Chapter 3: Diffusion Flashcards

1
Q

Explain Fick’s Law

A

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

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2
Q

Explain why CO is a diffusion limited gas

A

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

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3
Q

Explain why NO2 is a perfusion limited gas

A

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

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4
Q

Is O2 diffusion or perfusion limited?

A

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)

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5
Q

How much time does a RBC usually spend in the pulmonary capillaries?

A

0.75 seconds

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6
Q

Fill in the blanks

A
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7
Q

What is a normal mixed venous and alveolar partial pressure of O2

A

~ 40 mm Hg mixed venous
~ 100 mm Hg alveolar

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8
Q

what is the difference between end-capillary and alveolar O2 pp?

A

under normal circumstances almost the same

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9
Q

What does it mean that there is a reserve capacity for O2 diffusion?

A

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

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10
Q

How does exercise affect oxygenation of capillary blood?

A

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

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11
Q

How does a decreaed inspired oxygen affect oxygenation of pulmonary capillaries?

A

with a lower alveolar PAO2, the pp gradient is lower –> less drive for diffusion –> takes longer

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12
Q

What is DL?

A

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

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13
Q

How do you measure and calculate DL?

A

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

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14
Q

What is the normal DL?

A

25 ml/min/mmHg

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15
Q

Why does DL increase during exercise?

A

recruitment and distension of pulmonary capillaries –> more surface area + increases the pulmonary blood volume –> more blood to take up CO

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16
Q

How fast is the reaction time of O2 and Hgb?

A

0.2 seconds

17
Q

Why does CO2 diffuse faster than O2? How much faster?

A

because of its higher solubility
20 times faster

18
Q

List 3 causes that can make O2 diffusion limited

A
  • exercise
  • diffusion barrier disease
  • alveolar hypoxia