Mechanics of Breathing

Question 1

What is the definition of compliance?

Answer 1

The change in volume of the lung relative to the change in pressure exerted on the lung. It is essentially the capacity of the lung to stretch.

• High compliance - indicates a “stretchy lung”, which indicates the lung volume is able to increase by large amounts for small decreases in intrapleural pressure
• Low compliance - indicates a “stiff lung” that the lung volume increases only by small amounts for large decreases in intrapleural pressure.

Question 2

What factors effect lung compliance?

Answer 2

• Elastance
• Surfactant production
• Height of the lung

Question 3

What is elastance?

Answer 3

The ability to resist being deformed/elastic recoil. A loss of elastance (for example in emphysema) result in a loss of lung recoil, resulting in a high compliance (stretchy), and therefore do not recoil to their resting position during expiration

Question 4

How does the height of the lung affect compliance?

Answer 4

When upright, the weight of the lung stretches alveoli in the apex of the lung, and compresses the alveoli in the base of the lung. This means that on inspiration the alveoli in the apex expand very little relative to the “squashed” alveoli in the base of the lung, which expand much more. This means that the compliance in the base of the lung is higher than that of the apex of the lung.

Question 5

What is the difference between pulmonary and alveolar ventilation?

Answer 5

Pulmonary ventilation refers to total air movement into/out of the lungs, whereas alveolar ventilation refers to the amount of fresh air getting into the alveoli and therefore available for gas exchange.

Question 6

What proportion of a tidal volume of 500 mls of fresh air actually reaches the alveoli?

Answer 6

350ml - approximately 150 mls of tidal volume is trapped in dead space

Question 7

What receptors are involved in bronchodilation?

Answer 7

B₂ adrenergic receptors - sympathetic

Question 8

What receptors are stimulated to cause bronchoconstriction?

Answer 8

M₃ muscarinic receptors - parasympathetic

Question 9

What are the 4 phases of the cough reflex and what is the physiology behind each of the stages?

Answer 9

Sensory phase - vagal pulmonary receptors (rapidly and slowly adapting receptors, C-fibres and others) sense mechanical +/- chemical stimulus in the airways and transmit signals back to the brainstem and cortex, initiating the cough reflex. Results from any irritation

Inspiratory phase - large breath in is stimulated to ‘stretch’ the expiratory muscles and allow them to produce greater positive intrathoracic pressure on expiration. This allows the body to push out more air faster with force.

Compressive phase - glottis is closed after inspiration to maintain lung volume, while intrathoracic pressure is building.

Expiratory phase - the glottis opens and air is pushed out because of the high positive intrathoracic pressure.