L1 - Respiration - Airflow Flashcards
Which 4 factors have a role in determining air flow?
Type of air flow
Resistance of the pathway
Pressure gradients
Links between lung volume, resistance and airflow
What is laminar flow?
Steady flow down a tube in a uniform direction and speed
Flow rate is maximal in the centre but reduces towards the edge
During laminar flow movement of air into and out of the lungs is?
Proportional to the pressure gradient
Inversely proportional to the resistance
What is turbulent flow?
If flow rate moves beyond a critical value, irregular currents – vortices develop
Greater pressure gradient needed to obtain the same flow seen under laminar conditions – more effort
During turbulent flow the rate of gas movement is?
Proportional to square root of the pressure difference
What is transitional flow?
High numbers of bifurcations in the airways disrupt flow - creating eddies
What is determination of flow type governed by?
Reynolds number o r - radius o v - velocity o ρ - density of the gas o η - viscosity
Reynolds number equation
Re = 2rvp/n
If Re < 1000 what flow is seen?
Laminar flow
If Re is between 1000 and 1500 what flow is seen?
Flow switching between laminar and turbulent
If Re is > 1500 what flow is seen?
Turbulent flow
What is Poiseuilles law?
Airway resistance is proportional to gas viscosity
Length of the tube but is inversely proportional to the fourth power of the radius
Small changes in airway diameter have a big impact on the resistance and therefore flow rate
In a normal individual what is the airway resistance throughout the lung?
Total airway resistance is 1.5cm H2O .s.litres-1
- Pharynx-Larynx - 40%
- Airways >2mm diameter – 40%
- Airways <2mm diameter – 20%
In a COPD patient what is the airway resistance throughout the lung?
Total airway resistance is 5.0 H2O .s.litres-1
- Pharynx-Larynx - 12%
- Airways >2mm diameter – 18%
- Airways <2mm diameter – 70% (huge difference from normal patients)
What are the 2 factors impacting airway resistance?
Airway diameter
Lung volume
How does airway diameter impact airway resistance?
Increased mucus secretion reduces airway diameter – increased resistance
Oedema – increased fluid retention in lung tissue – increased resistance
How does lung volume impact airway resistance?
At a given lung volume, airway resistance is higher in COPD
At smaller volumes you have higher resistance
What is the effect of dynamic pressures on airflow during inspiration?
During inspiration - forced expansion of higher airways
Dilation of airways – decrease in resistance
What is the effect of dynamic pressures on airflow during expiration?
During expiration – forced collapse of higher airways
Constriction of airways – increase in resistance
What impact does emphysema have on airway compression?
Airway compression is exaggerated
- Loss of elastic tissue and break down of alveolar walls
- Tethering between the walls of airspaces reduced
- Airways less able to resist the collapse
How are the issues of emphysema overcome?
Slow exhalation
Breathing takes place at a higher lung volume
Exhale through pursed lips
When resistance is increased what is impaired about the lungs?
Inflation
In normal patient, after 1s of inspiration, lungs have reached tidal volume (500ml)
In COPD patients, after 1s of inspiration, lungs have only reached 60% of tidal volume
How do you calculate interpleural pressure?
Alveolar pressure - transpulmonary pressure
How do you calculate transmural pressure?
Airway pressure - intrapleural pressure
