Mechanics of Breathing Flashcards
Why is the fall in alveolar pressure large enough to be observed?
Delay in pressure change as it takes time for the air to move
What does impaired airway function lead to?
Insufficient ventilation
What does the rate of airflow depend on?
Pressure gradient and level of airway resistance
What happens to resistance as airway radius decreases?
Greater resistance to airflow
What is airway resistance increased by?
Turbulent airflow
What is patency?
State of being open
What can cause airway obstruction?
Loss of airway patency due to degradation of structure
What maintains patency in a healthy alveoli?
Elastin in surrounding alveoli
What happens to the alveoli in COPD?
Reduction in radial traction, so the bronchioles collapse
What does lung compliance quantise?
The relationship between the level of expansive force applied to the lung and the resulting change in lung volume
What is transpulmonary pressure?
The level of force acting to expand the lung
What is transpulmonary pressure made up of?
Alveolar pressure - intrapleural pressure
How is lung compliance calculated?
Dividing a change in lung volume by the associated change in pressure
What is compliance expressed as on a graph of lung volume against transpulmonary pressure?
Gradient
On a graph of lung volume against transpulmonary pressure, what does it mean if the curve is steeper?
Greater lung compliance
What does it mean if there’s a greater lung compliance?
Easier to inflate lung
- Less elastic recoil
- Less force required to inflate
What does a low lung compliance mean?
It’s harder to inflate the lung
- More elastic recoil
- More force required to inflate
What factors affect lung compliance?
Chest wall mechanics
Alveolar surface tension
Elastin fibres (density)
What diseases affect chest wall mechanics?
- Scoliosis
- Muscular dystrophy
- Obesity
What disease affects the alveolar surface tension?
Neonatal respiratory distress syndrome
What diseases affect the elastin fibre density?
Fibrosis (increases density - decreases compliance)
COPD (decreases density)
What is the effect of air-liquid interfaces?
- 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. Lungs become stiffer
- In order to expand the alveoli we must overcome these forces along with other factors
Why are alveoli lined with fluid?
Enable gas exchange
What does Laplace’s law describe?
The pressure generated by the surface tension within a given area
What would happen if two different sized alveoli were connected via airways and what is this force overcome by?
- Pressure gradients being created between the alveoli, resulting in smaller alveoli emptying into larger ones
- Overcome by pulmonary surfactants
What is a pulmonary surfactant?
Mixture of different phospholipids and phospholipoproteins
What are pulmonary surfactants secreted by?
Type 2 pneumocytes
What do pulmonary surfactants do?
- Reduce surface tension using ampipathic molecules to disrupt H-bonds (so lungs expand easier)
- Equalise pressure and volume across varying alveoli
What happens to the concentration of pulmonary surfactant as the alveoli expand and why does it happen?
- Decreases, which increase surface tension
- Set amount of surfactant in each alveoli
What does surfactant mean for the distribution of air in the alveoli?
Larger alveoli collapse into smaller alveoli
How does surfactant prevent alveolar oedema?
- Surface tension produced at the air-liquid interface reduces hydrostatic pressure. It is then pulled out of the surrounding capillaries into the alveoli
What is neonatal respiratory distress syndrome caused by?
Insufficient production of pulmonary surfactant
What is the pathway of neonatal respiratory distress syndrome?
- insufficient surfactant production
- stiff lungs, alveolar collapse, oedema
- respiratory failure
- hypoxia
What are the causes of death for neonatal respiratory distress syndrome?
Pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary and cerebral haemorrhage
Describe the different sub-divisions of the lungs.
- Trachea
- Primary bronchi
- Smaller bronchi
- Bronchioles
- Alveoli
How do the airways change as they get closer to the alveoli?
Air passes down a series of increasingly narrow and numerous airways
What is Ohm’s Law?
Airflow (V) = ΔPressure (P)/ Resistance (R)
Using Ohm’s Law, suggest how airflow can be increased?
Increasing ΔP and decreasing R
What is the Hagen-Poiseuille Law?
Resistance (R) ∝ 1/(r)^4
What can be inferred from the Hagen-Poiseuille Law?
As the radius of an airway decreases, the resistance increases which in turn causes the airflow to decrease dramatically
What is laminar flow?
When air flows in one plane and in one uniform direction
What is turbulent flow?
- Air flow becomes multi-directional and doesn’t move in an efficient manner
- Caused by obstruction in airways causing the pattern of flow to change from laminar to turbulent or around branching of airways
Define lung compliance.
Stiffness of the lung
What is Laplace’s law of pressure?
- 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
How can insufficient surfactant production be treated?
- Supplying mothers carrying babies with maternal glucocorticoid which increases the development of cells in the lungs
- Artificial surfactant supplementation to infants
What is the difference between static and dynamic compliance?
- Static compliance is the measurement taken whilst there is no airflow.
- Dynamic compliance is the measurements taken during the movement of air.
How can the level of airway obstruction be investigated?
Using a spirometry graph demonstrating forced vital capacity (FVC) and forced expiratory volume in one-second (FEV1) values.
If a lung condition is obstructive, what can be observed about the FEV1/FVC ratio?
FEV1/FVC <70% (for example, asthma, with increased resistance)
If a lung condition is restrictive, what can be observed about the FEV1/FVC ratio?
FEV1/FVC >80% (for example, fibrosis, with decreased compliance)
How is resistance generated in the airways?
As air passes through airways, it generates resistance as it comes into contact with the airway surface.
Why is an increase in lung compliance bad?
- Easy to expand the lungs but they cannot recoil
What happens with an increased concentration of surfactant molecules?
- The surface tension decreases as water molecules do not have space to bond.
What happens with a decreased concentration of surfactant molecules?
- Surface tension increases as water molecules are free to bond.
What happens when the lumen diameter increases?
- Increase in luminal area
- Decrease in resistance
- Increase in flow
What happens when the lumen diameter decreases?
- Decrease in luminal area
- Increase in resistance
- Decrease in flow
Name some factors that will decrease lumen diameter
- Contraction of airway smooth muscle,
- Excessive mucus secretion
- Oedema
- Loss of patency
