Exam #3: Gas Exchange

Question 1

What is effective respiration?

Answer 1

Enough oxygenation to meet tissue demand

Question 2

What is the main driver of gas exchange?

Answer 2

• The gradient in partial pressure of gases

- Gases diffuse “downhill” along the gradient of decreasing partial pressure

Question 3

What is Dalton’s Law of Partial Gas Pressures?

Answer 3

Pressure of a gas is directly proportional to the concentration of its molecules

Ptotal= P1 + P2 +P3
Pb= PN2 + PO2 + PCO2 + Pothers

Pb= 760 mmHg

Px=Fx (PB-PH20)

Question 4

How is the the total dry gas pressure obtained?

Answer 4

Subtract water vapor pressure from the total barometric pressure

Px= Fx (PB- PH20)

Question 5

What is Henry’s Law?

Answer 5

When gas is DISSOLVED in fluid & encounters a surface, it exerts its own partial pressure

Partial pressure= Concentration of dissolved gas/ solubility coefficient

Question 6

How does the solubility of CO2 compare to O2?

Answer 6

CO2»>O2

• This means CO2 dissolves easily.
• At EQUAL concentrations, oxygen exerts a higher partial pressure than CO2

Question 7

What is the ideal gas law?

Answer 7

PV=nRT

Question 8

What is the functional implication of the ideal gas law?

Answer 8

If P1>P2 or vice versa, then gas exchange will take place until:

P1V1= P2V2

E.g. in anesthesiology, when gas is administered, gas exchange will occur until Palveoli = Pblood

Question 9

How do gases diffuse in fluids & tissues?

Answer 9

Gas molecules diffuse proportionally to gas partial pressure difference

Question 10

What is Fick’s law of diffusion?

Answer 10

The amount of gas that moves across a tissue sheet is:

• Proportional to area
• Inversely proportional to thickness

Question 11

Write the equation for net rate of diffusion.

Answer 11

Diffusion is proportional to:

• Pressure
• Area
• Solubility

And inversely proportional to:

• distance through which the gas must diffuse
• square root of molecular weight of gas

Question 12

Describe gas exchange at the respiratory membrane.

Answer 12

Oxygen
- 100 P02 in alveoli
- 40 PO2 in venous circ.
Net into circulation

Question 13

What two things are required for adequate respiration?

Answer 13

Diffusion

Perfusion

Question 14

What is the difference between PAO2 & PaO2?

Answer 14

A= alveoli 
a= arterial

Question 15

What is the respiratory unit?

Answer 15

Location of gas exchange

• Bronchioles
• Alveolar ducts
• Atria
• Alveoli

Question 16

How is diffusion effected in atelectasis?

Answer 16

Collapse of lung tissue

• Decreased dP
• Increased distance
• Decreased SA

Question 17

How is diffusion effected in pneumonia?

Answer 17

Markedly increased distance = decreased diffusion

Question 18

How is diffusion effected in pulmonary edema?

Answer 18

Increased capillary hydrostatic pressure

• Increased distance
• Diffusion effectively blocked

Hydrostatic pressure increases & can cause pink frothy sputum (capillary blood)

Question 19

How is diffusion effected in pulmonary fibrosis?

Answer 19

Increased distance due to increase collagen & elastin deposition

Question 20

Describe oxygen diffusion between the alveoli & pulmonary capillary.

Answer 20

Arterial end= pulmonary arterial end
- Low PO2 (40 mmHg)
- Alveoli= 100mmHg
Oxygen in

Equilibration occurs by the time the blood reaches the venous end i.e. to go into the left side of circulation

Question 21

During exercise & tachycardia how does equilibration of oxygen change?

Answer 21

Takes longer

Question 22

What happens to the diffusion of oxygen in the tissues?

Answer 22

Arterial PO2» Venous & capillary= net diffusion of oxygen into the tissues

• Opposite occurs with CO2

Question 23

Draw the change in PO2 as blood travels through the circulation.

Answer 23

N/A

Question 24

What causes the drop in PO2 from pulmonary capillaries to systemic arterial blood?

Answer 24

Pulmonary shunting occurs i.e. deoxygenated blood from bronchioles returns straight to the left side of the circulation

Question 25

Draw the interdependence between tissue PO2, blood flow, & tissue metabolism.

Answer 25

Blood flow has to adapt to metabolic rate to maintain tissue oxygenation

Less metabolism= less flow

More metabolism= more flow

Question 26

Why is the small pressure difference between capillary CO2 & interstitial CO2 sufficient to drive diffusion?

Answer 26

High solubility of CO2

Question 27

What is the Diffusing Capacity (DlCO)

Answer 27

• Measurement of the lungs ability to transfer gases
• Patient breathes small amount of CO & initial value is measured
• Diffusion occurs
• Measure CO in alveoli afterward

Determine capacity to diffuse

Question 28

How do you predict the diffusing capacity of the lung for oxygen?

Answer 28

DLCO x 1.23

Question 29

What does a decrease in diffusing capacity tell you?

Answer 29

1) Thickened or damaged respiratory membrane
2) Anemia (less Hb)

• E.g. emphysema, pulmonary fibrosis, interstitial lung disease, pulmonary HTN, chronic PE

Question 30

What causes an increase in diffusing capacity?

Answer 30

1) Polycythemia

Question 31

What is the alveolar gas equation?

Answer 31

Relationship between oxygen in the alveoli, fraction of inspiratory oxygen & arterial pressure of Co2.

Question 32

List the pathway that oxygen takes as it is inspired & then exchanged for CO2.

Answer 32

• O2 inspired into the alveoli
• Exchange across the respiratory membrane
• Pulmonary capillaries
• Pulmonary vein
• Left atrium
• Left ventricle
• Systemic circulation