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Pulmonary Week 2 > ICL 1.4: Gas Exchange/Diffusion > Flashcards

ICL 1.4: Gas Exchange/Diffusion Flashcards

1
Q

what are the 3 steps in pulmonary gas exchange?

A
  1. alveolar ventilation
  2. pulmonary diffusion
  3. pulmonary blood flow
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2
Q

what is alveolar ventilation?

A

the process by which oxygen is brought into the alveoli from the external environment and carbon dioxide is expelled from the lungs

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3
Q

what is pulmonary diffusion?

A

net movement of gas molecules from an area of higher partial pressure to an area of lower partial pressure across the alveolar-capillary membrane

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4
Q

what is pulmonary blood for?

A

the blood that undergoes gas exchange with the alveolar air

it constitutes the entire output of the right ventricle

it supplies the lung with the mixed venous blood draining all of the tissues of the body

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5
Q

what is the normal arterial blood gas for O2, CO2, and pH?

A

PaO2 = 100 mmHg

PaCO2 = 40 mmHg

pH = 7.4

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6
Q

what is barometric pressure at sea level?

A

760 mmHg

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7
Q

what is the composition of dry inspired air?

A

O2 = 21%

CO2 = 0%

N2 = 79%

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8
Q

what is Dalton’s law?

A

Ptotal = P1 + P2 + P3 etc.

the total pressure of a gas mixture is the sum of the pressure contributed by each gas

partial pressure of a gas = (%gas/100)(Ptotal) = Fgas * Ptotal

Ptotal = barometric pressure = 760 mmHg

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9
Q

what is Henry’s law?

A

Cgas = k*Pgas

Cgas = concentration of gas in the liquid (mol/L)

k = solubility constant; dependent on each gas/liquid

Pgas = partial pressure of gas

it’s the relationship between the partial pressure of a gas within a liquid and its concentration within that liquid

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10
Q

what is Fick’s law?

A

Vgas = DAΔP/T

the rate of diffusion of a gas across a permeable membrane is determined by the chemical nature of the membrane itself, the surface area of the membrane, the partial pressure gradient of the gas across the membrane, and the thickness of the membrane

Vgas = rate of gas diffuse across permeable membrane

D = diffusion coefficient of that particular gas for that membrane

A = surface area of the membrane

ΔP = difference in partial pressure of the gas across the membrnae

T = thickness of the membrane

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11
Q

what is the PO2 of humidified tracheal air at sea level?

A 
PO2(atm) = Fgas*Ptotal
PO2 = 0.21*760 mmHg = 160 mmHg

PO2(inspired) = FgasPtotal = Fgas(Ptotal-PH2O)

PO2(inspired) = 0.21(760-47) = 150 mmHg

so once you’ve inspired air it gets humidified and now the air is no longer 100% oxygen; so you have to subtract the partial pressure of H2O from 760 to get the pressure of oxygen in the inspired air

so the blood in the capillaries has PO2 = 40 and PCO2 = 40 so both of these gases will be driven via diffusion across the alveolar surface into the alveoli

so CO2 will diffusion from the capillaries into the alveoli and PAO2 = PiO2 - PACO2 = 150-40 = 110 mmHg

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12
Q

what is the water vapor pressure at body temperature?

A

47 mmHg

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13
Q

if PAO2 is 105 mmHg and PACO2 = 40 mmHg, assuming equilibration of O2 across alveolar-pulmonary capillary barrier:

  1. what is the PaO2?
  2. how does this equilibrium occur?
  3. what is the concentration of dissolved O2 in the blood?

pulmonary artery: PO2 = 40 mmHg and PCO2 = 46 mmHg

pulmonary vein = PO2 = 100 mmHg and PCO2 = 40 mmHg

A
  1. 1-5 mmHg because A-a for O2 should be <5mm Hg
  2. diffusion!
  3. ΔP =O2 gradient = 105 - 40 = 65 mmHg

ΔP =CO2 gradient = 46-40 = 6 mmHg

oxygen has a much higher pressure gradient needed to successfully diffuse oxygen across the alveolar membrane

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14
Q

what is the proportion of O2 taken up by the lungs?

A

O2 uptake by the lungs matches O2 consumption by tissues

so if we consume 300 mL/minute then the pulmonary capillaries will uptake 300 mL/minute of oxygen from the alveoli

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15
Q

what is the proportion of CO2 taken up by the lungs?

A

CO2 production by the tissues equals the CO2 laminated by the lungs

so if we’re producing 300 mL/minute of CO2 then the pulmonary capillaries will give up 300 mL/minute of CO2

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16
Q

what is the concentration of O2 in the blood?

A

Henry’s law: Cgas = kPgas

C(O2) = (0.003mL/100mL/mmHg)*100 mmHg = 0.3

CO = 5 L –> 5000*(0.3/100) = 15 mL O2

how does it makes sense that the concentration of O2 in the blood is only 15 mL if we consume 300 mL/min?

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17
Q

why does oxygen require a higher pressure gradient than CO2 to diffuse across the alveolar membrane?

A

ΔP = 65 mmHg for O2 vs. 6 mmHg for CO2….why?

CO2 is 22 times more soluble in blood than O2 at body temperature! this allows for a lower pressure gradient to diffuse the same amount of molecules as oxygen

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18
Q

what is PACO2 normally? what factors can change it?

A

40 mmHg

it’s not a constant factor though and it varies based on 2 factors:

  1. metabolic CO2 production
  2. alveolar ventilation
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19
Q

what factors change alveolar PCO2?

A

normally it’s 40 mmHg but it can change based on metabolic CO2 production in the tissues and alveolar ventilation n

PACO2 = metabolic CO2 production/alveolar ventilation

alveolar ventilation is equivalent to the volume of alveolar space

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20
Q

what is the alveolar gas equation?

A

PAO2 = [(Patm-47)*0.21] - PACO2/RQ

47 is the pressure of water and .21 is the % composition of O2 i the air

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21
Q

how can you change the alveolar oxygen pressure?

A

PAO2 = [(Patm-47)*0.21] - PACO2

  1. increase atmospheric pressure –> hyperbaric chambers, climbing a mountain
  2. increase fraction of oxygen –> breathing 100% oxygen
  3. changes with alveolar PCO2
  4. changes with respiratory exchange ratio = CO2produced/O2 consumed = 0.8; this entity describes the type of fuel being used for metabolism
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22
Q

how does metabolism effect the alveolar oxygen concentration?

A

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

RQ is the respiratory quotient which is the amount of CO2 produced per O2 consumed

standard diet RQ = 0.8

fatty diet RQ = 0.7

starchy diet RQ = 1

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23
Q

how does alveolar ventilation effect the alveolar oxygen concentration?

A

alveolar ventilation is the alveolar volume

if you double the ventilation, that means the concentration of PACO2 will decrease because the concentration of CO2 is constant but with a larger volume, concentration will decrease

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

with double ventilation, PACO2 drops from 40 to 20 so then PAO2 will be 125 instead of the normal 100 which means alveolar oxygen concentration increased!

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24
Q

what happens to PaCO2, PaO2 and alveolar ventilation during moderate exercise vs. severe exercise?

A

during moderate exercise alveolar ventilation increases while PaO2 and PaCO2 don’t really change

during severe exercise, alveolar ventilation really increases, PaO2 increases and PaCO2 decreases –> when there is more ventilation, there’s less CO2 concentration in the alveolar space so the partial pressure gradient for CO2 has increased; this is because in the blood CO2 is 40 mmHg and now since the alveolar ventilation has increased the PaCO2 decreases!

if you have decreased ventilation, space has decreases which means concentration of CO2 increases! this decreases the alveolar concentration of O2

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25
Q

what happens to alveolar PCO2 and PO2 when you’re breathing air with low PO2?

A

PCO2 doesn’t change

PO2 decreases

this is because FO2 is decreased

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

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26
Q

what happens to alveolar PCO2 and PO2 when there’s increased alveolar ventilation and unchanged metabolism?

A

PCO2 decreases

PO2 increases

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

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27
Q

what happens to alveolar PCO2 and PO2 when there’s decreased alveolar ventilation and unchanged metabolism?

A

PCO2 increases

PO2 decreases

decreased alveolar ventilation increases CO2 which then causes O2 to drop since there’s more CO2

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

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28
Q

what happens to alveolar PCO2 and PO2 when there’s increased metabolism and unchanged alveolar ventilation?

A

PCO2 increases

PO2 decreases

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

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29
Q

what happens to alveolar PCO2 and PO2 when there’s decreased metabolism and unchanged alveolar ventilation?

A

PCO2 decreases

PO2 increases

PAO2 = [(Patm-47)*0.21] - (PACO2/RQ)

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30
Q

what happens to alveolar PCO2 and PO2 when there’s a proportional increase in metabolism and alveolar ventilation?

A

no change in PCO2 or PO2

31
Q

how long does it take for oxygen and CO2 exchange to occur between the alveoli and the capillaries?

A

most of it happens in .25 seconds but the RBCs are in contact with the alveoli for .75 seconds and that’s important

even though CO2 is more soluble than O2, CO2 is a little heavier than O2 so it will diffuse a little slower

32
Q

what is Graham’s law?

A

when gases are dissolved in liquids, the relative rate ofdiffusionof a given gas is proportional to its solubility in the liquid and inversely proportional to the square root of its molecular mass

(diffusion rate of CO2/diffusion rate of O2)*sqrt(mass O2/mass of CO2)

33
Q

what is Fick’s law of diffusion?

A

Vgas = kAΔP/T

the netdiffusionrate of a gas across a fluid membrane is proportional to the difference in partial pressure, proportional to the area of the membrane and inversely proportional to the thickness of the membrane

34
Q

what conditions decrease the rate of diffusion?

A
  1. interstitial or alveolar pulmonary edema
  2. pulmonary fibrosis
  3. pulmonary hypertension
35
Q

how does pulmonary fibrosis effect the rate of diffusion?

A

thickness of the tissue around the alveoli is increased which decreases the rate of diffusion!

Vgas = FAΔP/T

36
Q

how does emphysema effect the rate of diffusion?

A

emphysema destroys the alveoli so surface area is decreased!

Vgas = FAΔP/T

37
Q

what would increase the the diffusion rate in the lungs by increasing surface area?

A

exercise!

it increases the surface area by:
1. greater ventilation

  1. greater pulmonary blood flow
  2. better matching of ventilation to blood flow at alveolar level
  3. greater surface area for diffusion
  4. increased rate of diffusion
38
Q

what conditions decrease the diffusion rate in the lungs by decreasing the pressure gradient?

A

Vgas = FAΔP/T

there’s decreased alveolar O2 in:
1. high altitudes

  1. hypoventilation
39
Q

what contributes to the partial pressure of oxygen in the capillaries?

A

ONLY the dissolved O2!!!!

the O2 bound to Hb doesn’t contribute to the pressure of O2!

40
Q

what are the 3 forms of CO2 in the capillaries?

A
  1. bound to Hb
  2. H2CO3
  3. dissolved CO2

only the dissolved CO2 is what contributes to the partial pressure of CO2!

41
Q

when does diffusion of a gas into/out of the alveoli stop?

A

diffusion of a gas will stop when the PA=Pa for the gas

for a gas like O2, as soon as it enters blood it is rapidly “sponged” up by the Hb – this keeps the gas from achieving a high PP in solution and the diffusion continues till all the Hb is saturated

once the Hb is saturated the dissolved gas rapidly achieves the same partial pressure as in the alveoli and the diffusion stops but Hb saturation takes a while so diffusion of O2 will continue for a while

42
Q

why is the RBC transit time in the pulmonary capillaries important?

A

so even though we pretty much diffuse everything across in .25 seconds, the RBCs are in the pulmonary capillaries for .75 seconds

this helps us maintain 100% saturation in case of high CO requirements like during exercise –> during exercise the transmit time of RBCs through the lungs is shorter because the flow increases so if at .25 seconds we’re getting 100% Hb saturation, then when we decrease transit time during exercise to .5 seconds or something, we still get fully saturated!

so diffusion is not decreased during exercise in persons with normal diffusing capacity because pulmonary capillary transit time is not less than equilibration time (if you’re diffusion impaired, you’ll be hypoxic with exercise or other low oxygen conditions like mountains)

43
Q

what are the relative rates of diffusion of O2 vs. CO2 across the alveolar/capilary membrane?

A

the partial pressure gradient (ΔP) for O2 (60 mmHg) is 10x greater than that for CO2 (6 mmHg)

but, CO2 is ~20x more diffusible than O2

the equilibration time (the time it takes for diffusion at the alveolar-capillary level) for both gases is not appreciably different (~0.25 sec)

for both, CO2 and O2, the diffusion of gas across the A-C membrane is perfusion limited

44
Q

what is diffusion-limited gas exchange?

A

diffusion-limited gas exchange is characterized by incomplete equilibration

ex. CO

45
Q

what is perfusion-limited gas exchange?

A

perfusion limited gas exchange describes the scenario in which the rate at which gas is transported away from functioning alveoli and into tissues is principally limited by the rate of blood flow through the pulmonary capillaries and thus across the alveolar membrane

ex. N2O, O2, CO2

46
Q

why is CO diffusion limited?

A

Because CO has a very high affinity for hemoglobin (240x that of oxygen), it combines with Hb almost as fast as it diffuses across the alveolar-capillary membrane

very little CO dissolves in the blood and thus there is only a small rise in the partial pressure of CO in the pulmonary capillary

the ΔP for CO between the alveoli and the pulmonary capillary is maintained throughout the capillary transit time and thus the diffusion of CO continues and depends on the properties of the A- C membrane, i.e., its surface area and thickness (“diffusion- limited”)

the “sponge” never lets up and prevents the gas from dissolving enough to achieve the same PP in blood as in alveoli

47
Q

is the transfer of CO2 and O2 diffusion or perfusion limited?

A

CO2 and O2, the diffusion of gas across the A-C membrane is perfusion limited

this means that the Pa=PA by the times blood leaves the capillary around the alveoli

if PAGas = PaGas its perfusion limited! if not, its diffusion limited.

48
Q

what are the 5 reasons that lead to hypoxemia?

A
  1. ventilation-perfusion (V/Q) mismatch –> ideal range is 0.8-1.2. It happens in pneumonia, asthma, COPD, ARDS, Heart failure. Respond well to supplemental O2
  2. right to left shunt –> blood bypasses the lung altogether. Due to anatomic shunt in heart, or physiologic shunt in severe pneumonia, ARDS, heart failure. Supplemental O2 doesn’t help
  3. hypoventilation –> not movingenough air. It’s associated with an increase in CO2, and causes include CNS causes (sedation, stroke, tumours), neuromuscular disorders, airway obstruction (COPD, asthma, laryngospasm), and dead space ventilation
  4. diffusion defect –> oxygen isn’t getting from the airto the blood. causes include emphysema, PJP, atypical pneumonias, and pulmonary fibrosis
  5. low inspired oxygen content –> high altitude
49
Q

what is hemoglobin?

A

a protein with four subunits, each of which has a heme moiety attached to a polypeptide chain (HbA: two α and two β chains)

heme: complex of a porphyrin and one iron molecule (Fe2+ or Fe3+). Each of the four iron atoms can bind reversibly one molecule of O2

50
Q

at what saturation of oxygen does Hb bind oxygen?

A

there are 4 oxygen binding sites on oxygen and the first site is ALWAYS bound to oxygen; so at no point ever are you below 25% oxygen saturation

the second site binds at PaO2 = 26mmHg

3rd site binds at PaO2 = 40 mmHg

4th site binds at PaO2 = 100 mmHg

the blood entering the alveolar capillaries is already at 40 mmHg which means it’s already at 75% saturation when it gets to the lungs and it will get saturated to 100% as it passes through the lungs

51
Q

what is blood doping?

A

the amount of oxygen in your blood is limited by the amount of Hb in your blood; once it’s totally saturated you can’t pick up any more oxygen!

if you give someone blood, you increase the %volume of Hb in the blood and therefore increase your oxygen –> this improves athletic performance

52
Q

how much oxygen does your body need at rest?

A

250-300 mL/minute

VO2(consumption of O2)= Cardiac Output * (PaO2-PvO2)10= 5L (20-15 vol %)*10=250ml/min

53
Q

describe the oxyhemoglobin dissociation curve for fetal Hb

A

fetal Hb has a higher binding affinity than normal O2 so it can operate at a lower PO2 than adults

54
Q

describe the oxyhemoglobin dissociation curve for myoglobin

A

myoglobin approaches full saturation at PO2 levels normally found in voluntary muscle (15–30 mmHg)

the bulk of its oxygen can only be released at very low PO2 during exercise

it’s not a sigmoidal curve

55
Q

describe the oxyhemoglobin dissociation curve for oxygen

A

the S-shape facilitates oxygen loading in the pulmonary capillaries, and oxygen unloading at the tissues

important safety factor because a relatively low arterial PO2 of 60 mm Hg still has relatively high O2 sat (90%)

steep curve between 10-40 mm Hg suggest small decrease in pO2 will cause substantial dissociation of O2- more unloading

56
Q

what factors shift the HbO2 curve to the right?

A

shift to the right = decreased affinity for Hb oxygen binding sites for O2 facilitates oxygen let off at the tissue level when the PO2 is falling = increased tissue O2 delivery

  1. increased H+
  2. increased DPG
  3. increased temperature
  4. increased PCO2
57
Q

what is DPG and how does it effect oxygen dissociation curve?

A

increased DPG shifts the curve to the right and increases O2 tissue delivery by decreased the affinity of Hb to O2

DPG is part of the glycolysis pathway and RBCs have a lot of DPG

DPG is a great compensatory mechanism for hypoxia! in hypoxia there’s more glycolysis which increases DPG levels since it’s an intermediate which shifts the curve to the right and increases oxygen delivery to the tissue!

great for COPD or high altitudes

58
Q

how does CO2 effect the oxygen binding curve?

A

Increases in the carbon dioxide partial pressure of blood or decreases in blood pH result in a lower affinity of hemoglobin for oxygen

this is the Bohr effect!!

this explains the unloading of oxygen in the tissues because tissues are making lactic acid so the conditions are acidic which leads to oxygen unloading in the tissues!!

59
Q

how does CO effect the oxygen binding curve?

A

CO has a really strong affinity to Hb so Hb never achieves maximum oxygen saturation since the CO binds irreversibly to the Hb so that oxygen can’t bind

CO makes oxygen binding sites unavailable to bind oxygen thereby reducing Vmax

the curve isn’t sigmoidal anymore, it’s shorter and it’s also kind of shifted to the left which makes the little oxygen that is bound to Hb really hard to deliver

60
Q

what happens when there’s a left shift in the oxygen binding curve?

A

there’s increased affinity of O2 for Hb which means that O2 will bind Hb at lower oxygen concentrations

this decreases delivery of oxygen to the tissues since it’s more tightly bound to Hb

61
Q

which Hb variants have high O2 affinity?

A
  1. fetal Hb
  2. Hb Rainier

these would cause a left shift

62
Q

which Hb variants have low O2 affinity?

A
  1. Hb Kansas
  2. Hb Seattle

these would cause a right shift

63
Q

what is the structure of fetal Hb?

A

Hb F has 2 α and 2 γ chains compared to 2 α and 2 β chains in Hb A

64
Q

why does HbF have a higher affinity for oxygen?

A

γ chains in HbF have less affinity for 2,3-DPG

this shifts the graph to the left

65
Q

how does pulse oximetry work?

A

when Hb is bound to oxygen it’s a different light wave

66
Q

what is the Haldane effect?

A

deoxygenated Hb binds CO2 more readily than oxygenated Hb which is known as the Haldane effect

as a result:
1. lungs (oxygen high) promotes CO2 release

  1. increased O2 binding results in H+ release from Hb, so more CO2 release
  2. more CO2 in venous side
67
Q

where does the Bohr vs. Haldane effect dominate?

A

Bohr effect: CO2 –> H+ + HCO3-

this promotes the release of oxygen because of the increased acidity so this is important in the tissues

Haldane effect is when deoxygenated Hb binds CO2 more than oxygenated Hb so that’s important in the lungs when Hb gets back to the lungs and starts binding to Hb, it promotes the release of CO2

68
Q

someone is ascending a mountain with an atmospheric pressure of 420 mmHg. her Hb O2 saturation is 65%. there’s no change in ventilation. what is the oxygen saturation in blood?

A

(420-47).21 = 78

PACO2 = unchanged

PAO2 = (78-40) = 38

PA - Pa = 38-40 = -2

so then the concentration of O2 in the blood is [Hb] x 1.34 x SaO2 = 15 x 1.34 x 0.65 = 13.06 mL/Dl of O2 in the blood compared to normal 19.7 in a normal person

69
Q

how do people ascend mountains?

A

hypoxemia increases the respiratory drive which increases the total V/Q ratio and O2 delivery to the alveoli which partially correct PAO2 and PaO2

70
Q

what are the 3 measurements needed to calculate blood oxygen concentration?

A
  1. Hb
  2. SaO2
  3. PaO2
71
Q

what are the 3 primary ways that CO2 is transported in the blood?

A
  1. HCO3-
  2. dissolved in plasma
  3. carbaminohemoglobin

bicarbonate ion is the primary way

72
Q

binding to topical chemical renders Hb unable to bid O2. what would be the partial pressure of O2 in the capillary?

A

100 mmHg

73
Q

the PCO2 of exhaled hair is ____the PCO2 of alveolar air because exhaled air is a mixture of _____ gas

A

lower

deadspace and alveolar

Pulmonary Week 2 (9 decks)

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