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5 - Diffusion of Gases


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What is the diffusion equation?
Vx = [(Permx)(SA)(Px)]/X
Permx = permeability of x
SA = surface area
Px = pressure gradient
X = wall thickness
What is the surface area of the blood:gas barrier? What is the average thickness?
*surface area is 70 square meters
*thickness is 2um
Name the layers a molecule of oxygen must pass through to go from the alveolar space to the interior of a RBC (there are 6).
2.Alveolar epithelium
4.Capillary endothelium
6.RBC membrane
What is scleroderma? What is asbestosis? What effect do these conditions have on gas diffusion? Note that each affects a different part of the blood-gas barrier!
*scleroderma is a thickening of the interstitium due to deposition of CT
*asbestosis is a thickening of the alveolar epithelium due to irritation
*both will increase the time required for diffusion
What is pulmonary edema? Name three general conditions that may cause it.
*movement of fluid from pulmonary capillaries into the interstitium
1.Decreased plasma oncotic pressure
2.Increased capillary hydrostatic pressure
3.Increase in capillary permeability
Describe the changes in PO2 and PCO2 between an alveolus and the blood entering the capillary perfusing it (the alveolar pressure gradient). What does this say about the movement of each gas?
*PAO2 = 100mmHg, PaO2 = 40, change = 60
*PACO2 = 46, PaCO2 = 40, change = 6
*O2 moves easily because of the large gradient; the low gradient for CO2 is offset by high permeability
How long does it take for an RBC to pass through an average pulmonary capillary? How long is required for an RBC to be unloaded of CO2 and loaded with O2? What do these values imply?
*0.75 sec
*0.25 sec
*the body has a large physiological reserve to accomodate for faster passage time during exertion
How is the diffusing capacity of the lung tested?
CO probe
Some oxygen trivia:
*What is the average oxygen consumption for a 70-kg man?
*What is normal PaO2?
*What percentage of O2 is carried bound to Hb vs. dissolved in plasma?
*95% vs 5%
How are PO2 and oxygen affinity of hemoglobin related?
They are inversely proportional: the lower the PO2, the greater the affinity of hemoglobin for oxygen.
How does pH affect the affinity of hemoglobin for oxygen? What is the physiological benefit of this relationship?
As pH decreases, affinty decreases. This facilitates unloading of oxygen in areas that have a high metabolic rate, high CO2 production, and thus, low pH.
What is the rate of CO2 production by metabolism?
In what three forms is CO2 carried in the blood? Give the percentage of total CO2 carried as each form. a dissolved gas (10%) carbamino form of hemoglobin (30%) HCO3- (60%)
In the tissues, what happens when CO2 enters a RBC?
1.It combines with water to yield H+ and HCO3-
2.HCO3- is extruded by a Cl-/HCO3- antiporter - the "chloride shift"
3.H+ combines with Hb-O2 and induces release of O2
4.CO2 combines with Hb-H+ to yield the carbamino
Describe the chemical reactions that facilitate the liberation of CO2 and loading of O2 in the pulmonary capillaries.
1.The carbamino form of Hb dissociates to yield CO2 and Hb-H+
2.Hb-H+ gives up H+
3.Hb picks up O2
4.H+ combines with HCO3- to yield CO2 and water
What is the time required for the reaction of O2 with hemoglobin?
~0.2 seconds
How do ARDS, O2 toxicity, and toxins cause pulmonary edema?
They increase the permeability of the pulmonary capillary endothelium which allows fluid to leak out.
How might hypertension or over-administration of IV fluids cause pulmonary edema?
In either case the hydrostatic pressure in the pulmonary capillaries is increased, which shifts Starling forces such that fluid leaks out.
How might protein starvation or proteinuria cause pulmonary edema?
In either case the colloid oncotic pressure of plasma is lowered, thus shifting Starling forces such that fluid leaks out of the pulmonary capillaries.
What is a cause of pulmonary edema that has nothing to do with the movement of blood through the capillaries?
Blockage of pulmonary lymphatic drainage by tumor or other means.
At what arterial PO2 should supplemental oxygen be administered?
When PO2 falls below 55mmHg.
How are PCO2 and hemoglobin affinity for oxygen related?
The are inversely proportional - as PCO2 increases, the affinity of hemoglobin for oxygen decreases.

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