Chapter 3: Diffusion Flashcards
Describe Fick’s Law as applies to pulmonary physiology
Fick’s Law of Diffusion of gasses describes the rate of diffusion of a gas through a tissue slice. The law states that the rate of diffusion is directly proportional to area, pressure gradient, and diffusion constant, while being inversely proportional to thickness.
Describe the definition of the gas diffusion constant in pulmonary physiology
D is directly proportional to gas solubility, and inversely proportional to the square root of molecular weight
Which gas has a higher diffusion constant, oxygen or carbon dioxide? Why?
Carbon dioxide, because it is much more soluble in water than oxygen.
At rest, how long does it take PaO2 to reach PAO2?
About 0.25 seconds
What is the capillary transit time of pulmonary blood during intense exercise?
About 0.25 seconds
In what ways is the diffusion process challenged?
Exercise, alveolar hypoxia, thickening of the blood-gas barrier
Why is carbon monoxide used to measure diffusing capacity?
Uptake of carbon monoxide in the lungs is diffusion-limited, not perfusion-limited
What is the normal diffusing capacity of CO?
~25 mL per min per mm Hg
What happens to diffusing capacity on exercise?
Increases
Why does diffusing capacity increase on exercise?
Extra blood flow from recruitment and distension of pulmonary blood vessels
Name the two components of resistance to oxygen uptake by RBCs
Diffusion across membranes and through plasma and reaction to bind with hemoglobin
Is nitrous oxide diffusion limited or perfusion limited?
Perfusion
Is oxygen diffusion limited or perfusion limited?
Usually perfusion, but diffusion limited under conditions of intense exercise, alveolar hypoxia, and thickening of the blood-gas barrier
Name four conditions that decrease diffusion of gases across the blood-gas barrier
1) Emphysema - loss of pulmonary capillaries, less area
2) Asbestosis - thickening of the blood-gas barrier
3) Pulmonary embolism - decreased blood supply
4) Severe anemia - no reservoir in RBCs to act as a straw sucking oxygen (or CO) across from the alveolus
Compare the saturation rates of nitrous oxide and carbon monoxide in arterial blood
Both gasses have good solubility in water. However, nitrous oxide does not bind to hemoglobin, while CO (and oxygen) do. Thus, CO is sequestered in RBCs very quickly, which maintains the pressure gradient across the alveolar interstitium. Thus, nitrous oxide saturates quickly, while CO is taken up continuously while capillary blood transits the alveoli.
