SM_156a: Diffusion and Gas Transport Flashcards
The oxygen cascade is driven by a ________ gradient
The oxygen cascade is driven by a partial pressure gradient
What is Fick’s Law of diffusion?
V·= (A/t) * D * ∆P
where D is proportional to solubility / √molecular weight
The diffusion coefficient D for CO2 is ____ times that of O2
The diffusion coefficient D for CO2 is 20 times that of O2
The diffusion path from alveolus to RBC includes the ______, _______, and _______
It is a ______ barrier to diffusion
The diffusion path from alveolus to RBC includes the alveolus, respiratory (alveolar-capillary) membrane, and capillary
It is a minimal barrier to diffusion
What are the two general categories of gas transfer between alveolus and capillary?
- Diffusion limited
- Perfusion limited
What is diffusion limited gas transfer between alveolus and capillary?
Gas transfer limited by rate of diffusion
Related to the Fick equation, V· = A/t * D * ∆P, which depends on the surface area available for diffusion and the diffusion distance
What is perfusion limited gas transfer between alveolus and capillary?
Gas transfer limited by the ability of the blood flow to deliver gas to the tissue
Related to rate of blood flow
What is diffusion limitation?
PA, g ≠ Pc’g where A is alveolar and c’ is end capillary
(example: rate of diffusion from alveolus to capillary is not sufficient to rase Pc‘O2 to PAO2 in a single pass through the lung)
What are the limiting factors to gas exchange described by Fick’s law of diffusion?
- Surface area (pus, water, atelectasis)
- Diffusion distance (emphysema, pulmonary fibrosis)
A patient briefly inhales 2 different gases, X and Y. Partial pressures of X and Y are then determined in alveoli, mixed venous blood, and end pulmonary capillary blood.
Y is diffusion limited, X is not
No equilibration between alveolus and capillary for Y, if increase surface area can increase diffusion rate
(X is perfusion limited)
For a gas (Y), that is diffusion limited, increasing cardiac output will __________
Decrease Pc’Y
Increasing cardiac output will decrease the amount of time blood spends in a pulmonary capillary, decreasing Pc’Y
What is perfusion limitation?
Rate of diffusion is sufficient so that end capillary partial pressure equals alveolar partial pressure (Pc’ = PA)
Example: nitrous oxide (PN20)
For a gas (Y), that is perfusion limited and with a constant PAY, a small increase in cardiac output will _______
For a gas (Y), that is perfusion limited and with a constant PAY, a small increase in cardiac output will have no effect on Pc’Y
Describe the diffusion of oxygen
Equilibration rate is between N2O and CO
Normally perfusion limited
What is the formula for diffusion capacity (DL)?
DL = V· / (PA - Pc)
1 / DL = resistance
What are the two components of resistance with regards to diffusing capacity?
1 / DL = 1 / Dm + 1 / (θ * Vc)
1 / Dm is resistance of alveolar-capillary membrane
1 / (θ * Vc) is resistance due to chemical reactions
What is the resistance due to chemical reactions for O2?
Binding of oxygen to hemoglobin
What is the resistance due to chemical reactions for CO2?
Conversion of bicarbonate to carbon dioxide
Diffusion rate of CO2 ______ that of O2
Diffusion rate of CO2 is about equal to that of O2
Describe the diffusion of carbon dioxide
Chemical reactions slow equilibration
Perfusion limited
How is oxygen transported through the blood?
- Dissolved O2
- Bound to hemoglobin (HbO2) - more common
Which formula describes the transport of dissolved O2?
Henry’s Law, C(O2, dissolved) = K * PO2
What is the formula for O2 delivery?
O2 delivery = O2 dissolved * cardiac output
Describe the oxygen dissocation curve
- Amount of O2 dissolved rises linearly w/ PO2 but is a small portion of total O2 (only dissolved O2 gives rise to partial pressure of oxygen)
- HbO2 rises quickly
What is the effect of hypoxia on HbO2 equilibrium?
Equilibrium: Hb + 4O2 ⇔ Hb(O2)4
At equilibrium:
- Free [O2] on left = free [O2] on right
- PO2 on left = PO2 on right
- Free [O2] on left < total [O2] on right
Hypoxia shifts the equilibrium
What is the blood O2 capacity?
Amount of O2 bound to Hb when Hb is saturated
One gram Hb maximally binds 1.34 mL O2 and Hb is 15 gm% O2
O2 capacity is 1.34 mL/gm * 15 gm/100mL = 20.1 mL/dL
What is oxygen content?
Amount of O2 carried by the blood, which varies with PO2
What is the formula for % saturation?
%O2 saturation (SO2) = (O2 content / O2 capacity) * 100
What is the HbO2 characterized by?
- O2 binding capacity
- P50, which is an expression of affinity of Hb for O2
Altering [Hb] changes _______ but not ______ or _______
Altering [Hb] changes O2 capacity but not P50 or % saturation at any given PO2
Which factors increase P50?
Curve shifted to the right by increases in
- Temperature
- H+ or PCO2 (Bohr effect)
- 2,3-BPG
What is the Bohr effect?
Deoxyhemoglobin is a weaker acid than oxyhemoglobin, so deoxyhemoglobin readily accepts H+, which lowers affinity of Hb for O2 in the tissue
What is the Haldane effect?
Oxyhemoglobin is a stronger acid than deoxyhemoglobin, so oxyhemoglobin has a lower affinity for H+ and CO2, so addition of O2 causes H+ to be released from Hb in the lungs
Where does the Bohr effect act?
In tissue
Where does the Haldane effect act?
In lungs
Compare O2 versus CO2 in the blood
- CO2 curve is much more linear (never plateaus at physiological levels of CO2)
- Much more CO2 than O2 in the blood
- Hb is about 90% saturated w/ O2 at about 60 mmHg
How is CO2 transported in the blood?
- Dissolved CO2
- HCO3- (mainly)
- Carbamino CO2
What is carbamino CO2?
CO2 bound to blood proteins (mostly hemoglobin)
Higher O2 corresponds to the ______ line for carbamino CO2
Higher O2 corresponds to the arterial line for carbamino CO2
Lower O2 corresponds to the ______ line for carbamino CO2
Lower O2 corresponds to the venous line for carbamino CO2
