Mechanics of Breathing

Question 1

Role of muscles in normal breathing?

Answer 1

Diaphgram and external intercostal muscles on inhalation to increase thoracic space and reduce pressure

Question 2

Role of muscles in forced breathing (exhalation)?

Answer 2

Abdominal muscles, Extracartilaginious internal intercostals, Pectoral girdle muscles

Question 3

Explain ‘work of breathing’

Answer 3

Energy expenditure of respiration (muscular work predominantly on inhalation at rest) - work done to overcome airway resistance and stretching of elastic tissue/wall

Question 4

Explain surface tension

Answer 4

Force water molecules exert on surface of lungs when bound together (strong inward pull), surfactant reduces surface tension so lungs are able to expand

Question 5

What is compliance?

Answer 5

Disposition of stretchability of lungs (ability to expand under traction/pressure)

Question 6

What is elastance?

Answer 6

Disposition of lungs to return/recoil to resting position (due to intrinsic elasticity)

Question 7

Hooke’s Law

Answer 7

Strain is proportional to the applied stress (extension is proportional to force)

Question 8

Laplace’s Law

Answer 8

The larger the vessel radius, the larger the wall tension to withstand pressure

Question 9

Where does the main part of airway resistance lie?

Answer 9

In medium sized bronchioles as smaller ones collectively have the lowest resistance

Question 10

What stops the lungs from collapsing from the inner thoracic wall?

Answer 10

Negative pressure within the pleural space

Fixed pleural seal between visceral and parietal pleura

Question 11

What’s the functional residual capacity?

Answer 11

When expansile and contractile forces are in balance (that of lungs recoiling and that of muscles expanding thoracic cavity)

= expiratory reserve volume + residual volume

Question 12

What’s a pneumothorax?

Answer 12

If integrity of pleural seal is broken (due to trauma etc), intrapleural pressure will rise and lung will collapse away from chest wall under its unopposed force of elastic recoil

Question 13

What’s the bedside test of airway resistance?

How does resistance change with disease?

Answer 13

Peak Expiratory Flow Rate

Flow is limited by resistance as radius of airways is reduced

Question 14

Which 2 things help prevent lung collapse?

Answer 14

Surfactant and alveolar interdependence

Question 15

How does lung compliance change in emphysema and fibrosis?

Answer 15

Emphysema - increased compliance as less tissue to expand

Fibrosis - reduced compliance as scar tissue

Question 16

What could phrenic nerve palsy lead to?

Answer 16

Respiratory distress/breathlessness as diaphragm paralysed and is the muscle doing most of the work on inspiration

Question 17

What’s surfactant and what’re its roles?

Answer 17

Secreted by Type II Pneumocytes, predominantly phospholipid

Antioxidant - surfactant proteins (SP-A and SP-D) recruit macrophages/neutrophils and can bind pathogens
Reduces likelihood of lung collapse and fluid transudation
Reduce surface tension and increase compliance

Question 18

How does Hooke’s law relate to lung compliance and the hysteresis curve?

Answer 18

Hooke’s law: extension is proportional to load and distance
Lung is stiffer at high and low pressures but expands well in the middle (hysteresis curve shows variation in lung compliance and volume)