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Pulm by Minnie: AH Update > SM 156a - Diffusion and Gas Transport > Flashcards

SM 156a - Diffusion and Gas Transport Flashcards

Some images and questions are borrowed from Dr. McCrimmon's lecture on Diffusion and Gas Transport (SM 156a)

1
Q

Under normal conditions, at what partial pressure of oxygen is hemoglobin in the blood 50% saturated with oxygen?

(What is the P50 for oxygen bound to hemoglobin under normal conditions?)

A

27 mmHg

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2
Q

What causes anemic hypoxia?

A

Low O2 capacity due to low hemoglobin

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3
Q

What is the equation for oxygen saturation (SO2)?

A

SO2 = O2 content / O2 capacity * 100

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4
Q

Which gas is perfusion limited?

How can you tell?

A

Gas X is perfusion limited

Pc’X reaches PAX in <0.75 seconds; this means that blood gets across the alveolar-capillary membrane with enough time to spare, and perfusion must keep up in order to maintain the concentration gradient

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5
Q

What causes circulatory hypoxia?

A

Shock or hypofusion of tissues

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6
Q

In the pulmonary capillary bed, is oxygen usually perfusion or diffusion limited?

A

Perfusion limited

In healthy lungs, diffusion occurs quickly; gas exchange depends on the rate of blood flow through the pulmonary capillary bed

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7
Q

Hypoxia due to shock or hypofusion of tissues is called ___________ hypoxia

A

Hypoxia due to shock or hypofusion of tissues is called circulatory hypoxia

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8
Q

How does gas content vary between the gas phase and the liquid phase?

A
  • Gas phase
    • Content: n = PV/RT
    • Governed by the ideal gas law
  • Liquid phase
    • Content: C = solubility * Partial pressure
    • Governed by Henry’s law
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9
Q

Which gas is diffusion limited?

How can you tell?

A

Gas Y is diffusion limited

In 0.75 seconds, Pc’Y has not reached PAY; Diffusion across the alveolar-capillary membrane is too slow for the partial pressures to equilibrate in the time that blood is in the pulmonary capillary bed

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10
Q

What happens to diffusing capacity when the individual alveolar air spaces become large?

Why?

A

Diffusing capacity decreases

  • Surface area for diffusion decreases
  • Distance that gas must diffuse increases
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11
Q

In which 3 forms is CO2 transported in the blood?

A
  1. Dissolved CO2
  2. Bicarbonate (hydrated CO2)
  3. Carbamino compounds (bound to hemoglobin)
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12
Q

Dissolved CO2 makes up ___% of the total CO2 in venous blood, and ___% of the CO2 released to the lungs

A

Dissolved CO2 makes up 5 % of the total CO2 in venous blood, and 10% of the CO2 released to the lungs

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13
Q

What is the normal difference in oxygen content between arterial and mixed venous blood?

A

20.1 vol% - 15.1 vol% = 5 vol% O2

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14
Q

Which enzyme catalyzes the rate limiting reaction in the formation of HCO3-?

A

Carbonic anhydrase

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15
Q

Describe the Bohr effect

A

H+ + HbO2 -> HHb + O2 -> HbNH2 + CO2 -> HbNH-COOH

When H+ is elevated in tissus, oxygen is displaced from hemoglobin by CO2, creating a carbamino compound. This causes a right shift in the oxygen-hemoglobin binding curve.

Occurs in the tissues

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16
Q

What causes histotoxic hypoxia?

A

The available oxygen cannot be utilized and oxygen extraction is lowered (ex: cyanide poisoning)

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17
Q

Oxygen capaity is a direct function of the amount of _________ present in the blood

A

Oxygen capaity is a direct function of the amount of hemoglobin present in the blood

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18
Q

What is the equation for blood O2 content?

A

CO2 = SO2 * [Hb] * O2 capacity

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19
Q

What are the two factors that contribute to resistance to oxygen diffusion into the pulmonary capillaries?

A
  1. Resistance to diffusion thought the blood-gas barrier (1/Dm)
  2. Chemical reaction rates of O2 and CO2 ( 1/(θ*Vc) )
    • θ = Volume of gas contributed by RBCs in 1 mL of blood per mmHg of pressure gradient between plasma and RBC
    • Vc = Pulmonary capillary blood volume
20
Q

Is carbon dioxide usually perfusion or diffusion limited?

A

Perfusion limited

21
Q

Suppose a tissue is not adequately perfused. What is likely to occur first: toxic CO2 levels or hypoxia?

A

Hypoxia

CO2 can typically be cleared away becaue it diffuses across membranes 20x faster than oxygen

22
Q

In which two forms is oxygen transported in the blood?

A

Dissolved O2

Chemically bound to hemoglobin

23
Q

For a gas (X) that is perfusion limited, a small increase in the cardiac output will…

  1. Increase Pc’X
  2. Have no effect on Pc’X
  3. Decrease Pc’X
A

b. Have no effect on Pc’X

Blood will spend less time in the pulmonary capillary, but perfusion limited gases typically have a large “safety margin;” their partial pressure equilibrates with PA long before blood leaves the pulmonary capillary

24
Q

What is the equation for Henry’s law?

What does this equation describe?

A

Cg = K * Pg

  • Cg = concentration of the dissolved gas (g)
  • K = solubility coefficient
  • Pg = partial pressure of the gas

Henry’s law describes the concentration of dissolved gas in a solvent

25
Q

What is the chloride shift?

A

A rate-limiting chemical reaction that slows CO2 equillibration at the lung

  • Increased HCO3- in the RBC activates the Cl-/HCO3- exchanger
  • Cl- comes into teh cell
  • H+ ions stay in the RBC, buffered by hemoglobin
  • This slows down CO2 offloading to the lungs
26
Q

Why is carbon monoxide dangerous when inhaled?

A

Carbon monoxide binds very stongly with hemoglobin, and only desaturates at very low PCO

Carbon monoxide causes poisoning because oxygen cannot bind hemoglobin, and therefore cannot be delivered to tissues

27
Q

Which form of O2 in the blood contributes to PaO2?

  1. Dissolved O2
  2. O2 Bound to hemoglobin
  3. Both a and b
  4. Neither a or b
A

a. Dissolved O2

=> Oxygen bound to hemoglobin does not contribute to PaO2

=> Oxygen must continue to bind to hemoglobin to maintain the concentration gradient

28
Q

What is the equation for Fick’s law of diffusion?

29
Q

What is the O2 uptake by the body per minute?

A

250 mL O2/min

Arterial O2 content = 20.1 vol%

Mixed venous O2 content = 15.1 vol%

Difference = 5 vol%

Cardiac output = 5000mL blood/min

5 mL O2 /100 mL blood * 5000 mL blood/min = 250 mL O2/min

vol% = mL O2 per 100 mL blood

30
Q

Hypoxia caused by low PO2 in arterial blood is called ________ hypoxia

A

Low PO2 in arterial blood is called hypoxic hypoxia

31
Q

Carbamino compounds make up ___% of the total CO2 in venous blood, and ___% of the CO2 released to the lungs

A

Carbamino compounds make up 5 % of the total CO2 in venous blood, and 30% of the CO2 released to the lungs

(High proportion of CO2 released to lungs due to the Bohr/Haldane effect)

32
Q

What is the normal value of mixed venous O2 content?

A

CO2 = SO2 * [Hb] * O2 capacity

Normal mixed venous: 0.75 * 15 * 1.34 = 15.1 mL O2 / 100 mL blood

33
Q

What binds more CO2: reduced hemoglobin or oxyhemoglobin?

A

Reduced hemoglobin (Deoxyhemoglobin)

This means that deoxyhemoglobin is a weaker acid than oxyhemoglobin

34
Q

CO2 diffuses across the alveolar-capillary membrane 20x faster than oxygen. Why then, is equillibration of CO2 only slightly faster than equillibration of O2?

A

Chemical reactions limit the equillibration of CO2

  • Formation of HCO3
  • Formation of carbamino compounds with blood proteins (hemoglobin)
  • Chloride shift
35
Q

Bicarbonate makes up ___% of the total CO2 in venous blood, and ___% of the CO2 released to the lungs

A

Bicarbonate makes up 90 % of the total CO2 in venous blood, and 60% of the CO2 released to the lungs

Release to the lungs is slowed by chemical reactions
(ex: chloride shift)

36
Q

Which gas represents the dissociation of CO2 in the blood?

37
Q

For a gas (Y) that is diffusion limited, increasing the cardiac output will…

  1. Increase Pc’Y
  2. Have no effect on Pc’Y
  3. Decrease Pc’Y
A

c. decrease Pc’Y

Blood will spend less time in the pulmonary capillary, thus decreasing Pc’Y

38
Q

Hypoxia caused by low oxygen capacity due to low hemoglobin is called __________ hypoxia

A

Hypoxia caused by low oxygen capacity due to low hemoglobin is called anemic hypoxia

39
Q

What is the normal value of arterial O2 content?

A

CO2 = SO2 * [Hb] * O2 capacity

Normal: 0.975 * 15 * 1.34 = 19.6 mL O2 / 100 mL blood

If you ignore dissolved oxygen:
1.00 * 15 * 1.34 = 20.1 mL O2 / 100 mL blood

40
Q

Which reaction is rate limiting for oxygen transfer from gas to blood?

A

Oxygen combining with hemoglobin

41
Q

Which gas represents the dissociaton of oxygen in the blood?

42
Q

What causes hypoxic hypoxia?

A

Low PO2 in arterial blood

43
Q

List the barriers that oxygen must cross between inspiration and the mitochondria of a tissue cell

A
  • Gas space within the alveolus
  • Alveolar-capillary membrane
  • Plasma capillary
  • Into and out of the RBC
    • Includes hemoglobin binding and unbinding
  • Plasma capillary
  • Interstitial fluid
  • Plasma membrane
  • Mitochondrial membrane
44
Q

In the blood, 1g of hemoglobin binds ___mL of O2

A

In the blood, 1g of hemoglobin binds 1.34 mL of O2​

45
Q

What is oxygen capacity?

A

The amount of oxygen bound to hemoglobin when hemoglobin is saturated

46
Q

Describe the Haldane effect

A

HHb + O2 -> HbO2 + H+

Increased free H+ pushes this equation to the right:
H+ + HCO3- -> H2O + CO2

As a result, H+ decreases, facilitating:
HbNH-COOH -> HbNH2 + CO2

This occurs in the lungs (lots of O2 around) and allows CO2 to be offloaded from the blood to the lungs

Pulm by Minnie: AH Update (34 decks)

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