respiration gas exchange
TRANSCRIPT
RESPIRATION
Gas Exchange
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PARTIAL PRESSURES
In a mixture of gasses, the total pressure distributes among the constituents proportional to their percent of the total
The concentration of a gas can therefore be expressed as its partial pressure
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Partial Pressures in air
Oxygen = 21%
Nitrogen = 79%
Po2 = 160 mm Hg
PN2 = 600 mm Hg
Total Pressure (at sea level) = 760mm Hg
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Effect of water vapor
As fresh air enters the nose and mouth it is immediately mixed with water vapor
Since the total pressure remains constant, the water vapor lowers the partial pressure of all other gases
For this reason, the PO2 is lowered to about 149 mmHg
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DEAD SPACE VOLUME
At the height of expiration, about 150ml of gas still occupies the respiratory tree
This “old gas” is necessarily mixed with the incoming fresh air and further lowers the PO2 to about 100 mmHg
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GAS EXCHANGE ACROSS PULMONARY CAPILLARIES
Both oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients
Inspired Air PO2 = 160mmHg
PCO2 = 0.03mmHg
LUNG PO2 = 100mmHgPCO2 = 40mmHg
PO2 = 40mmHgPCO2 = 46mmHg
PULMONARY CAPILLARIESPO2 = 100mmHgPCO2 = 40mmHg
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GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES
Both oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients
TISSUE PO2 < 40mmHgPCO2 > 46mmHg
PO2 = 40mmHgPCO2 = 46mmHg
SYSTEMIC CAPILLARIESPO2 = 100mmHgPCO2 = 40mmHg
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Carbon dioxide/Bicarbonate Relationship
CO2 + H2O <---> H2CO3 <---> H+ + HCO3-
Carbon dioxide dissolved in water readily combines with water to form carbonic acid. The carbonic acid then dissociates into the hydrogen ion and bicarbonate ion. The former reaction is catalized by and enzyme called Carbonic Anhydrase in many tissues.
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GAS TRANSPORT IN BLOOD
Oxygen physically dissolved = 1.5%Oxygen bound to hemoglobin =
98.5%Carbon dioxide physically dissolved =
10%Carbon dioxide bound to hemoglobin
= 30%Carbon dioxide as bicarbonate = 60%
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HEMOGLOBIN/OXYGEN DISSOCIATION
PO2 of blood (mmHg)
% HemoglobinSaturation
Resting PO2
SystemicNormal PO2
Capillaries
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Agents which shift the Hb/O Dissociation curve: The Bohr Effect
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UNDERSTANDING THE HB/O DISSOCIATION CURVE
The plateau: Provides a margin of safety in the oxygen carrying capacity of the blood
The steep portion: Small changes in Oxygen levels can cause significant changes in binding. This promotes release to the tissues.
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Agents which shift the Hb/O Dissociation curve: The Bohr Effect
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Carbon dioxide/Bicarbonate Relationship
CO2 + H2O <---> H2CO3 <---> H+ + HCO3-
Carbon dioxide dissolved in water readily combines with water to form carbonic acid. The carbonic acid then dissociates into the hydrogen ion and bicarbonate ion. The former reaction is catalized by and enzyme called Carbonic Anhydrase in many tissues.
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Carbon Dioxide Transport in the Blood: At the tissues
Tissue Cell
Red Blood Cell
CO2 + H2O ---> H2CO3 ---> H+ + HCO3
Carbonic Anhydrase
+ Hb --->HbH+ Hb ---> HbCO2
HbO2 -----> Hb + O2
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Carbon Dioxide Transport in the Blood: At the lungs
Alveolus
Red Blood Cell
CO2 + H2O <--- H2CO3 <--- H+ + HCO3-
Carbonic Anhydrase
+ Hb <---HbH+ Hb <--- HbCO2
HbO2 <--- Hb + O2
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The Haldane Effect
Removal of oxygen from hemoglobin increases hemoglobin’s affinity for carbon dioxide
This allows carbon dioxide to “ride” on the empty hemoglobin
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RESPIRATORY CONTROL
Pons: Pneumotactic centerPons: Apneustic centerMedulla: Dorsal respiratory groupMedulla: Ventral respiratory group
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Medulla: Dorsal respiratory group
Inspiratory neurons
Pacemaker activity
Expiration occurs when these cease firing
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Medulla: Ventral respiratory group
Both inspiratory and expiratory neurons
Inactive during normal quiet breathing
Rev up inspiratory activity when demands for ventilation are high
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Pons: Pneumotactic center
Fine tuning over medullary centers
Switches off inspiration
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Pons: Apneustic center
Fine tuning over medullary centers
Blocks switching off of inspiritory neurons
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CARBON DIOXIDE CONTROLLS RESPIRATION
High carbon dioxide generates acidity of blood in brain
Acidity of blood in systemic circulation is prevented from directly influencing the brain due to the blood/brain barrier’s impermeability to H+
CO2 + H2O <---> H2CO3 <---> H+ + HCO3
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OXYGEN LEVELS MUST FALL DRASTICALLY TO AFFECT BREATHING
Receptors in carotid bodies
Below 60 mmHg for oxygen partial pressure, breathing is stimulated
This is a last-ditch, fail-safe mechanism only!
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