Respiratory Physiology - Mechanics of Breathing

Question 1

Muscles of inspiration

Answer 1

Diaphragm
External intercostal muscles

Accessory muscles under exertion also used

Question 2

Form of respiration in health

Answer 2

Negative pressure ventilation

Question 3

How is negative intrathoracic pressure generated

Answer 3

Diaphragm contracts + flattens and intercostal muscles contract to lift rib cage

Increases lung volume and therefore decreases intrathoracic pressure

Generates pressure gradient from atmosphere in to the lungs

Question 4

Innervation of diaphragm

Answer 4

2 phrenic nerves
C3/4/5

Question 5

Method of expiration in normal circumstances

Answer 5

Passive expiration

Question 6

Method of expiration in exertion

Answer 6

Element of active expiration with contraction of abdominal muscles + contraction of internal intercostal muscles

Question 7

Pressure-volume curve of lungs

Answer 7

At atmospheric pressure, there is still small volume of gas in the lung
This is called minimal volume - difficult to remove due to closure of small airways

Non-linear relationship

Hysteresis - behaviour in expiration lags behind behaviour in inspiration

Get the same curve if using negative or positive pressures

Question 8

Compliance definition

Answer 8

Change in volume / Change in pressure

Units = ml/cmH2O

Question 9

Normal compliance of lung

Answer 9

~ 200 ml/cmH2O

Question 10

Examples where compliance is reduced

Answer 10

• Pulmonary fibrosis
• Pulmonary oedema

Lung tissue stiffer

Question 11

Examples where compliance is increased

Answer 11

• Emphysema
• Increased age

Loss of lung architecture and loss of elastic recoil

Question 12

Proteins contributing to elastic properties / compliance of the lung

Answer 12

Collagen
Elastin

Question 13

Factors contributing to elastic properties / compliance of the lung

Answer 13

Change in lung protein geometry

Surface tension of alveolar lining fluid

Question 14

Surface tension definition

Answer 14

Total force across imaginary line 1cm across the surface

Picture A

Molecules of water attract each other more than they attract molecules of air

Question 15

Units of surface tension

Answer 15

dynes / cm

or

milli-Newtons / m

Question 16

Features of surface tension on a curved surface

Answer 16

On a curved surface, surface tension produces pressure

Picture B

Question 17

Laplace’s law

Answer 17

Relates to pressure in a soap bubble

Pressure = 4 x surface tension / radius

P = 4T / r

Question 18

Implication of Laplace’s law

Answer 18

Pressure is higher in spheres of smaller radius, and therefore they inflate the larger spheres

Picture C

Question 19

Pressure-volume curve of lung when inflated with fluid vs air

Answer 19

Easier to inflate lung with fluid than with air, with less hysteresis

Due to loss of surface tension when fluid used to inflate lung

Question 20

Surfactant definition

Answer 20

Material which reduces surface tension of alveolar layer

Question 21

Effect of surfactant on surface tension

Answer 21

Surface tension does not change with changes in area with water

Detergent reduces the surface tension but again unaffected by area changes

Surfactant results in surface tension altering with changes in area.
Surface tension very low at lower area %
Therefore aids inflation with air and elastic recoil at higher area %

Question 22

Main constituent of pulmonary surfactants

Answer 22

Dipalmitoylphosphatidylcholine
(DPPC)

Phospholipid

Question 23

What produces pulmonary surfactants

Answer 23

Type 2 alveolar epithelial cells

Secreted as lamellar inclusion bodies

Question 24

3 functions of surfactant

Answer 24

Increases compliance of lung

Increases stability of the lung

Reduces tendency to alveolar oedema

Question 25

How does surfactant increase pulmonary compliance

Answer 25

Reduced surface tension at lower areas so easier to inflate

Higher surface tension at higher areas so elastic recoil

Question 26

How does surfactant increase stability of lung

Answer 26

Due to lower surface tension at lower areas and higher at higher areas, you do not see smaller alveoli inflating larger alveoli as you might expect according to Laplace’s law

Question 27

How does surfactant reduce tendency to alveoli oedema

Answer 27

Reduced pressure in alveoli
Therefore reduced capillary all tension surrounding alveoli

Therefore reduced hydrostatic pressure of capillary and fluid more likely to move into capillary and less likely to form oedema

Question 28

Effect of lung if no surfactant was present

Answer 28

Significant atelectasis

As seen in premature new-borns where surfactant not yet produced

Question 29

Infant respiratory distress syndrome

Answer 29

No surfactant - commonly in premature babies

Treat by giving artificial surfactant

Question 30

Regional differences of lung ventilation at FRC

Answer 30

Weight of lung results in different pressures in apex vs base of lung

Greater negative pressure at apex so on different part of curve

Base at a steeper part so change in pressure results in greater change in volume compared with apex

Question 31

Regional differences of lung ventilation at residual volume

Answer 31

Pressures are less negative
Pressure at base above atmospheric pressure

Therefore no ventilation at bases occur until pressure falls below atmospheric pressure

More likely to have airway closure at higher volumes with increasing age or diseased lungs

Question 32

Patterns of gas flow in tubes

Answer 32

Move from A -> B -> C as flow rate is increased

Question 33

Poiseuille equation for laminar flow

Answer 33

Note resistance is inversely proportional to radius to 4th power

Question 34

Velocity profile of laminar flow

Answer 34

Flow rate at centre of tube is higher than elsewhere in the tube

Question 35

Reynold’s number

Answer 35

Likelihood of turbulent flow

Question 36

Pressure / Volume / Flow changes during breathing cycle

Answer 36

Question 37

Number of airways with each generation

Answer 37

Exponentially increases
Therefore very large number of smaller airways

Question 38

Where in airways is there the most resistance

Answer 38

Medium sized airways

Higher resistance with smaller radius but due to vastly high numbers, the resistance is shared more and therefore less for each airway

Question 39

What happens to airway resistance as volume increases

Answer 39

Airway resistance decreases as volume increases

This is because radial traction from lung parenchyma increases radius of airways and pulls them open (similar concept to blood vessel effects)

Explains why patients with lung disease (eg COPD) breath with higher lung volumes

Question 40

Flow-Volume curves with spirometry

Answer 40

A = maximal effort
C = not maximal effort
B = low effort then maximal effort at the end

Cannot exceed downslope flow, always meets it

Question 41

Isovolume pressure-flow curves

Answer 41

At different lung volumes, expiratory flow is independent of effort exerted

I.e, reach a maximum expiratory flow which cannot be exceeded

Question 42

Reason that expiratory flow is independent of effort exerted at fixed lung volumes

Answer 42

Dynamic compression

During inspiration as intrapleural pressure decreases, airways are pulled open with radial traction

This is not present in expiration and in fact has opposite effect as transpulmonary pressure increases inwards causing airway compression / collapse

Increase in transpulmonary pressure matches increase in alveolar pressure and therefore flow remains constant regardless of effort

Question 43

Tissue resistance

Answer 43

Resistance from tissue sliding over another

Not as important / notable as airway resistance

Question 44

Work done by the lung during inspiration

Answer 44

In absence of airway resistance, work done would be area from line AEC to D to 0

Additional work done with airway resistance is represented by the shaded curve