Alveolar Gas Exchange

Question 1

Define Alveolar Ventilation

Answer 1

• Volume of air reaching the alveoli
• if per minute: VA(dot) = VA x Frequency
• 4 L/min is average value

Question 2

Define Perfusion

Answer 2

• from right ventricle

* 5 L blood/min

Question 3

What is the equation for diffusion rate of a gas?

Answer 3

J = ((S.A.) x D x (P1- P2))/distance

J = diffusion rate in ml/min
D: Diffusion coefficient for each gas
(P1- P2) = pressure gradient across alveolar membrane
S.A. = surface area available for diffusion
distance = diffusion distance (thickness of alveolar barrier)

Question 4

What is the diffusion rate of O2 and CO2 under normal resting conditions?

Answer 4

250 ml O2 and 200 ml CO2 exchanged every minute

Question 5

What happens to “J” as surface area increases?

Answer 5

it increases; conversely, a decrease in SA will decrease J, such as COPD

Question 6

What happens to “J” as distance increases?

Answer 6

J decreases

layers to cross include fluid, alveolar epithelium, interstitial space, blood vessel wall; average = 0.6 microns

deposition of collagen can cause an increase in diffusion distance, think fibrosis

Question 7

What is “D” in the diffusion rate equation?

Answer 7

Diffusion coefficient for each gas, depends on solubility of the gas in water, the molecular weight of the gas;
-Dco2 is 20x Do2

Question 8

What is a normal pressure gradient for O2?

What about CO2?

Answer 8

PAO2 - PvO2 = 104-40 = 60 mm Hg

PACO2 - PvCO2 = 40-45 = -5 mmHg

Question 9

How long does a RBC spend in a pulmonary capillary on average? How long does it take O2 to equilibrate?

Answer 9

0.75 seconds under resting conditions; O2 requires 0.25 seconds to equilibrate in the capillary

Question 10

How does exercise change equilibration of O2?

Answer 10

Now the RBC spends 0.25 seconds in the pulmonary capillary, just enough time to equilibrate.

Question 11

What is the diffusion capacity of the lung for oxygen?

Answer 11

In a normal person DLO2 is about 21 ml O2/min/mm Hg

Question 12

How do we measure DLO2?

Answer 12

By using CO due to its affinity for Hb

• the PaCO is 0 mm Hg
• patient inhales single breath with small amount of CO
• DLO2 = 1.23 x DLCO

Question 13

How long does it take CO2 to equilibrate in the pulmonary capillary?

Answer 13

It is almost immediate; DLCO2 has been estimated at 400 ml CO2/min/mm Hg

Question 14

Why does surfactant matter?

Answer 14

Surface tension; LaPlace’s law: Pressure = 2T/r

In a large alveolus, radius is large so pressure is low; in a small alveolus, radius is small so pressure is high. Shared airways means air wants to move from high to low pressure. Surfactant reduces T in small alveoli more than larger and keeps them from collapsing

Question 15

Where is surfactant produced and what is it made of?

Answer 15

• Produced by Type II pneumocytes
• Composed of several constituents
  
  • Phospholipid: Dipalmitoylphosphatidylcholine
  • Multiple proteins (SP = surfactant protein)
    * SPA, SPB, SPC, SPD
    * SPB is particularly important to the function
• Stored in the intracellular lamellar bodies
• Secreted into alveolus