Lung mechanics

Question 1

How does TLC differ between a person with normal, obstructive and restrictive lung disease?

Answer 1

• TLC increases for obstructive and decreases for restrictive
• Vital capacity decreases in both
• residual volume is higher in obstructive and lower is restrictive

Question 2

What does obstructive lung disease mean?

Answer 2

flow in and out of the lungs is restricted

COPD, emphysema, bronchitis

Question 3

What does restrictive lung disease mean?

Answer 3

inflation and deflation of lung/chest wall is restricted

lung fibrosis, neuromuscular disease, obesity

Question 4

What is the transrespiratory system?

Answer 4

lung and chest wall combo

Question 5

Explain the arrangement of the pleura

Answer 5

The two pleura have fluid inside and if the layer is moved in one direction the other will move too. The tension between will increases or decrease depending on whether the force is pulling or pushing.

Question 6

What is the pressure in between the pleura at FRC and why?

Answer 6

The lungs are pulling in and the chest wall is pushing out the same amount so there is negative pressure in the the pleural space as they layers are being pulled in both directions and so there is a little vacuum.

Question 7

What is the transrespiratory pressure at FRC?

Answer 7

It is 0 between the outside and inside so there is no net flow of air

Question 8

What happens to the pleural pressure during inspiration?

Answer 8

Pleural pressure is more negative than at FRC because the muscle force pulling the pleura is greater than the lung recoil

Question 9

During normal and forced expiration what happens to pleural pressure?

Answer 9

normal - returns to same at FRC

forced - becomes more positive than at FRC (pleura are pushed closer together) as the muscle force is greater

Question 10

Pressure volume graph - around the middle of the graph what happens and its importance

Answer 10

Around the volumes in the middle of the graph, a small change in pressure will cause a large change in volume.
This is why it is easy for us to maintain tidal breathing when at rest

Question 11

How does the pressure volume graph differ for someone with obstructive/restrictive lung disease compared to normal?

Answer 11

• default starting point for restrictive is lower (whole graph is lower)
• for obstructive the whole graph is higher
• FRC differ for both too

Question 12

How does flow rate change?

Answer 12

it is determined by how we change alveolar pressure because flow depends on atmospheric and alveolar but atmospheric we cannot control.

Question 13

What causes air flow in?

Answer 13

The respiratory muscles work to expand the chest wall and increase the capacity of the thoracic cavity so we create a negative pressure inside which makes air flow in

Question 14

When the lungs fill up and you hold your breath, what happens to the air flow and why?

Answer 14

No air flow as there is equilibrium in pressure inside and out

Question 15

During expiration what happens to air flow and why?

Answer 15

The tension in the inspiratory muscles that were being used to hold your breath is released - it compresses the gas molecules and creates a positive pressure which forces the air out and then you’re back where you started

Question 16

What happens if you superimpose the graph of volume changes to pleural pressure (inverted) and look at the differences between the lines?

Answer 16

It is equal to the difference between the volume and flow rate graphs

Question 17

What is compliance?

Answer 17

The willingness of a structure to change shape when pressure is applied

Question 18

What is elastance?

Answer 18

The tendency of something to recoil to its original volume

Question 19

Compare the elastance of a condom and a tyre

Answer 19

condom has high compliance and tyre has low compliance and high elastance

Question 20

Ventilating fluid filled lungs vs air filled lungs

Answer 20

It is harder to inflate air filled lungs than to deflate.
When lungs are fluid-filled they are much more compliant than when air-filled. There is surfactant lining the lungs and the air water interface exhibits surface tension where the fluid - water interface does not

Question 21

Surface tension

Answer 21

The water molecules all interact to create a layer.
At the top layer, there is air on one side so there is no matching force on one side
This makes some of the upper water molecules disappear down and causes a tension across the top - the molecules come under strain

Question 22

Surface tension in the alveoli and its shape

Answer 22

Because of the shape of the alveoli, the distribution of water is more dense around the outside
On the inside, the water molecules are all attracted across the alveolar space to water molecules on the opposite side.
This means that if the alveolus is too small, these forces will pull the alveoli together and collapse them

Question 23

Pulmonary surfactants and what it does

Answer 23

Type II Pneumocytes produce surfactant which breaks up the surface tension. The surfactant splits the water molecules on the surface and reduces the surface tension between them. Surfactant prevents collapse of small airways and increases compliance.
This means that you have to work less hard to breathe.
(Law of Laplace)

Question 24

Ventilation and perfusion

Answer 24

Intrapleural pressure is more negative at the top of the lung due to gravity and then alveoli are larger and less compliant so less ventilation up.
The interpleural pressure is less negative and smaller transmural pressure so alveoli smaller and more compliant and more ventilation.
Lower intravascular pressure at the top due to gravity, greater resistance and less flow rate. Near the bottom the intravascular pressure is higher due to gravity and there is less resistance and more flow.

Question 25

Resistance and tub size

Answer 25

Normally it is assumed that the smaller the airways, the smaller the tube, the greater the resistance.
However, flow must be considered. If you have low flow through a small tube and really high flow through a large tube then the large tube may have greater resistance

Question 26

What is conductance?

Answer 26

how well will the airways conduct and allow air to pass through

Question 27

Airway generation/number of airways and resistance

Answer 27

You breathe in air and the airways then rapidly bifurcate and the air gets split across smaller and smaller bronchioles so flow rapidly decreases.
Peak resistance occurs around the fourth generation. After that as the number of airways increase, the resistance decreases.

Question 28

How does conductance change with lung volume?

Answer 28

Conductance steadily increases with increasing lung volume

Question 29

What happens to resistance as lung volume increases?

Answer 29

It decreases as they airways expand when you are breathing so radius of airways is larger.

Question 30

What equation can be used to express resistance?

Answer 30

Poiseuille’s Law
resistance = 8nl /pir^4

n=viscosity

Question 31

What is patency?

Answer 31

whether the airways are open and active and able to allow air through

Question 32

Do all airways have cartilage?

Answer 32

Some of the airways have a structural support (e.g. cartilage rings) but not all.

Question 33

What is transmural pressure?

Answer 33

difference between airway and pleural space

Question 34

Flow of air in collapsible tubes

Answer 34

In the preinspiratory lung the transmural pressure is positive and the airway is patent. It is patent during mid-inspiration and end-inspiration as well.

However, if you try forced expiration where you rapidly go from TLC to RV then there is going to be a massive increase in intrapleural pressure. During forced expiration, the intrapleural pressure is very high and exerts a pressure inwards on the collapsible tubes. If this inwards pressure is greater than the outwards pressure exerted by the tubes themselves, then the tube will collapse.

(There is a gradient of internal pressure between the lungs and the atmosphere)