31 - Gas Exchange

Question 1

1) Understand the concept of partial gas pressure and how it applies to respiratory gases

Answer 1

• Gradient in partial pressure is the main driver of gas exchange.
• Dalton’s Law of partial pressure
o Pressure of gas in an environment is directly proportional to the concentration of its molecules
o To get dry gas pressure, water vapor pressure must be subtracted from total pressure.

Question 2

Dalton’s law

Answer 2

Px = Fx (PB - PH2O)

Px = partial pressure of breathing gas x
PB = barometric pressure
Fx = concentration

Question 3

Henry’s law

Answer 3

o Partial pressure of gas dissolved in an environment depends on its concentration and the solubility coefficient.
o For a given partial pressure, the higher the solubility of the gas, the higher the concentration of the gas in solution.

Partial pressure = concentration of dissolved gas / solubility coefficient

Question 4

2) Understand the concept of the ideal gas law and how it applies to respiratory gas exchange

Answer 4

• Ideal gases
o Ideal gasses do not condense, evaporate, or sublime at operating range
o O2 and N2 are ideal
o CO2 is considered ideal in operating range
o PV=nRT
• Because nRT is essentially the same b/t the two environments, we focus on: P1V1= P2V2
• Because of the above equation, if pressure is altered, gas exchange will occur until equilibrium is returned (works for anesthesia)

Question 5

3) Understand the process of gas exchange b/t the gas phase in the alveoli and the aqueous phase of the blood

Answer 5

• Net diffusion is determined by the difference between the two partial pressures.
• Diffusion occurs from [↑] to [↓] of the particular gas.

Question 6

Fick’s law

Answer 6

• Fick’s Law:
o When there is a tissue sheet b/t the two body fluids, the amount of gas that moves across the area is proportional to the surface area and inversely proportional to the thickness of sheet.

Question 7

• Diseases affecting diffusion rate:

Answer 7

Atelectasis
Pulmonary fibrosis
Pulmonary edema
Pneumonia

Question 8

o Atelectasis

Answer 8

 Decreases the diffusion rate

 There is a change in pressure because affected alveoli collapse and no air is coming in, decreasing pressure

Question 9

o Pulmonary fibrosis

Answer 9

 Decreases the diffusion rate

 Interstitial thickness is increased

Question 10

o Pulmonary edema

Answer 10

 Decreases the diffusion rate

 Interstitium is flooded with exudate from capillaries/veins

Question 11

o Pneumonia

Answer 11

 Decreases the diffusion rate

 Interstitial tissues/alveoli are inflamed

Question 12

5) Understand the relationship between diffusion and perfusion in the effectiveness of gas exchange in the lungs

Answer 12

• Poor perfusion/ventilation will decrease the partial pressure difference between the two compartments.
• Can lead to diminished or no gas exchange.

Question 13

O2 gas exchange at alveoli-pulmonary capillaries

Answer 13

o 2 main factors:
 Diffusion between alveoli or capillaries and blood perfusion through capillaries
o Pressure gradient is 104 Hg  40 Hg (The gradient difference is 64 Hg)
o Normal:
 By the time blood has moved 1/3 of the way thru the capillary, it has became about 104 mm Hg O2.

Question 14

O2 gas exchange at alveoli-pulmonary capillaries during exercise

Answer 14

 20x O2 required
 Time of blood in capillaries is reduced due to ↑ cardiac output.
 Having said that, diffusing capacity for O2 increases almost 3x.
 By the time blood leaves capillaries it’s almost saturated to 104 mm Hg O2

Question 15

• Tissue capillary-interstitium-cell O2 exchange

Answer 15

o Systemic blood ~ 95 mm Hg O2 due to bronchial circulation shunt.
o Interstitial fluid ~40 mm Hg O2.
o This difference allows O2 to diffuse rapidly.
o Capillary pressure falls to around 40 mm Hg O2 by the end of the capillary.
o Intracellular pressure of O2 is roughly 23 mm Hg O2 because its constantly being used by the cell.

Question 16

7) Understand the relationship between tissue PO2, blood flow and rate of metabolism

Answer 16

• If blood flow ↑, then pressure of PO2 also ↑
• If highly metabolizing cells use up a lot of O2, the interstitial PO2 ↓.
• Essentially, interstitial PO2 is determined by balance of rate of metabolism and rate of blood flow

Question 17

8) Describe the process of CO2 exchange at different interfaces: cell-interstitium and blood-alveoli under normal and strenuous exercise.

Answer 17

Cells-interstitium:
o CO2 is much more diffusible than O2. This means the partial pressure gradient does not need to be as large.
o 46 mm Hg CO2 in cell, 45 mm Hg CO2 in interstitium, 40 mm Hg CO2 in capillary blood

Blood-alveoli:
o Normal: CO2 diffuses to air within short time frame
o Exercise: CO2 diffuses by the end of capillary flow

Question 18

9) Understand the basis for measurement of DLCO as a pulmonary function test

Answer 18

• Lung diffusion capacity is a common measurement of lung’s ability to transfer gases.
• Because it is difficult to obtain accurate expiratory [O2] and [CO2], carbon monoxide (CO) is used.
• CO binds immediately to hemoglobin so the partial pressure difference is determined by the alveolar pressure of CO.
• We can then determine diffusion capacity based on how much CO has left the alveolar area in a certain amount of time. So…

DLCO = JCO/PACO

DLCO = CO diffusing capacity
JCO = CO uptake
PACO = alveolar PCO

• To convert CO diffusion capacity to O2 diffusion capacity, the value must be multiplied by the diffusion coefficient for O2.

Question 19

• Polycythemia

Answer 19

o ↑ DLCO is found in polycythemia because of the large ↑ in RBC and Hb

Question 20

• Other lung conditions

Answer 20

o ↓ DLCO is found in emphysema, pulmonary fibrosis, interstitial lung disease, pulmonary hypertension, chronic pulmonary thromboembolism, and ↓ blood flow thru lungs (anemia).

This is a sensitive test for lung disease, but does not show the specificity of what disease is the antagonist.

Question 21

10) Understand the alveolar gas equation and apply it to predict the status of gas exchange.

Answer 21

• This equation can be used to asses the status of gas exchange or quality of the respiratory membrane.

PAO2 = FiO2 (PB - PH2O) - (PaCO2 / RQ)

PAO2 = alveolar O2 pressure (measured @ hosp)
FiO2 = fraction of inspiratory O2 (0.21)
PB = barometric pressure (760)
PH2O = partial pressure of water vapor (47)
PaCO2 = arterial pressure of CO2 (measured @ hosp)
RQ = respiratory quotient (0.8)