17) Mechanics of Breathing

Question 1

Why is the fall in alveolar pressure not instantaneous neglected back to normal?

Answer 1

• There is a delay due to the time taken for air to move.

Question 2

Describe the different sub-divisions of the lungs.

Answer 2

• Trachea
• Primary bronchi
• Smaller bronchi
• Bronchioles
• Alveoli

Question 3

How do the airways change as they get closer to the alveoli?

Answer 3

• In order for gas exchange to take place the air must pass down a series of increasingly narrow and numerous airways

Question 4

What can impairment of airway function lead to?

Answer 4

• Insufficient ventilation

Question 5

What two equations determine rate of airflow?

Answer 5

• Ohm’s law: Airflow (V) = ΔPressure (P)/ Resistance (R)
• Airflow can be increased by increasing ΔP and decreasing R
• Hagen-Poiseuille equation: Resistance (R) ∝ 1/(r)^4
• As the radius of an airway decreases, the resistance increases which in turn causes the airflow to decrease dramatically

Question 6

What is laminar flow?

Answer 6

• When air flows in one plane and in one uniform direction

Question 7

What is turbulent flow?

Answer 7

• Air flow becomes multi-directional and doesn’t move in an efficient manner
• This can be caused by obstruction in airways causing the pattern of flow to change from laminar to turbulent or around branching of airways
• Can produce wheezing sound

Question 8

What does patency mean?

Answer 8

• The state of being open

- If an airway looses patency it is no longer open and collapsing shut

Question 9

What causes a loss of patency in airways?

Answer 9

• Occurs through degradation of airway structure which help keep the airway open.
• This occurs as a result of long term chronic diseases like COPD

Question 10

What is lung compliance?

Answer 10

• The stiffness of the lung. (I.e. it is the amount of force required to expand the lung)
• A lung with a higher compliance is less stiff and so is easier to expand
• A lung with a lower compliance is more stiff and so is harder to expand
• It is a way of quantifying the relationship between the level of expansive force applied to the lung and the resulting change in lung volume

Question 11

What is transpulmonary pressure and what is its equation?

Answer 11

• The level of force acting to expand the lung

- Transpulmonary pressure = Alveolar pressure - Intrapleural pressure

Question 12

What is the equation for compliance?

Answer 12

• Compliance = ΔVolume/ ΔPressure

Question 13

What are the different factors that affects lung compliance?

Answer 13

• Chest wall mechanics: Properties of the chest wall and upper body (Obesity). Can cause compliance to decrease
• Alveolar surface tension (NRDS): Can cause compliance to decrease
• Elastin fibres (Fibrosis): In some diseases the level/density of elastin increases (due to collagen deposition) which results in scarring of the lungs as stiff tissue is deposited. Hence compliance gets lower. Can cause compliance to increase and decrease

Question 14

What is alveoli lining fluid?

Answer 14

• Fluid that lines the inside of the alveoli to enable gas exchange to be more efficient as the gas dissolves in the fluid before diffusing into blood vessels
• A bubble is formed of water molecules and air in the alveoli through a water-air interface
• Surface tension is created due to H-bonds between the water molecules pulling them together
• This exerts a collapsing force towards the centre of the bubble/alveoli
• It causes the alveoli to collapse. In order to expand the alveoli we must overcome these forces along with other factors
• This makes the lungs stiffer

Question 15

What is Laplace’s law of pressure?

Answer 15

• Pressure (P) = 2Surface Tension (T)/radius of bubble/alveoli (r)
• If T is constant then P∝ 1/r
• The smaller the alveoli the larger the pressure/collapsing force generated

Question 16

What happens when a smaller alveoli is attached to a larger alveoli via an airway?

Answer 16

• According to Laplace’s law the smaller bubble would empty into the larger bubble as it generates a larger collapsing force
• This can be problematic in lungs as we will end up with a small number of large alveoli and smooth inflation of the lungs will be more difficult

Question 17

How are the lungs adapted to ensure the smooth inflation of alveoli and the lungs despite the difference in sizes of alveoli?

Answer 17

• The lungs produce pulmonary surfactant (a cocktail of phospholipids and phospholipid proteins) which is secreted by pneumocytes found in the epithelial wall
• Due to their amphipathic nature pulmonary surfactant sit in the interface between the air and the lining fluid
• Here it disrupts the H-bonds between the water molecules which decreases the level of surface tension and decreases the collapsing force
• This makes inflation of the lungs easier

Question 18

How does pulmonary surfactant help equalise the pressure and volume of larger alveoli?

Answer 18

• As alveoli expand, the concentration of the surfactant molecule decreases as it is spread over a larger volume
• This increases surface tension
• This means larger alveoli will collapse into smaller ones to help with the consistent inflation of the lungs

Question 19

How does pulmonary surfactant prevent alveolar oedema?

Answer 19

• Surface tension produced at the air-liquid interface also reduces hydrostatic pressure
• This causes fluid to be pulled out of surrounding capillaries and into alveoli leading to oedema
• By reducing surface tension, pulmonary surfactant prevents the oedema

Question 20

How do neonatal respiratory distress occur?

Answer 20

• It can occur due to insufficient production of pulmonary surfactant
• Premature births, maternal diabetes and congenital developments can lead to the insufficient surfactant production
• This leads to stiffer lungs which are harder to expand (more likely for the lungs and alveoli to collapse) along with oedema.
• This causes a failure in the respiratory system
• This eventually leads to hypoxia and eventually pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary and cerebral haemorrhage

Question 21

How do we treat insufficient surfactant production?

Answer 21

• Supplying mothers carrying babies with maternal glucocorticoid which increases the development of cells in the lungs
• Artificial surfactant supplementation to infants