79 - The Physiological Consequences of Disruption or Destruction of the Alveolar Capillary Membrane

Question 1

Number of alveoli in lungs

Answer 1

~200-300 million

Question 2

Interstitium in lungs

Answer 2

Space between alveolar membranes and capillaries

Question 3

What is the alveolar-capillary membrane composed of?
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2
3
4

Answer 3

Layer of surfactant
Type I alveolar cells (forms alveolar membrane)
Basement membrane
Vascular endothelial cell

Question 4

Features of A-C membrane 
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4

Answer 4

Thin (0.5 microns)
Large surface area (50-100M2)
Alveolar volume is ~3-6L
Capillary volume is ~80L (increases is increased cardiac output)

Question 5

Examples of diseases/processes that can disrupt A-C membrane
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Answer 5

Acute H1N1 (pandemic influenza)
Emphysema
Lymphangitis carcinomatosis (infiltrating cancer)
Drug-induced pneumonitis
Idiopathic pulmonary fibrosis 
Tuberculosis

Question 6

Pneumonitis

Answer 6

Inflammation of alveoli (also called alveolitis).

Pneumonia is a subtype of pneumonitis (implies an infectious aetiology)

Question 7

Likely physiological effects of disrupting A-C membrane
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3

Answer 7

Abnormal gas exchange
Abnormal lung mechanics
Pulmonary vascular complications

Question 8

What affects partial pressures of gasses in the alveoli?

Answer 8

Ventilation of alveoli

Question 9

Equation that describes rate of diffusion of a gas

Answer 9

Fick’s law

Question 10

Fick’s law

Answer 10

Rate of ventilation is proportional to (surface area x gas constant x (difference in partial pressures)) / Thickness of membrane

Question 11

Amount of time that an average RBC spends in contact with alveolar membrane at rest

Answer 11

~0.75 second

Question 12

Amount of time that an average RBC spends in contact with alveolar membrane at rest

Answer 12

~0.25 second

Question 13

Rough amount of time required for RBC Hb molecules to become oxygenated

Answer 13

~0.25 second

Question 14

What doesn’t limit gas exchange in a normal person?

Answer 14

Diffusion (only takes ~0.25 second to oxygenate Hb in an RBC)

Question 15

When is O2 transfer diffusion-limited at rest?

Answer 15

Only when there is a gross abnormality of the AC membrane (EG: thickening).
With exercise, can become apparent with less-severe disease).

Can desaturate O2 with exercise

Question 16

Effect of thickened A-C membrane on O2 saturation

Answer 16

Takes a longer time to saturate RBC Hb

Question 17

Difference in diffusion between CO2 and O2

Answer 17

The same, except CO2 diffusion is 20 times faster.

Question 18

V/Q mismatch of CO2

Answer 18

Doesn’t really affect CO2.

If hypercapnic, is from poor alveolar ventilation, not from poor diffusion, as CO2 diffuses so well

Question 19

What can cause low PaO2

Answer 19

Low PiO2 (low inspired oxygen, EG: altitude)
Low ventilation
Abnormal gas exchange (low V/Q, shunt, diffusion impairment)

Question 20

What does abnormal gas exchange for O2 mean?

Answer 20

Wide A/a gradient for O2

Question 21

What can cause high PaCO2?

Answer 21

Low ventilation

Question 22

Effect of low ventilation

Answer 22

Not breathing in enough O2, not breathing out enough CO2.

Question 23

Causes of low PaO2 with normal A-a gradient

Answer 23

Low PiO2 and low ventilation

Question 24

Most common cause of abnormal gas exchange

Answer 24

V/Q mismatch.
Particularly units with inadequate ventilation for the amount of perfusion (amount of blood going through segment). Extreme form is shunts.

Question 25

When is diffusion impairment most obvious?

Answer 25

During exercise

Question 26

Mechanical effects of restrictive lung disease

Answer 26

Like having a tight band around your chest (lungs stiffer, harder to inflate).
–> breathless, increased work of breathing, reduced lung volumes, altered patterns of breathing

Question 27

Why do alveolar-capillary membrane diseases increase work of breathing ?

Answer 27

Because the inspiratory muscles need to generate higher

pressures to overcome the stiffness (reduced compliance) of the lungs (elastic work of breathing)

Question 28

Important consequences of increased WOB
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Answer 28

1. Recruitment of accessory muscles of inspiration (scalene and sternomastoid muscles)
2. Increased oxygen consumption by respiratory muscles
3. Risk of respiratory muscle fatigue, if the airway obstruction is severe

Question 29

Effect on lung volumes of stiff lungs

Answer 29

All lung volumes reduced

Question 30

Effect on FEV1 and FVC of stiff lungs

Answer 30

Decreased FEV1, FVC.

Normal FEV1/FVC ratio

Question 31

Effect on FEV1 and FVC of narrowed airways

Answer 31

Decreased FEV1, normal FVC.

Decreased FEV1/FVC raito

Question 32

FER

Answer 32

Forced expiratory ratio (FEV1/FVC)

Question 33

Only constrictive lung disease without stiff lungs

Answer 33

Emphysema

Question 34

Components of lungs leading to compliance
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2

Answer 34

1) Tissue composition (elastic, fibrous)

2) Surface tension in alveoli. Reduced by surfactant.

Question 35

Example of a disease leading to reduced surfactant

Answer 35

Premature baby hyaline membrane disease

Question 36

What determines elastic properties of the lungs

Answer 36

Lung compliance

Question 37

Lung resistance in emphysema

Answer 37

Decreased, because of destruction of elastic tissue,

Emphysematous lungs hyperinflate

Question 38

Breathing pattern of those with restrictive lung disease

Answer 38

Prefer rapid, short breaths (to try to maintain alveolar ventilation)

Question 39

Breathing pattern of those with airflow obstruction

Answer 39

Prefer fewer, longer, deeper breaths (because difficult to get air through airways, so try to do this less)

Question 40

Effect of restrictive lung disease on maximum lung volume

Answer 40

Decreases

Question 41

What often limits exercise capacity of those with restrictive lung disease

Answer 41

Hypoxia with or without pulmonary hypertension

Question 42

Gas exchange and mechanical effects of restrictive lung disease
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Answer 42

1. Increased sensation of breathing
2. Increased elastic WOB
3. Reduced lung volumes
4. Altered pattern of breathing
5. Reduced maximum ventilation
6. Abnormal gas exchange, which
   worsens with exercise