Mechanics of breathing Flashcards
Airflow equation ?
Airflow (V) = pressure gradient / resistance (R)
What does increased resistamce mean?
Decreased airflow
an increased pressure gradient = increased airflow
What is the Hagen-Poiseuille equation?
It describes the relationship between resistance and the various properties of airflow
Resistance ∝ 1/ Radius^4
Explain what will happen if radius is halved
Airway resistance will increase by a factor of 16
Pathological features that can decrease lumen size
Contraction of airway smooth muscle,
excessive mucus secretion,
oedema/swelling of the airway tissue,
damage to the integrity of the airways structure (i.e. loss of patency)
What is airway patency?
patency refers to the state of being open or unobstructed; a ‘loss of patency’ = closing/obstruction
How are the open structures of the airways maintained?
Elastic fibres and radial traction
Why is airway obstruction more noticeable during expiration?
During expiration, the lung tissue and airways are compressed.
Does intrapleural pressure increase or decrease during inspiration?
Decrease, it becomes more negative
Does intrapleural pressure increase or decrease during expiration?
Increase, it becomes more positive
what is the term given to the difference between the pressure within the alveoli and intrerpleual space?
Transpulmonary pressure
What does the transpulmonary pressure determine?
The level of force acting to expand or compress the lungs
How do bronchodilators work?
Sympathetic beta 2 agonists
Beta 2 adrenergic stimulaton
Increased cAMP
Causes bronchodilation
Increased radius
Decreased resistance
Increased air flow
Collapsing pressure of alveoli equation?
P=2T/r
Pressure=2 X surface tension / radius of bubble
If T remains constant
p ∝ 1/r
Lung compliance equation?
Compliance =change in volume/ change in pressure
How does emphysema change compliance?
Increases compliance by degradation of the elastin fibres
How does fibrosis affect compliance?
scarring and deposition of structural fibres such as collagen making the lung stiff and less compliant
What causes surface tension in the alveoli?
Hydrogen bonds between water molecules
They combine to exert an overall collapsing force rowards centre of bubble
The collapsing force in the bubble can generate pressure. What can be used to measure the amount of pressure?
Law of laplace
if bubbles of varying size are connected (such as different size alveoli connect by airways) which bubble will empty out into the other, the smaller or larger one?
the smaller bubble will empty into larger ones due the pressure gradient (smaller volume=larger pressure_
How is the problem of bubbles collapsing resolved?
Surfactant
they remove surface tension
What is pulmonary surfactant made of?
Phospholipoprotein
secreted by type II pneumocytes
Surfactants are amphipathic
What does this mean?
hydrophilic head and hydrophobic tail regions, and so will naturally position themselves at the air-liquid interface.
Why does the amount of surfactant decrease during inflation?
surface tension (and thus pressure generated) increases with increasing alveolar surface area. This means that air will naturally flow from larger (more inflated) alveoli to smaller ones, helping to distribute air across the lung during inspiration
What is NRDS
Neonatal respiratory distress syndrome
Infants born prematurely have insufficient levels of surfactant
What is the effect of NRDS
Alveoli collapse, decreasing lung compliance, increasing likelihood of alveolar oedema
How is NRDS treated?
supplementation of affected infants with artificial surfactant
glucocorticoids to mothers deemed high risk
