Chp. 3: Diffusion Flashcards
Fick’s Law
Rate of transfer of a gas through a sheet of tissue is proportional to tissue area and difference in gas partial pressures between the two sides and inversely proportional to tissue thickness
Also proportional to a diffusion constant: constant is proportional to solubility of gas and inversely proportional to square root of molecular weight
Why is CO2 more diffusible than O2?
Much higher solubility but similar molecular weight
Is CO diffusion perfusion or diffusion limited?
Diffusion
Is N2O diffusion perfusion or diffusion limited?
Perfusion
Is O2 diffusion perfusion or diffusion limited?
Perfusion
What determines diffusion vs. perfusion limitation?
Solubility in blood-gas barrier compared with solubility in blood
What is normal mixed venous PO2?
40mmHg
Why is there a considerable reserve capacity for diffusion?
Because equilibration occurs in only a fraction of the time the red cell spends in the pulmonary capillary
In what situation is diffusion impairment convincingly demonstrated?
Severe exercise at very high altitude
At rest, how quickly does PO2 of blood reach that of alveolar gas?
1/3 of time in capillary
Diffusing capacity of lung
Area, thickness, and diffusion properties of sheet and gas concerned
Single-breath method for diffusing capacity
Measurement of inspired and expired CO concentrations with infrared analyzer
CO used because uptake is diffusion limited
Normal is 25mL/min/mmHg
Diffusing capacity INCREASES on exercise
What are the two stages of uptake of O2?
1) Diffusion of O2 through blood-gas barrier (including plasma and red cell interior)
2) Reaction of O2 with Hb
What decreases measured diffusing capacity of lung?
Reduction of capillary blood volume or concentration of Hb by disease
KEY CONCEPTS of diffusion
1) Fick’s law states rate of diffusion of a gas through a tissue sheet is proportional to area of sheet and partial pressure difference across it, and inversely proportional to thickness of the sheet
2) Examples of diffusion and perfusion limited gases are CO and N2O, respectively. Oxygen transfer is normally perfusion limited, but some diffusion limitation may occur with intense exercise, thickening of blood-gas barrier, and alveolar hypoxia
3) Diffusing capacity of lung is measured using inhaled carbon monoxide. Value increases markedly on exercise.
4) Finite reaction rate of oxygen with hemoglobin can reduce its transfer rate into blood, and the effect is similar to that of reducing the diffusion rate.
5) CO2 transfer across blood-gas barrier is probably not diffusion limited
