Download - RESPIRATORY GAS TRANSPORT
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RESPIRATORY GAS TRANSPORT
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OXYGEN TRANSPORT• 98.5% oxygen in arterial blood is bound to
hemoglobin and 1.5% is dissolved in plasma• Each heme group of 4 globin chains in a
hemoglobin molecule may bind O2
• After binding with O2, hemoglobin changes shape to allow further uptake (positive feedback)
• An oxyhemoglobin dissociation curve shows relationship between hemoglobin saturation and oxygen partial pressure - PO2.
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Oxyhemoglobin Dissociation Curve
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CARBON DIOXIDE TRANSPORT
• 90% as carbonic acid in plasma CO2 + H2O H2CO3 HCO3
- + H+
• 5% as carbaminohemoglobin (HbCO2)- binds to amino groups of Hb (and plasma proteins)
• 5% as dissolved gas in plasma
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SYSTEMIC GAS EXCHANGE• CO2 loading
– carbonic anhydrase in RBC catalyzes• CO2 + H2O H2CO3 HCO3
- + H+
– chloride shift• keeps reaction proceeding, exchanges HCO3
- for Cl- (H+
binds to hemoglobin)• O2 unloading
– H+ binding to HbO2 causes affinity for O2
• Hb arrives 97% saturated, but leaves 75% saturated
(venous reserve) – Utilization coefficient (amount of oxygen Hb has
released) is 22%
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ALVEOLAR GAS EXCHANGE
• Reactions are the reverse of systemic gas exchange
• O2 loading & CO2 unloading– as Hb loads O2 its affinity for H+ decreases, H+
dissociates from Hb and binds with HCO3-
• CO2 + H2O H2CO3 HCO3- + H+
– reverse chloride shift• HCO3
- diffuses back into RBC in exchange for Cl-
and free CO2 diffuses into alveoli to be exhaled
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Alveolar Gas Exchange
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Systemic Gas Exchange
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FACTORS FAVORING OXYGEN UNLOADING
• Metabolic needs of tissues affect O2 unloading (HbO2 releases O2)– Low ambient PO2
: tissue has PO2
– Increased temperature of tissue– Bohr effect: tissue has CO2, which raises H+ and lowers
pH– bisphosphoglycerate (BPG):BPG produced by RBCs as a
metabolic intermediate binds to Hb and causes HbO2 to release O2
body temp. (fever), TH, GH, testosterone, and epinephrine raise BPG and cause O2 unloading
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Oxygen Dissociation & pH
Bohr effect: release of O2 in response to low pH
Active tissue - more O2 released
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• Metabolic needs of tissues affect CO2 loading– Haldane effect: low level of oxyhemoglobin
(HbO2) enables blood to transport more CO2
• HbO2 does not bind CO2 as well as deoxyhemoglobin (HHb) does.
• HHb binds more H+ than HbO2 - The CO2 + H2O HCO3
- + H+ reaction therefore
shifts to the right
FACTORS FAVORING CARBON DIOXIDE LOADING
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CHEMORECEPTORS
• Monitor pH, PCO2, PO2
of body fluids
– Peripheral chemoreceptors• aortic bodies - signal medulla by vagus
nerves• carotid bodies - signal medulla by
glossopharyngeal nerves– Central chemoreceptors on surface of medulla
• primarily monitor pH of cerebrospinal fluid
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Peripheral Chemoreceptor Pathways
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DIRECT CARBONDIOXIDE EFFECT ON CHEMORECEPTORS
CO2 may directly stimulate peripheral chemoreceptors and trigger ventilation more quickly than central chemoreceptors
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DIRECT EFFECT OF OXYGEN ON CHEMORECEPTORS
• Usually, oxygen has little effect on chemoreceptors.
• Chronic hypoxemia, PO < 60 mmHg, as is associated with emphysema & pneumonia may cause increased ventilation .