The Mechanics of breathing

Question 1

Define resistance

Answer 1

Resistance is an opposing force that acts to reduce the flow of a gas or fluid through a conduit (pipe).

Question 2

How is resistance generated in the airways?

Answer 2

As air passes through airways, it generates resistance as it comes into contact with the airway surface.

Question 3

What is the equation for airflow?

Answer 3

Airflow (V) = Pressure (P) / Resistance (R)

Question 4

Why might we not be able to increase airflow?

Answer 4

We can increase airflow; however this can sometimes not happen e.g the effort/force required to do so may not be able to be generated, or the airway might be completely obstructed.

Question 5

What factors determine the level of resistance?

Answer 5

• The cross-sectional area of the airway lumen (the empty section in the middle of the airway, through which air passes).
• The airflow patterns.

Question 6

What does the Hagen-Poiseuille equation describe?

What is the equation?

Answer 6

The relationship between resistance and the various properties of airways and airflow.

Resistance (R) is proportional to 1/ Radius^4

Question 7

Describe some aspects of respiratory diseases that generate pathological features which reduce the size of the lumen of the airway

Answer 7

Contraction of airway smooth muscle, excessive mucus secretion, oedema/swelling of the airway tissue, and damage to the integrity of the airways structure (i.e. loss of patency) will all reduce the size of the airway lumen, increasing airway resistance and decreasing airflow.

Question 8

Describe what happens when airflow changes of linear from turbulent and how it impacts on airflow

Answer 8

• Where airflow changes from a linear to a turbulent pattern, increased airway resistance is generated.
• Turbulence occurs where high velocities of airflow are achieved (e.g. during forced breathing manoeuvres) or if there is a sudden decrease in luminal area such as in obstructed airways.
• The vibration generated by the turbulent airflow is responsible for the wheezing sound produced in patients with obstructed airways.

Question 9

Define airway patency

Answer 9

Patency refers to the state of being open or unobstructed; a ‘loss of patency’ = closing/obstruction

Question 10

What maintains the open structure of the airway?

Answer 10

The open structure of the airways is maintained by elastic fibres within the wall of the airway and radial traction. Most airways are positioned within surrounding lung tissue which has elastic properties - the airways are pulled open by their connections to the surrounding tissue

Question 11

In terms of collapsing forces what occurs during forced expiration and what prevents lung collapse during healthy individuals

Answer 11

• When intrapleural pressure becomes positive (as can occur during forced expiration), collapsing force will be exerted onto the airways. In healthy individuals, the structural integrity of the airways is sufficient to prevent collapse, however in diseases involving impaired airway structure (e.g. COPD), this can be problematic .

Question 12

What is transpulmonary pressure and what does it determine?

Answer 12

The difference between the pressure within the alveoli and intrapleural space (termed ‘transpulmonary pressure) determines the level of force acting to expand or compress the lungs.

Question 13

What happens with a high lung compliance?

Answer 13

Higher lung compliance = less elastic recoil = less force required to inflate = ↑ volume change per pressure change (↑gradient on volume-pressure curve)

Question 14

What happens with a low lung compliance?

Answer 14

Lower compliance = more elastic recoil = more force required to inflate = ↓volume change per pressure change (↓ gradient on volume-pressure curve)

Question 15

What is the equation that links lung compliance, volume and pressure

Answer 15

Compliance (Cl) = change in volume/ changes in pressure

Question 16

Describe the lung volume and transpulmonary pressure graph

Answer 16

This relationship is demonstrated by a graph of lung volume vs. transpulmonary pressure, as lung compliance = the gradient of the curve. For static compliance (measurements taken whilst airflow =0), the steepest part of the curve is used, whereas for dynamic compliance (measurements taken in the presence of airflow), the gradient between the end tidal inspiratory and end tidal expiratory points is used:

Question 17

Give some examples of diseases that affect the recoil or structure of the lungs and chest wall can lead to altered lung compliance

Answer 17

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Question 18

Describe the graph of these diseases (lung volume vs interplural pressure)

Answer 18

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Question 19

Describe alveoli surface tension and the role of pulmonary surfactant (4)

Answer 19

• The internal surfaces of alveoli are lined with fluid
• The water-air interface formed between the lining fluid and pseudo-spherical alveolar airspace essentially creates a bubble
• Within the bubble, surface tension arises due to the relative strength of hydrogen bonds between water molecules combining to exert an overall collapsing force toward the centre of the bubble.
• The collapsing force produced at the water-fluid interface generates pressure.

Question 20

Give two equations for pressure and explain there meaning

Answer 20

Pressure and bubble radius are inversely proportional (pressure increases as radius decreases), which means that smaller bubbles generate greater pressure than larger ones

Question 21

What happens when two alveoli are connected

Answer 21

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Question 22

What is neonatal respiratory distress syndrome caused by?

Answer 22

Insufficient production of pulmonary surfactant

Question 23

Give the steps in neonatal respiratory distress syndrome

Answer 23

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