Disruption/Destruction of the A-C Membrane

Question 1

What are the likely physiological effects of disrupting the A-C membrane?

Answer 1

1. Abnormal gas exchange
2. Abnormal lung mechanics
3. Pulmonary vascular complications

Question 2

What are the normal values of PAO2 and PACO2?

Answer 2

PAO2 = 90-100mmHg
PACO2 = ~40mmHg

Question 3

What is Fick’s Law?

Answer 3

Describes diffusion
- the rate of exchange of a gas depends on the surface area of the membrane, the difference in the partial pressures and a gas constant all divided by the membrane thickness

Question 4

How does diffusion occur?

Answer 4

Blood cell spends ~3/4 of a second in contact with alveolar capillary membrane
- uptake of O2 occurs in ~1/4 of a second

Question 5

Why is PaCO2 likely to be high?

Answer 5

Inadequate alveolar ventilation

- not very likely to be diffusion problem, unless the A-C membrane is very severely abnormal

Question 6

What are some causes of low PaO2?

Answer 6

Low PiO2 
- altitude, no A-a gradient
Low ventilation 
- high CO2
Abnormal gas exchange
- problem exchanging O2 - mismatch V/Q
- low V/Q
- shunt 
- diffusion impairment 
- wide A-a gradient

Question 7

What are the mechanical effects of restrictive lung diseases?

Answer 7

• Breathlessness, increased WOB, reduced lung volumes, altered pattern of breathing, reduced maximum ventilation

Question 8

What elastic properties of the lungs affect compliance?

Answer 8

• Tissue composition (inflamm/fibrotic tissue = stiff)

- surface tension in alveoli (reduced by surfactant)