Diffusion and Gas Transport Flashcards
What two factors affect PO2 at the level of inspired gas?
barometric pressure and inspired oxygen fraction
What are the two factors that affect PO2 at the level of alveolar gas?
oxygen consumption and alveolar ventilation
What are the two factors that affect PO2 at the level of arterial blood?
venous admixture and ventilation perfusion ratio
What are the two factors that affect PO2 at the level of the cell?
blood flow and Hg concentration
The rate of transfer of a gas through a tissue sheet is proportional to what?
the area of the tissue sheet and the pressure gradient
The rate of transfer of a gas through a tissue sheet is inversely proportional to what?
thickness
What is Fick’s law of diffusion?
the rate of gas diffusion by volume per unit time
Vg: volume of gas diffusing
A: surface area available for diffusion
D= diffusion coefficient
t= thickness of diffusion barrier
deltaP: pressure gradient driving diffusion
The diffusion coefficient (D) in Ficks law of diffusion is proportional to what variable? What is it inverse to?
solubility of gas in the medium
square root of the molecular weight
Why is the increased size of air space in emphysema a problem for gas diffusion?
it increases the distance required to diffuse from center to capillary of alveolus, reducing diffusion across the membrane
What is the most relevant rate limiting factor for alveolar gas transfer to blood?
Hb binding reaction rate
equilibration and diffusion between gas and blood takes a finite time that is generally less than the time the RBC spends in the pulmonary capillaries
What does the rate of diffusion of a gas WITHIN a given solvent depend on?
concentration gradient
Define diffusion limited gas transfer
In diffusion limited gas transfer, the diffusion pathway provides a barrier that limits gas transfer. Blood flow has NO effect on the exchange process
What is the effective solubility (capacitance) for CO? and how does that affect diffusion?
binding between CO and Hb is very high,
In non smokers what is the baseline level of CO?
none
When comparing a more soluble v. less soluble gas, which one will have a slower rate of rise in blood tension?
less soluble gas
Are low solubility gases more likely to be diffusion or perfusion limited?
Perfusion limited, because their partial pressure rises very quickly in blood
Is oxygen generally perfusion or diffusion limited? why?
Perfusion limited
What 2 processes limit diffusion?
blood-gas barrier movement
chemical reactions
What is the diffusion Capacity (DL?)
the rate of gas transfer divided by the pressure gradient across the membrane
Is diffusion resistance arrange in series or in parallel?
in series
Why is carbon monoxide generally used to determine diffusing capacity? What equation does this use?
Its mean pulmonary capillary partial pressure is normally zero
What two factors limit CO2 exchange, thus offsetting the fact that diffusion of CO2 is generally faster than O2 when looking just at solubility?
- formation of bicarbonate
- formation of carbamino compounds
At any given time, what is the solubility of oxygen in plasma? If PO2 = 100 mmHg, what is the dissolved oxygen concentration?
.003 ml 02/100 ml blood/mmHg
.3 Vol%
Define oxygen capacity in the Hb dissociation curve.
the maximum amount of O2 that can bind to Hb
What is P50? what does it describe?
Partial pressure of O2 at which 50% is bound to Hb
Expresses affinity of Hb for oxygen
If Hb concentration varies, which changes: O2 capacity or P50?
O2 capacity only
How do you calculate SO2 ?
Is SO2 dependent or independent of oxygen capacity?
independent
If P50 drops, what direction does the SO2 v. PO2 relationship go?
Right shift
If PO2 is normal, do you still have to worry about CO toxicity?
Yes.
Oxygen is not the only gas that has an affinity for Hb. CO binds very strongly with Hb. Its affinity is about 200 times greater than O2 and desaturates only at very low PCO. This combination of factors makes CO potentially lethal even if PO2 is normal, since CaO2 is drastically reduced for a significant duration even after a transient exposure to CO.
Does increased CO2 in the blood cause an increase or a drop in pH?
drop in pH - produces acid
Define the Bohr effect
tissues are more acidic than lungs, facilitating O2 release to tissues that are metabolically active
Tissues are more acidic due to production of CO2
What are three conditions that raise 2,3 DPG? What does this mean?
Chronic hypoxia due to:
altitude
lung disease
severe anemia
Increases P50 and shifts curve to the right
HOw do you calculate total oxygen delivery to tissues?
O2 delivery = arterial O2 concentration * blood flow
Define hypoxic hypoxia
hypoxic hypoxia (a low PO2 in the arterial blood);
Define anemic hypoxia
(when oxygen carrying capacity is lowered by a low hemoglobin concentration in the blood);
Define circulatory hypoxia
e.g., shock or hypoperfusion of the tissues
Define histotoxic hypoxia
such as cyanide poisoning; this is the only form of tissue hypoxia where available oxygen cannot be utilized and the O2 extraction is lowered
What enzyme is responsible for the production of bicarbonate?
carbonic anhydrase
Define chloride shift in bicarbonate metabolism
When chloride bicarbonate exchanger moves bicarb out of the cell in exchange for chloride to preserve electric neutrality
Does reduced or oxyhemoglobin bind more CO2?
Reduced Hb
Define the Haldane effect
When O2 displaces HHb infavor of HbO2 in the lungs; this results in release of CO2 from bicarbonate and additional carbamino Hb breakdown
What are the two main drivers of the Bohr-Haldane effect?
- deprotonated Hb binds O2 more strongly than protonated Hb
- deoxyHb binds protons more strongly than oxyhemoglobin
Across a small range of PCO2, what does the shape of the CO2 dissociation curve mean ?
larger changes in CO2 without saturation effects
What factors determine the rate of diffusion across the alveolar-capillary barrier at a given instant? If the diffusing capacity is constant can the rate of diffusion vary?
How does rate of diffusion affect oxygen consumption?
The rate of diffusion of O2 should not affect the O2 consumption unless the diffusion falls below the minimum required to maintain V O2 .
Calculate diffusing capacity using CO. What’s a normal value? how much does it increase during exercise?
DL = ventilation rate CO/PACO
normal: 25
increases by 2-3x
With falling FiO2, will diffusion limitation value increase or decrease?
increase
In health, what effect does exercise have on diffusion capacity?
increases it
How do you calculate the O2 content of blood?
fractional HbO2 saturation x [Hb] x amount o2 bound in Hb
i. e. 100% O2 sat, 15 gm Hb/100mL blood, 1.34 mL O2/gm Hb
1. 0 x 15 x 1.34 = 20.1 mL o2/100 ml blood = 20.1 vol% O2
How do you calculate O2 uptake or consumption per 100 mL of blood?
arterial O2 content - venous o2 content = O2 removed from blood by metabolism
How do you calculate the amount of O2 consumed per minute?
(arterial O2 content - venous O2 content) X cardiac output
