Breathing mechanism

Question 1

What is resistance within the airways?

Answer 1

As air passes through airways, it generates resistance as it comes into contact with the airway surface. Resistance is an opposing force that acts to reduce the flow of a gas or fluid through a conduit (pipe)

Question 2

What is the relationship between the airflow, pressure gradient and airway reisistance?

Answer 2

Airflow (V) = Pressure gradient (P) / Resistance (R). As the pressure gradient increases the airflow increases. As the resistance increases the airflow decreases

Question 3

What is the Hagen-Poiseuille equation and what does it describe?

Answer 3

This equation shows that Resistance (R) is proportional to 1/radius^4. It describes the relationship between resistance the various properties of airways and airflow.

Question 4

Give examples of pathological features which impact the cross-sectional area of the lumen of the airway.

Answer 4

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 5

what is airflow turbulence and what causes it to form?

Answer 5

It is the change in direction of the air in the airway from linear to random directions. 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

Question 6

What is the effect of turbulent airflow?

Answer 6

It causes increased airway resistance. The vibration generated by the turbulent airflow is responsible for the wheezing sound produced in patients with obstructed airways.

Question 7

What is airway patency?

Answer 7

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

Question 8

How is airway patency maintained?

Answer 8

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

Question 9

How does patency vary during the breathing cycle?

Answer 9

As the lungs expand during inspiration, the lung tissue and airways contained within are stretched upon. During expiration, the lung tissue and airways are compressed.

Question 10

Effects of pressure differentials in the interpleural space

Answer 10

When the 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.

Question 11

What is transpulminary pressure?

Answer 11

the difference between the pressure within the alveoli and the intrapleaural space. It determines the level of force acting to expand or compress the lungs.

Question 12

What is lung compliance

Answer 12

A measure of the lungs ability to stretch and expand

Question 13

Effects of higher lung compliance

Answer 13

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

Question 14

Effects of lower lung compliance

Answer 14

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

Question 15

Lung compliance equation

Answer 15

Compliance = change in volume/change in pressure

Question 16

How is lung compliance demonstrated

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

Effect of emphysema on the lungs

Answer 17

Emphysema involves degradation of elastin fibres making the lung less stiff and more complaint but reducing recoil.

Question 18

Effect of pulmonary fibrosis on the lungs

Answer 18

Pulmonary fibrosis involves scarring and deposition of structural fibres such as collagen making the lung stiff and less compliant.

Question 19

What is alveolar surface tension?

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.

Question 20

What is the law of Laplace

Answer 20

The collapsing force produced at the water-fluid interface generates pressure. The amount of pressure within a specific bubble can be calculated by the Law of Laplace, which described the relationship between collapsing pressure, the radius of the bubble, the surface tension.
Pressure = Surface tension/radius of bubble

Question 21

What is pulmonary surfactant?

Answer 21

A phospholipoprotein secreted by type II pneumocytes (alveolar cells). Surfactant molecules are amphipathic, with hydrophilic head and hydrophobic tail regions, and so will naturally position themselves at the air-liquid interface.

Question 22

What is the role of pulmonary surfactant?

Answer 22

To stop the small alveoli from collapsing into the larger ones due to a difference in pressure. This would thus make inflation of the lung very difficult

Question 23

How does the pulmonary surfactant do its job?

Answer 23

The presence of the surfactant molecules then acts to disrupt the attractive forces between water molecules, reducing surface tension and the collapsing pressure generated.

Question 24

What is Neonatal respiratory distress syndrome (NRDS)?

Answer 24

A condition that occurs in infants born prematurely, and who develop and produce insufficient levels of pulmonary surfactant (surfactant production at week 24-28).

Question 25

What does NRDS result in?

Answer 25

This deficiency results in respiratory failure due to the alveoli collapsing, decreasing lung compliance (‘stiffer’ lungs), and alveolar oedema reducing gas exchange. The increased forces and pressures involved within the lung also damage alveoli and innervating capillaries.

Question 26

How is NRDS treated.

Answer 26

NRDS is treated by either:
Supplementation of affected infants with artificial surfactant,
and/or by administering glucocorticoids (which increase surfactant production via maturation of type 2 pneumocytes) to mothers deemed high risk (e.g. mothers with poor diabetic control – insulin appears to affect pneumocyte maturation - or those at risk of premature birth).