pk3 Surfactant and Resistance

Question 1

What generates the distending pressure of the lung?

Answer 1

The distending pressure of the lung is generated by the elastic recoil of the lungs and thoracic cage.

Question 2

What determines the volume for any given distending pressure?

Answer 2

Compliance determines the volume for any given distending pressure.

Question 3

What determines the compliance of the thoracic cage?

List 3 conditions in which the compliance of the thoracic cage is decreased.

Answer 3

• The compliance of the thoracic cage depends on its rigidity and shape.

1 - Arthritic spondylitis.

2 - Kyphoscoliosis.

3 - Spasticity of thoracic or abdominal musculature.

*There are no conditions which cause an increase in compliance of the thoracic cage.

Question 4

What determines the compliance of the lung?

Which of these properties has the most significant effect on elastic recoil?

Answer 4

1 - Elastic properties of lung tissues (e.g. collagen, elastin etc.).

2 - Surface tension due to the air-liquid interface. Surface tension acts to collapse the lung and so decreases compliance.

• E.g. a fluid-filled lung is much more compliant than an air-filled lung because the air-liquid interface is removed, so surface tension decreases.
• 2/3 of the total elastic recoil of the lungs is caused by surface tension.

Question 5

What is surface tension?

How does it act to collapse the lung?

Answer 5

• Surface tension is the force generated by the air-interstitial fluid interface that results from the tendency of water to shrink to the minimum possible surface area.
• This is due to the attractive intermolecular forces between water molecules, pulling them together.
• In a spherical alveolus, the surface tension generated by the air-liquid interface has a tangential component that pulls the alveolus inwards, collapsing it.

Question 6

Describe the shape of a normal lung compliance curve.

Answer 6

• X axis: distending pressure.
• Y axis: volume.
• The graph is a straight line until high distending pressures, where the curve plateaus as it approaches total lung capacity (compliance begins to decrease).

Question 7

What is Laplace’s Law?

What is the implication of Laplace’s law?

Answer 7

• For spherical vessels:

P = 2T / r

P = Pressure.

T = Tension.

r = Radius.

• The implication of Laplace’s law is that smaller alveoli must have a greater internal pressure to keep them inflated (because the change in surface tension is negligible). *But in reality, it is more complicated, as explained in a later card.

Question 8

What is alveolar interdependence?

What is the advantage of alveolar interdependence?

Answer 8

• Alveoli are said to be interdependent because each alveolus is mechanically coupled to other adjacent alveoli (as they share a common wall).
• This is advantageous because this allows alveoli to prevent the collapse of adjacent alveoli.

Question 9

What is surfactant?

Why is surfactant needed in the lungs?

Answer 9

• Surfactant is a detergent-like substance that acts to greatly lower the surface tension in alveoli.
• Surfactant is needed in the lungs because if the lungs where solely lined with interstitial fluid, the surface tension would be so great that breathing would be impossible.

Question 10

What is the surface tension of pure interstitial fluid?

How does this change with surfactant?

Answer 10

• Pure interstitial fluid has a surface tension of 70mN/m.

- With lung surfactant, surface tension of interstitial fluid drops to 2mN/m.

Question 11

Which component of surfactant is responsible for its surface tension-lowering effect?

Answer 11

DPPC is responsible for the surface tension-lowering effect of surfactant.

Question 12

Which cells secrete surfactant?

Answer 12

Surfactant is secreted by type 2 alveolar epithelial cells.

Question 13

Describe the structure of surfactant.

How does its structure relate to its function?

Answer 13

• Surfactant has a glycerol backbone.
• It has one hydrophobic tail containing a palmitate molecule.
• It also has one hydrophilic tail containing a phosphate molecule and a choline molecule.
• In the air-interstitial fluid interface, the palmitate tail moves to the air and the phosphate-choline tail moves to the interstitial fluid.
• This causes surfactant molecules to line the air-liquid interface, displacing interstitial fluid from the interface.
• This reduces the collapsing radial forces generated by the interstitial fluid.

Question 14

How does surface area affect the surface tension-lowering effect of surfactant?

Why does this happen?

Answer 14

• High surface area increases the surface tension-lowering effect of surfactant.
• This is because a higher surface area will allow for a higher density of surfactant in the alveoli.

Question 15

How does the internal pressure of small alveoli compare to that of large alveoli?

Answer 15

• According to Laplace’s law, P = 2T / r, smaller alveoli should have a higher internal pressure.
• However, a smaller radius means that surfactant density increases.
• This increases the surface tension-lowering effect of surfactant, balancing out Laplace’s law.
• The net difference in pressure between small and large alveoli is therefore the same.

Question 16

What is hysteresis?

What causes it?

Answer 16

• Hysteresis is the phenomenon in which the pressure-volume curve of the lung is different on inhalation as compared to expiration.
• It is caused by the changing density of surfactant as the radius of the alveoli increase during inspiration.

Question 17

Give an example of a disease in which lung compliance is increased, and a disease in which lung compliance is decreased.

Answer 17

• Emphysema increases lung compliance (due to loss of elastic tissue).
• Pulmonary fibrosis decreases lung compliance.

Question 18

What is infant respiratory distress syndrome?

What causes it?

How is it treated?

Answer 18

• Whilst in the womb, a foetus has no air-liquid interface, and therefore there is no need for surfactant.
• Surfactant only appears in late gestation.
• In infant respiratory distress syndrome, infants are born with a lack of surfactant.
• This can also be accompanied by fibrous hyaline membranes that form on the alveoli, hindering diffusion.
• The infant must therefore expend a lot of energy to inflate the lungs, which also deflate to low volumes at expiration.
• Infants become hypoxic, hypercapnic, acidotic and exhausted.
• It has various aetiologies, including maternal diabetes and prematurity.
• Assayed immature lungs can be treated with glucocorticoids, which accelerate maturation.
• After birth, it can be treated by administering surfactant directly into the trachea.

Question 19

What causes adult respiratory distress syndrome?

Answer 19

• Adult respiratory distress syndrome is caused by damage to the alveolar-capillary interface, and to type 2 alveolar epithelial cells.
• This can be due to a large number of disorders, including pneumonia, sepsis and smoke inhalation.

Question 20

Define airflow resistance.

Answer 20

Airflow resistance is the pressure difference required to achieve a given flow.

Question 21

What is the normal airflow resistance in the lung?

Answer 21

Normal airflow resistance is 0.2kPaL/s.

Question 22

List the resistances that must be overcome in order to achieve airflow in the lungs.

Answer 22

1 - Work is required to stretch the lung against its compliance.

2 - Work is required to move air against resistance to flow by friction.

Question 23

List 2 equations for flow.

From this, derive the equation for resistance.

Answer 23

F = ΔP / R

or

F = ΔP π r^4 / 8 L η

therefore

1 / R = π r^4 / 8 L η

R = 8 L η / π r^4

F = Flow.

P = Pressure.

R = Resistance.

r = Radius.

L = Length of the airway.

η = Viscosity.

Question 24

How does airway resistance change throughout the lung?

Why?

Answer 24

• Resistance is highest in the trachea and is lowest in the alveoli.
• This is because resistance is proportional to 1/r^4, so as radius increases, resistance decreases exponentially.
• Even though the radius of the trachea is greater than that of a single alveolus, the total cross sectional area is much greater in the alveoli compared to the trachea, therefore the resistance is lower in the small airways.
• 80% of the total resistance is in the large airways.

Question 25

Describe the physiological control of bronchial diameter.

Answer 25

• Bronchial diameter is under control of smooth muscle in the airways, which itself is under the control of the autonomic nervous system:
• Bronchoconstriction is caused by:

1 - Increased vagal parasympathetic activity (also induces mucus secretion).

2 - Local hormonal mediators such as histamines and leukotrienes.

3 - Decreased airway CO2.

• Bronchodilation is caused by:

1 - Activation of beta 2 adrenoceptors by adrenaline or sympathomimetic drugs.

2 - Non-adrenergic, non-cholinergic innervation.