Mechanics of Respiratory System Flashcards
Turbulent vs. Laminar airflow
Turbulent flow in larger airways (turbulent flow inversely proportional to square root of density)
Laminar flow in smaller airways (laminar flow inversely proportional to viscosity)
So lower density and lower viscosity means greater flow
Resistance vs. conductance
Resistance: pressure encountered when you try to drive flow (R = P/V)
Conductance: flow you get when you use a given pressure to drive air down the tube (G = P/V)
Resistance is opposite of conductance!
What determines expiratory flow (V dot)?
Pleural pressure (high Ppl means high expiratory flow) Note: use accessory muscles to increase Ppl during exhalation
Lung recoil pressure (high Pst(L) means high expiratory flow)
Atmospheric/mouth pressure (low Patm means high expiratory flow)
Resistance of airway (low Raw means high expiratory flow)
V dot = (Ppl + Pst(L) - Patm)/Raw
= (Palv - Patm)/Raw
= P/R (Ohm’s Law)
Equal pressure point
EPP: point in airway at which intralumenal (airway) pressure equals pleural pressure (falls to this point from alveoli out airway to mouth)
During forced expiration!
Intralumenal (airway) pressure always has to fall by Pst(L) (lung elastic recoil pressure) to get to equal pressure point, so lung elastic recoil is primarily what determines expiratory flow
When pressure of airway (intraluminal space) becomes less than pleural pressure (passed EPP), the airway will collapse
Equation for expiratory flow (V dot)
Flow = Pressure/Resistance
V dot = P/R
Ohm’s Law
What can cause expiratory air flow to be low?
1) Ppl is low: poor expiratory effort; muscle weakness
2) Pst(L) is low: low lung volume; emphysema (lowered lung recoil)
3) Raw is high: airway narrowing from asthma or bronchitis
Starling resistor
When airway collapses, resistance increases and intraluminal pressure increases and airway opens again
(airway fluctuates open and closed)
Why do airways collapse in people with emphysema?
1) Less lung recoil –> EPP moves down airways toward alveoli into unsupported airways
2) Intra-mural (airway) pressure falls if patient also has increased airway resistance from chronic bronchitis
3) Connective tissue support of airways itself is deficient so airways collapse easier
Note: increased expiratory effort does NOT increase flow in people with emphysema (or others?)
What is the time constant (tau)?
Tau = Resistance x Compliance
Tau = 0.4 sec
Passive emptying described by time constant where lung colume falls by 63% each time constant (0.4 sec)
Passive recoil returns lung to original volume in 4 time constants (1.6 sec)
Full breath cycle takes 1.6 (expiratory) + 0.8 (expiratory = 2.4 sec –> 25 breaths per minute
What happens to tau in emphysema, and what does this cause?
R (airway resistance) and C (compliance) both increase in emphysema, so tau increases too. This means it takes longer to passively breathe/exhale, but can’t get enough oxygen that way! Use accessory muscles to breathe –> barrel chest
What happens to lung emptying when time constants are asymmetrical?
Gas flow becomes asymmetric
Unit with larger time constant (empties slower) will only empty a fraction of its air at a breathing rate that the unit with the smaller time constant dictates
This is how most lung diseases present, not uniformly
“Heterogeneity of disease”
Total lung capacity (TLC)
Total amount of air lungs can hold
6 L
Functional residual capacity (FRC)
What is left in your lungs after exhaling normal tidal volume
“Equilibrium volume of the lungs”
ERV + RV
2.4 L
Vital capacity (VC)
What you can expire after maximal inspiration
IC + ERV
4.7 L
Forced vital capacity (FVC)
Total volume of air that can be forcibly expired afer maximal inspiration
4.7 L (same as VC!)
