24B01: Exam Report
(a) Describe the work of breathing and its components (85% of marks). (b) Briefly outline the efficiency of the lung (15% of marks).
29% of candidates passed this question.
This question required candidates to provide a detailed description of the work of breathing and its components.
A good answer divided the components into elastic and inelastic/resistive work and provided an overview of the factors that affected each of them.
This description was allocated the majority of marks. A thorough answer also discussed the role of expiration on work of breathing, whilst usually a passive process it can be active under some conditions and good answers touched upon these.
The second section of the question required an overview of the fate of the energy utilised by the respiratory muscles, with a large proportion lost as heat, or through hysteresis and airway resistance.
F3xi / 24B01: Describe the work of breathing and its components
Definition
Product of pressure and volume or force and distance
Units of measurement
Joules
Components
Elastic and Inelastic / Resistive work
Normal tidal breathing:
- Inspiration- active (most of the WOB)
- Expiration- passive (can become active in certain conditions)
Inspiration
Resistance work
- ~ 35% of total work – lost as heat
- > Mainly- frictional forces on gas molecules
- > 10%- tissue resistive work
Elastic work
- ~65% of total work
- > elastic forces- outwards- chest wall, inwards- surface tension, lung parenchyma
- > stored as potential energy
- > released and is used to overcome frictional airway and tissue forces
Expiration
- > overcome non-elastic forces- tissue and airway resistance
- > accounted for by stored potential energy
Factors affecting
Resistance work
- Increased respiratory rate
- Increased airway resistance
Elastic work
- Increased minute volume
- Lung diseases eg., emphysema
Diagram
Efficiency
Defined as the mechanical work of breathing divided by the corresponding energy expenditure
Fate of the energy utilised by the respiratory muscles, with a large proportion lost as heat, or through hysteresis and airway resistance.
Author: Nazma Navilehal Rajasab