L16. Physiological Consequences of Disruption of the Alveolar-Capillary Membrane

Question 1

What is the alveolar-capillary membrane made up of? [4]

Answer 1

1. Type 1 alveolar cells
2. Capillary Endothelial cells
3. A Shared Basement Membrane
4. Layer of surfactant

Question 2

What characteristics of the AC membrane makes the diffusion of gases efficient? [4]

Answer 2

1. Thickness (very low)
2. The large surface area
3. Alveolar volume (3-6L)
4. Capillary volume (80mL) that is able to increase markedly with increasing CO

Question 3

What processes can disrupt AC membrane? [5]

Answer 3

1. Inflammation (and infection)
2. Fibrosis
3. Emphysema
4. Fluid
5. Cancer

Question 4

What are the shared pathophysiological consequences of disrupting the AC membrane? [3]

Answer 4

1. Abnormal gas exchange
2. Abnormal lung mechanics
3. Pulmonary vascular complications (ie. pulmonary hypertension)

Question 5

What is the rate of diffusion of gases depend upon?

Answer 5

Fick’s Law:
diffusion is proportional to surface area, a constant (solubility and size of gas), different in partial pressures and inversely proportional to the thickness of the membrane.

Question 6

Why is the pCO2 less affected by gas exchange problems?

Answer 6

Because the rate of diffusion of CO2 is about 20x more efficient than oxygen.

Question 7

At rest, the Cardiac Output is 5L/min, what is the efficiency of the diffusion of oxygen at this rate?

Answer 7

At 5L/min and a capillary capacity of about 80 mL means that the velocity of blood through the pulmonary capillaries is quite high. Thus any one RBC in the blood is in contact with the alveolar membrane (gas exchange) for about 0.75 seconds.

Question 8

What does it mean that there is a redundancy in the diffusion capacity of oxygen? How does the compensatory mechanism occur in exercise states?

Answer 8

The efficiency of oxygen diffusion means that it only requires 0.25 seconds for a Hb molecule to be completely saturated at rest. Thus there is 0.5 seconds of redundancy (extra oxygenation time) that can be used when the CO (and thus the flow) is increased.

Question 9

How do clinicians differentiate diffusion problems with obstructive (V/Q) mismatch problems?

Answer 9

Diffusion problems often don’t affect the CO2 as much as it does O2. However obstructive problems (poor ventilation) lead to V/Q mismatch with hypercapnoea and hypoxia occurring.

Question 10

What is the different for causes of low PaO2?

Answer 10

Obstruction (ventilation abnormality)
low ventilation/hypoventilation
low/dysfunctional gas exchange

Question 11

What is the major cause of high PaCO2?

Answer 11

Hypercapnoea: likely due to a problem with ventilation (obstruction) and rarely due to diffusion problems at the level of the AC membrane (damage needs to be very severe in order to cause a high PaCO2)

Question 12

What does it mean to alter the mechanical effects of breathing?

Answer 12

Compliance of the lungs decreases: stiffer, less elasticity

Increase the load of breathing which means more work is required to breathe

Question 13

What are the consequences associated with increasing the mechanical effects of breathing?

Answer 13

Breathlessness
Increased work of breathing
Reduced lung volume
Altered patterns of breathing
Reduced maximum ventilatoin

Question 14

An increase in the respiratory muscle effort is a direct consequence of increasing the elastic work of breathing. What happens as a result of increased respiratory muscle effort? [3]

Answer 14

1. Recruitment of accessory muscles of respiration
2. Increased oxygen consumption by respiratory muscles
3. Risk of respiratory muscle fatigue

Question 15

What is the clinical definition of Respiratory Failure?

Answer 15

PaO2 < 60 mmHg

PaCO2 > 50 mm Hg

Question 16

What happens to the lung volumes (FEV1 and FVC) when there is reduced compliance of the lungs?

Answer 16

Reduced compliance means more work needs to be done to allow flow.
FEV1 is reduced
FVC is reduced
Depending on the severity of the restriction, the FER is either normal or decreased

Question 17

What two elastic properties of the lungs affect the compliance of the lungs?

Answer 17

1. Tissue composition: inflammation and fibrosis decrease elastin concentrations and increase the stiffness = decreased compliance
2. Surface tension in the alveoli: the less surfactant the less compliance the lungs have (pathology is only really hyaline membranes in premature babies)

Question 18

What is the relationship between compliance and volume and pressure of the lungs?

Answer 18

Compliance = change in volume/change in pressure

Compliance is given by the pressure-volume curve (a starling relationship)

Question 19

Give an example of the pathologies that increase the compliance and that decrease compliance in the lungs

Answer 19

Emphysema: destruction of tissues leads to an decrease in alveolar surface area and an increase in compliance: overinflation

Pulmonary fibrosis: fibrosis decreases elastic tissue composition in the lungs leading to a decrease in compliance

Question 20

Describe the altered pattern of breathing for restrictive lung disease compared to that of obstructive lung disease

Answer 20

Both want to achieve minimal work of breathing
Restrictive: shallow and rapid breathing to get maximal minute ventilation (Ve) with minimal work
Obstructive: slow, deep breaths to get maximal Ve (minimise the need overcome the obstruction)