Mechanics Of Ventilation 2 Flashcards
Describe airflow through tubes
Laminar flow- uniformly in one direction, parallel stream lines
Slow flow rate
What is the equation for flow rate ?
Flow rate (V) = Pressure gradient (P) / Airway resistance (Raw)
What is Poiseuilles law ?
It describes how fluid flows through a narrow tube.
V=P x Pi x r^4 / 8 x n x L
n is viscosity of the fluid
L is the length of the tube
Describe turbulent flow
Eg. After exercise or an asthma attack
High flow rate
Disorganised stream lines
Describe transitional flow
Intermediate flow rate
Eddy currents
Where is the majority of resistance to inhaled air ?
Resistance to air movement through conducing airways- bulk flow.
Low resistance in terminal bronchioles to allow diffusion to occur.
What effect does lung volume have on airway resistance ? Why is this ?
Airway resistance decreases as lung volume increases
This is due to radial traction. As lung expands, connective tissue pulls on bronchioles, so their diameter expands and resistance falls.
What occurs during forced expiration?
Dynamic compression- airways are compressed and may close due to increased pressure
Bronchioconstriction
Increased airway resistance
Parasympathetic stimulation
Fall in CO2 pp
Bronchiodilation
Lowered airway resistance
Autonomic stimulation
How does infection increase airway resistance?
- Inflammation of the tissues lining the upper airways
- Overproduction/ accumulation of mucus
Effect of airway resistance on PEFR
Greater airway resistance means slower peak expiratory flow rate
What is elastic recoil determined by ?
- Elastic properties of the lung tissue
- Surface tension in the alveoli
What forms elastic tissue in the lung ?
Elastin and collagen fibres
What problems does the presence of air-fluid interface create ?
- Attractive forces in liquid oppose expansion by inspired air
- Promotes collapse of smaller alveoli
- Causes transduction of fluid from capillaries
Minimised by surfactant.
What is pulmonary surfactant ?
Phospholipoprotein secreted from Type II alveolar cells
What does pulmonary surfactant do ?
- Lowers surface tension in the liquid layer
- Prevents alveolar collapse at low pressure
When is compliance reduced ?
- When surface tension is increased
- When elasticity is impaired
