The Mechanics of Breathing

Question 1

What does the rate of airflow depend on?

Answer 1

Pressure gradient and level of airway resistance

Question 2

State ohms laws and describe it

Answer 2

Ohm’s law -
Airflow (V) = change in pressure (P) / resistance (R)

• Hence if the change in pressure increases the airflow will increase
• But if the resistance increases then the airflow will decrease

Question 3

State the Hagen-Poiseuille equation and describe it

Answer 3

Hagen-Poiseuille equation -
Resistance (R) is proportional to 1 / the radius to the power of 4

• As the airways radius decreases the resistance increases dramatically

Question 4

Describe how airway resistance comes about

Answer 4

Airway resistance relates to the fact that as air molecules move through the airways some of the air molecules will come into contact with the airways surfaces which creates friction - causes energy to be dissipated and speed at which the molecules of air move then slows down

Question 5

Describe turbulent flow and what it does to airway resistance

Answer 5

• Airway resistance is increased by turbulent flow
• During normal air movement the air moves in a laminar flow meaning it moves in one direction smoothly through the airways
• When the airway is obstructed e.g. from mucus build up turbulent flow can occur where the air starts to move through the airways in multiple directions - causes wheezing noise

Question 6

Describe how airway obstruction can be caused by degradation of structures

Answer 6

• In a healthy individual the elastin surrounding alveoli provides radial traction to splint bronchioles against positive pressure
• In COPD, the elastin is not correctly formed so there is no radial traction and therefore the bronchioles collapse as they can no longer cope with positive pressures so obstruction occurs

Question 7

What is lung compliance?

Answer 7

Refers to the stiffness of the lung - high compliance = easy to expand, low compliance = stiffer

Question 8

What does lung compliance quantify?

Answer 8

The relationship between the level of expansive force applied to the lung and the resulting change in lung volume

Question 9

How can lung compliance be quantified?

Answer 9

Compliance is quantified by transpulmonary pressure (the level of force acting to expand the lung)
Transpulmonary pressure = alveolar pressure - intrapleural pressure

Question 10

Describe how compliance can be shown in a graph format

Answer 10

On a graph of lung volume (%) against transpulmonary pressure (mmHg), as the transpulmonary pressure increases the lung volume increases - the steeper the line is the more compliance there is as less pressure is needed to reach a given lung volume as more compliant lungs are easier to expand

Compliance = the change in volume / the change in pressure

Question 11

State the factors and associated diseases that can affect lung compliance

Answer 11

• Chest wall mechanics e.g. scoliosis, muscular dystrophy and obesity make compliance lesser so it becomes harder for the lungs to inflate
• Alveolar surface tension e.g. neonatal respiratory distress syndrome decreases compliance
• Elastin fibres e.g. fibrosis decreases lung compliance and COPD increases lung compliance

Question 12

What is pulmonary surfactant?

Answer 12

A fluid which enables gas exchange (the gas molecules dissolve into water before they diffuse)

Question 13

Describe what is formed within the alveoli due to water molecules and what this causes

Answer 13

Within the bubble formed by the air-water interface surface tension is created due to hydrogen bonds forming between water molecules which exert a collapsing force towards the centre of the bubble - resists inflation

Question 14

State what the law of laplace suggests and give the equation

Answer 14

The law of laplace -
- Describes the pressure generated by the surface tension within a bubble
- The collapsing force generates pressure - the amount within a specific bubble can be found using the law of laplace

P = 2 x T / r

P = pressure
2T = 2 x surface tension
R = radius of the bubble e.g. the alveoli

Hence the smaller the alveoli is the larger the collapsing pressure generated is

Question 15

Explain why the fact that smaller alveoli having a greater collapsing pressures is a problem

Answer 15

• This means that if there are 2 different sized alveoli next to each other, the smaller alveoli will empty into the larger one
• This is a problem because it would result in a small number of very large alveoli which would reduce surface area and make inflation of the lungs very difficult

Question 16

Describe how this is overcome and include all the roles of lung surfactant

Answer 16

To overcome this alveolar surface tension pulmonary surfactant is secreted by type 2 pneumocytes

• The surfactant molecules are amphipathic so have hydrophobic and hydrophilic components
• These surfactant molecules once secreted sit between the alveolar air and the lining fluid which disrupts the hydrogen bonds between water molecules which reduces surface tension and collapsing pressure so makes inflation easier by increasing compliance
• Surfactant also acts to equalise pressure and volume across varying alveoli
• As the alveoli expand the concentration of surfactant molecules decreases increasing surface tension - now larger alveoli will collapse into smaller ones which aids in helping consistent inflation of the lungs
• Surfactant also helps to prevent alveolar oedema because by reducing surface tension at the air liquid interface hydrostatic pressure is increased so fluid isn’t pulled out of the surrounding capillaries and into the alveoli so it helps to prevent oedema which is seen in patients with less surfactant

Question 17

Describe neonatal respiratory distress syndrome as an example of a lack of surfactant production

Answer 17

• May occur due to a premature birth, maternal diabetes or congenital developmental issues
• Causes insufficient surfactant production
• Causes stiff (low compliance) lungs, alveolar collapse and oedema
• Leads to respiratory failure and hypoxia
• There is then pulmonary vasoconstriction, endothelial damage, acidosis and pulmonary/cerebral haemorrhage
• Can be overcome by artificial surfactant supplementation of infant or maternal glucocorticoid supplementation