Chapter 3: Diffusion Flashcards
Explain Fick’s Law
Fick’s law describes that the diffusion through tissues is proportional to the surface area and partial pressure gradient, and inversely proportional to the thickness of the tissue
Also diffusion constant (D) - proportional to solubility, and inversely proportional to the root of molecular weight
Explain why CO is a diffusion limited gas
CO maintains a high partial pressure gradient and maintains it as it moves and binds to the RBCs rapidly, alveolar to capillary gradient maintained/does not get saturated
Explain why NO2 is a perfusion limited gas
NO2 does not bind to Hgb - partial pressure in capillaries rises rapidly - gradient is reduced fast and blood needs to continue to flow to continue absorbing NO2
Is O2 diffusion or perfusion limited?
under normal conditions O2 is perfusion limited, as it equilibrates relatively fast (~ 1/3 of the time RBC spends in the capillaries)
this may change to diffusion limited if the diffusion barrier is compromised (i.e., needs longer to equilibrate)
How much time does a RBC usually spend in the pulmonary capillaries?
0.75 seconds
Fill in the blanks
What is a normal mixed venous and alveolar partial pressure of O2
~ 40 mm Hg mixed venous
~ 100 mm Hg alveolar
what is the difference between end-capillary and alveolar O2 pp?
under normal circumstances almost the same
What does it mean that there is a reserve capacity for O2 diffusion?
in normal circumstances, O2 pp gradient equillibrates within 1/3 of the time RBC spends on the capillary
if the blood flow was eccalerated (e.g., exercise) or the pp gradient was decreased (e.g., low inspired O2) the body can still compensate for this to a degree
How does exercise affect oxygenation of capillary blood?
increased blood flow, less time for RBC in capillaries - if there is a diffusion impairment or decreased inspired O2, i.e., O2 takes longer to move into capillaries –> body may not be able to compensate
How does a decreaed inspired oxygen affect oxygenation of pulmonary capillaries?
with a lower alveolar PAO2, the pp gradient is lower –> less drive for diffusion –> takes longer
What is DL?
diffusion capacity of the lung
measurement including the surface area, thickness, and diffusion constant
needed because thickness and surface area cannot be measured in an alive person/animal
How do you measure and calculate DL?
Fick’s law Vgas = A/T x D x P1-P2
Vgas = DL x P1-P2
DL = Vgas/P1-P2
* carbon monoxide is used for this measurement –> alveolar and end-capillary CO almost the same so
* DL = Vgas/PACO
measured with the single-breath method
* single inspiration dilute CO, then 10 second breath hold –> measure rate of dissappearance of CO with an infrared analyzer
What is the normal DL?
25 ml/min/mmHg
Why does DL increase during exercise?
recruitment and distension of pulmonary capillaries –> increases the pulmonary blood volume –> more blood to take up CO
