Lecture 1: Lung mechanics

Question 1

How much is the decrease in intrapleural pressure during inspiration?

Answer 1

Usually -2.5 -6 mmhg

Question 2

At what point during breathing are pressures equal?

Answer 2

At the end of inspiration

Question 3

What is the transpulmonary pressure?

Answer 3

The chest wall exerts a distending pressure on the pleural space, which is transmitted to the alveoli to increase its volume, lower its pressure, and generate airflow inwards

Question 4

Which pressure is always positive?

Answer 4

Transpulmonary pressure

Question 5

Which pressure is always negative?

Answer 5

Intra-pleural pressure

Question 6

What happens to alveolar pressure during breathing?

Answer 6

Moves from slightly negative to slightly positive

Question 7

What muscles are used during inspiration?

Answer 7

Diaphragm (75% change in volume)

External intercostals
- bucket-handle

Accessory muscles:

• scalene
• sternomastoids

Question 8

What is compliance?

Answer 8

Static measure of lung “stretchiness”

Volume change per unit pressure change

Question 9

What is the balance point (where the lung and chest are in equilibrium, after exhaling)?

Answer 9

Functional Residual Capacity (FRC)

Question 10

What are the endpoints of breathing?

Answer 10

Total lung capacity (TLC) and residual volume (RV)

Question 11

What happens to the compliance of the lung at higher volumes?

Answer 11

It decreases

Question 12

Describe normal lung tissue in terms of compliance.

Answer 12

Compliance is just right.

Good compliance for low work of inhalation, and good retention of elasticity of alveolar units to allow effective exhalation.

Question 13

Describe interstitial fibrosis in terms of compliance.

Answer 13

Compliance is decreased due to more stiff alveolar walls from scarring (called fibrosis).

Difficult to inflate.

Question 14

Describe emphysema in terms of compliance.

Answer 14

Compliance is increased due to loss of alveolar interdependence.

Difficult to exhale.

Question 15

What is surface tension?

Answer 15

Cohesive forces between molecules
Molecules on the surface have no atoms above them

Results in stronger attractive forces on nearest neighbours on the surface

Liquid surface area becomes as small as possible i.e. sphere

Tends to collapse the alveolus

Surface tension increases with

• Emphysema
• Age

Question 16

What is the main component of pulmonary surfactant?

Answer 16

Dipalmitoyl phosphatidylcholine (DPPC)

Question 17

How does surfactant reduce surface tension?

Answer 17

Hydrophilic and hydrophobic ends repel each other and interfere with liquid molecule attraction

Question 18

Why is surfactant needed?

Answer 18

Increases lung compliance
- because surface forces are reduced

Promotes alveolar stability

Prevents alveolar collapse

• small alveoli are prevented from getting smaller
• large alveoli are prevented from getting bigger

Surface tension tends to suck fluid from capillaries into alveoli
- reduction of surface tension reduces hydrostatic pressure in tissue outside capillaries and keeps lungs dry

Question 19

When are babies able to produce enough pulmonary surfactant?

Answer 19

Over 30 weeks

Question 20

What does Laplaces law tell us?

Answer 20

Pressure in a spherical compartment is:

• Proportional to the tension
• Inversely proportional to the radius of that sphere

Surfactant causes surface tension (which contributes to alveolar wall tension, along with elastic recoil), to be lower in smaller spheres

Question 21

What will occur when there are two alveoli, one bigger than the other, with no surfactant?

Answer 21

Both alveoli will have equal surface tension

The smaller will have higher pressure

The smaller is likely to collapse and be harder to inflate

Question 22

What will occur when there are two alveoli, one bigger than the other, with surfactant?

Answer 22

The smaller will have less surface tension (more per surface area)

They will have equal pressure

The smaller will inflate at a faster rate than the bigger until they are equal in size

Question 23

What is airway resistance?

Answer 23

Force needed to inflate lung is greater than elastic recoil

Airway resistance originates from friction between air and mucosa

Is the pressure difference between the alveoli and mouth, divided by the flow rate

Resistance = Pressure1-Pressure2 / Flow

Question 24

What is laminar flow?

Answer 24

Smooth flow

Resistance generated is proportional to the radius (r4).

Question 25

What is turbulent flow?

Answer 25

Irregular, chaotic, with eddie currents

Good for transferring heat (radiators try to enhance turbulent flow), but the resistance is high

Question 26

What is predicted by Reynolds number?

Answer 26

When laminar flow converts to turbulent flow

Question 27

As the airway generation increases, what happens to resistance?

Answer 27

It decreases