to
form carbamino compounds.
• Haldane Effect: Hb interaction with O2
reduces its ability to combine with CO2.
• This aids in releasing CO2 in the lungs.
