respiratory mechanisms Flashcards
airflow in tube definition
the movement of air in a tube where there is a difference of pressures between the ends
what does the pressure difference depend upon?
the rate and pattern of airflow
two basic types of airflow
laminar flow and turbulent flow
laminar flow definition
when the stream lines are parallel to the sides of the tube and is found at low flow rates
flow rate definition
the volume/mass of a gas/liquid that moves per unit time
what does Poiseuille’s law state?
that flow is related to viscosity, the pressure gradient, length and radius of the tube
flow rate equation
Pᴨr^4/8nl
resistance equation
8nl/ᴨr^4
what is flow rate proportional to?
the pressure gradient
why is the tube radius critical?
halving the radius can increase the resistance by x16
what happens when flow rate is increased?
the unsteadiness of the laminar flow increases, stream lines separate from the tube walls and local eddies (swirling and reverse currents) form at branches
forms a transitional flow
turbulent flow definition
when complete disorganisation of the stream lines is seen at high flow rates
what affects the turbulent flow?
viscosity of gas becomes unimportant, however an increase in gas density matters and affects the pressure gradient
what determines whether air flow is turbulent or laminar?
Reynolds number
How is Reynold’s number calculated?
2rvd/n
r= radius
v= average velocity
d= density
n= viscosity
when does turbulence occur?
Re>2000
Explain types of flow in the lungs
mainly laminar flow
but the lungs highly complicated branching pattern means that there may be eddies and that the flow is non steady,
complete laminar flow only occurs in the terminal bronchioles
true turbulence may occur in the trachea during exercise, when velocity/flow rate is high
what causes wheezing?
occurs during the expiratory phase of respiration due to an obstruction that produces turbulent air flow
what does the resistance determine?
the rate of laminar flow down a pressure gradient?
causes of airway resistance
- decrease in lung volume increase airway resistance
- upper most parts of the bronchial tree, as in total these have less divisions and thus a smaller radius than the total branches further down. Although the smallest bronchioles have the smallest radius, the very large cross sectional area compensates for this
- segmental bronchi are the site of maximal resistance- 1/3 of total resistance arises from the nose, pharynx and larynx
what reduces the resistance caused by the nose, pharynx and larynx?
mouth breathing
what is the silent zone?
peripheral airways that contribute very little to resistance, therefore airway disease that begins here is unlikely to produce a difference in resistance that could be detected
what does anaphylaxis cause?
bronchiolar contraction that increases airway resistance by reducing the radius
decreases in lung volume compress the airways and increase resistance further
what is radial traction?
a force that keeps airways open during lung expansion and volume increases
sympathetic control of resistance explained
B2 adrenergic receptor agonists cause bronchodilation, reducing resistance, such as adrenaline and salbutamol
can be used as treatments for asthma and COPD
parasympathetic control of resistance explained
M2 GPCR, where ACh acts and causes bronchocontriction
anticholinergic agents used in COPD
how is airway resistance measured?
subject sealed in plethysmograph
breathes through a pneumotachograph to measure flow
when they breathe, the alveolar pressure falls and the pressure in the box rises
alveolar pressure can then be estimated by P1V1=P2V2
peak expiratory flow definition
the maximum air flow during forced expiration beginning with the lungs fully inflated
what happens to the peak expiratory flow in asthma patients?
lower, due to high resistance
what happens to the peak expiratory flow in emphysema?
low lung elasticity, resulting in poor airway support, easy collapse and thus reduced peak expiratory flow
what limits the peak expiratory flow rate?
the person’s maximum rate of expiration is limited by the compression of the airways by intrathoracic pressure
forced expiration intrapleural pressure exceeds airway pressure so the airways would collapse due to the greater positive pressure outside than inside
FEV1 definition
the volume of air exhaled in the first second
how is peak expiratory flow measured?
spirometry
is FEV1 different to forced vital capacity?
yes it is less, however not that much less as the majority of air is expired in the first second
what happens to FEV1 and FVC in restrictive diseases?
both are reduced
what happens to FEV1 and FVC in obstructive diseases?
both reduced but FEV a lot more than FVC
Obstructive disease definition + examples
respiratory disease characterised by airway obstruction
asthma, COPD, chronic bronchitis
restrictive disease definition + examples
extra pulmonary, pleural or parenchyma respiratory diseases that restrict lung expansion, resulting in a decreased lung volume
decreased force vital capacity as physically cannot inhale more and thus expel more
pulmonary fibrosis
what diseases affect airway resistance
COPD, asthma, emphysema and chronic bronchitis
