Mechanics Of Breathing

Question 0

Define the term ‘compliance’ of the lungs and state how, in principle it is measured

Answer 0

Compliance - stretchiness of the lungs

Change in volume/change in pressure

Question 1

Describe the mechanical system of the lungs and thorax

Answer 1

Resting expiratory level - inward force of elastic recoil = outward recoil of chest wall (equilibrium)
Functional residual capacity - elastic forces of lung and chest wall are equal and in opposite directions
Pleural space + fluid = pleural seal
Pleural pressure remains negative - resting -ve, inspiration more -ve

Question 2

Describe the factors which affect the compliance of the lungs, including the role of surfactant

Answer 2

Surfactant increases compliance by reducing surface tension
It disrupts the interactions between molecules
As alveoli get bigger, surface tension increases and surfactant is less effective

Question 3

Explain Respiratory Distress Syndrome

Answer 3

Babies born premature with too little surfactant –> stiff lungs, few and large alveoli –> unable to expand alveoli
Treatment: mother given steroid (e.g. dexamethasone) which stimulates baby to produce surfactant

Question 4

Describe the factors which influence airway resistance in the normal lung and how airway resistance changes over the breathing cycle

Answer 4

Resistance = pressure/rate of flow
Resistance increases with falling radius
Trachea = increased resistance, alveoli = decreased resistance due to alveoli in parallel
Breathing in is easy

Question 5

Explain resistance in terms of Obstructive Airway Disease

Answer 5

Small airways narrowed –> increased resistance –> difficulty breathing out

Question 6

Explain Laplace’s law

Answer 6

P = 2T/r (pressure = 2 x surface tension/radius of alveolus)
Big alveoli = low pressure
Small alveoli = high pressure
Small alveoli would collapse into big alveoli
Alveoli roughly same size