Mechanics Of Breathing 2

Question 1

Why is the fall in alveolar pressure large enough to be observed?

Answer 1

There is a delay due to the time taken for the air to move.

Question 2

What does rate of airflow depend on?

Answer 2

Pressure gradient and the level of airway resistance.

Question 3

What is the equation for airflow?

Answer 3

Airflow (V) = change in pressure/resistance

Question 4

What does increasing the pressure gradient do to airflow?

Answer 4

Increases blood flow

Question 5

What does increasing resistance do?

Answer 5

It reduces airflow.

Question 6

What effect does breathing harder do?

Answer 6

It increases the pressure gradient which in turn increases airflow.

Question 7

Why does increasing the pressure gradient to compensate for increased resistance have fixed limits?

Answer 7

Because the respiratory system may not have the force or effort required to cause this massive increase in pressure gradient. Or it could be due to obstruction in the airway.

Question 8

What is airway resistance increased by?

Answer 8

1. It is increased by turbulent flow which occurs due to high velocities of airflow (e.g by forced expiration). The vibration created by turbulent airflow is responsible for wheezing sound in patients with obstructed airways.
2. Decrease in luminal area

Question 9

What is the relationship between airflow and radius?

Answer 9

Resistance is directly proportional to 1/r^4
Resistance= 8nL/Pir^4
Increased radius increases blood flow.

Question 10

Name 4 things that would increase resistance and reduce blood flow by reducing lumen.

Answer 10

1. Contraction of airway smooth muscle
2. Excessive mucus secretion
3. Oedema/swelling of airway tissue
4. Damage to integrity of airway structures

Question 11

What does ‘loss of airway patency’?

Answer 11

Obstruction of airway

Question 12

What are open structure of airway maintained by?

Answer 12

By elastic fibres within the wall of the airway and by radical traction (force keeping the airways open).

Question 13

Why are airways compressed in expiration?

Answer 13

Because majority of the airways are surrounded within lung tissue (which have elastic tissue). During inspiration, lung tissue expand which stretches the airways allowing air in.
During expiration, the lung tissue and airways are compressed.

Luckily, we have elastic fibres surrounding the alveoli to allow radial traction that keep the bronchioles open at expiration when the lungs and airways are compressed (at positive alveolar pressure).

In COPD, there is less elastic fibres so reduced radial traction at expiration so the bronchioles collapse at the positive alveolar pressure hence they close causing obstruction. So it’s more difficult to expire.

Question 14

What is spirometry?

Answer 14

Spirometry is when you breathe out as much as you can. We can find FVC and FEV1. We can then find the ratio.
We use it to see airflow and to see if there’s any constriction.

Question 15

What does FVC stand for?
What does FEV1 stand for?
How do you find the ratio and what does it mean?
What does less than 80% mean?

Answer 15

FVC = forced vital capacity
FEV1 = forced expiration volume in one second
Ratio = FEV1/FVC
It is the total lung capacity an individual can exhale in the first second
Less than 80% indicates obstructive airway disease.

Question 16

Why do we take the ratio rather than just looking at FVC and just FEV1?

Answer 16

We take the ratio to take into account of the fact e.g someone with a bigger FVC will a bigger FEV1 so this on its own doesn’t tell us much. So takes into account different lung capacities (because e.g someone who is smaller will have a smaller FVC).

Question 17

What is airway obstructive disease? Give examples.

Answer 17

Obstructive airway is due to increased resistance. Example would be asthma.
Reduced FEV1 (less than 80% expected value) but FVC is the same (because the maximum amount of air is eventually breathed out but just takes longer).
The FEV1/FVC ratio will be less than 70%.

Question 18

What is restrictive lung disease?

Answer 18

This is caused by decreases compliance. So the airway isn’t affected but lung capacity is. As a result, the FVC is reduced (less than 80% of expected value). FEV1 is also reduced.
The FEV1/FVC ratio is more than 70% so it’s pretty normal.
An example is fibrosis.

Question 19

What is transpulmonary pressure?

Answer 19

Transpulmonary pressure = alveolar pressure - intrapleural pressure
Transpulmonary pressure = the level of force acting to expand the lung

Question 20

What is lung compliance?

Answer 20

Relationship between transpulmonary pressure and lung volume.
Compliance = change in volume/change in pressure
The steeper the graph, the greater the compliance. Compliance is caused by density of elastic and collagen fibres.
Lung compliance is how much force is needed to explain the lungs.

Question 21

What does high compliance mean?

Answer 21

Means less elastic recoil so less force is required to inflate the lungs so lower increase in transpulmonary pressure is needed to increase volume. So gradient of graph increases.

Question 22

What does low compliance mean?

Answer 22

Means more elastic recoil so more force is needed to inflate lungs so greater increase in transpulmonary pressure is needed to increase volume. So gradient of graph decreases.

Question 23

Example of disease with high compliance?

Answer 23

Emphysema. There is elastic tissue degradation reducing elastic recoil of lungs.
Can be caused by smoking cigarettes.

Question 24

Example of disease with low compliance?

Answer 24

Fibrosis. This is caused by lung scarring and deposition of collagen, making it more difficult to increase lung volume so more force is needed.

Question 25

How do diseases affect lung compliance?

Answer 25

Scoliosis reduces lung compliance due to progressive atelectasis. Muscular dystrophy reduces compliance due to breakdown of skeletal muscles (e.g diaphragm) so greater force is needed to expand lungs. Obesity also reduces lung compliance due to increased weight on thoracic cage and abdomen. Neonatal respiratory distress syndrome decreases compliance because lack of pulmonary surfactant means surface tension is greater so it takes more force to inflate lungs. COPD increases compliance as it reduces elastic fibres for lung recoil so less force is needed to increase volume.

Question 26

What are alveoli lined with?

Answer 26

Alveoli are lined with alveolar lining fluid to enable gas exchange and the inside of the alveolar are lined by the same thing. This causes surface tension to arise due to hydrogen bonding between water molecules. This exerts a collapsing force towards the centre of the bubble (alveolus).

Question 27

What does Laplace’s law show?

Answer 27

It describes the pressure generated by the surface tension (and radius) within a bubble. Pressure= 2T/r So if the tension remains constant, pressure is directly proportional to 1/r. This means that the smaller the alveoli, the larger the pressure generated.

Question 28

What would happens if 2 alveoli of different sizes were connected to each other via airway?

Answer 28

The tension would probably be the same but the smaller alveolus has a smaller radius which generates a larger pressure. This creates a pressure gradient between the small and big alveoli, causing air to move from the small alveoli to the big one. This is a problem because this movement of air will cause the radius of the smaller alveolus to decrease even more causing greater pressure so air will continuously move into the bigger one, eventually causing the small alveolus to collapse which is bad. So this makes it difficult to inflate the lungs.

Question 29

What reduces the alveolar surface tension?

Answer 29

Pulmonary surfactant produced by pneumocytes II reduces surface tension. It has hydrophobic and hydrophilic properties so will sit between the air and water which reduces the surface tension. This as a result makes lungs easier to inflate and less likely to collapse.

Question 30

What happens to surface tension when alveoli expand?

Answer 30

As alveoli expand, the concentration of surfactant molecules decrease which increases surface tension, so basically, as surface area increases, surface tension increases. So this means larger alveoli have greater surface tension, which in turn increases the pressure. This allows the bigger alveoli to pass air into smaller ones to allow consistent inflation of lungs.

Question 31

How does pulmonary surfactant help reduce oedema?

Answer 31

Surface tension decreases hydrostatic pressure (as it creates a collapsing force). So adding pulmonary surfactant reduces surface tension hence increasing hydrostatic pressure. This creates a lower pressure gradient between the alveoli and blood so less blood is forced out into alveoli so less fluid builds up in alveoli.

Question 32

Explain Neonatal Respiratory Distress Syndrome (NRDS)

Answer 32

1. Insufficient surfactant production due to premature birth/maternal diabetes/developmental issues 2. Leads to stiff and low compliance lungs due alveolar collapse and oedema 3. This causes respiratory failure 4. This causes hypoxia 5. This causes pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary and cerebral haemorrhage.

Question 33

Does pulmonary surfactant increase compliance?

Answer 33

Yes

Question 34

How would you treat NRDS?

Answer 34

If the mother was at risk of producing child with this, she would be supplied with glucocorticoid. This reduces the chance of the child getting NRDS. If the child gets NRDS, then we give them artificial surfactant.

Question 35

What is static compliance?

Answer 35

Measurements taken when airflow is zero. Steepest part of curve is used. Lung measurements are taken at specific lung volumes where the patient pauses inspiration at a certain point therefore airflow falls to zero.

Question 36

What is dynamic compliance?

Answer 36

Measurements taken at movements of air. To calculate dynamic compliance, patient breathes normally at tidal volume . It is the gradient of the line from the end of expiration to end of inspiration.