Physiology Lecture 2: Lung Dynamics Flashcards
2 factors that govern lung dynamics
- Physical characteristics of the airways
- Lung parenchyma (accomodates changes in lung volume)
Describe Poiseuille flow
Perfectly laminar flow in which the gas in the center of the tube moves fastest (minimum energy)
When is laminar flow most likely to occur
When the flow rate is low and the tube diameter is small
In laminar flow, if the tube radius is halved, how much must the driving pressure be increased by in order to maintain the same flow?
16 times
Mean forward velocity of fluid or gas at any point in tubular flow
Equal at all points
How does the driving pressure vary in turbulent flow?
Varies with the square of the flow
When is turbulent flow most likely to occur
When the flow rate is high and the tube diameter is large
How to determine likelihood of laminar vs. tubular flow
Reynold’s number (using airway size “r” and speed of air “v”)
Higher reynold’s number = more turbulent flow
Reynold’s number equation
2rvd/n
Where:
- r = radius
- v = velocity
- d = gas density
- n = gas viscosity
Driving pressure across the airways in the lungs (equation)
ΔP = k1V + k2V2
Resistance equation
R = ΔP/V
2 types of pulmonary resistance
- Airway resistance
- Tissue resistance
Define airway resistance
Loss of energy as air flows through the airways
Define tissue resistance
Loss of energy as lung tissue changes length and volume (elastic energy)
Energy is dissipated as fibres and molecules move past each other
Equatioin for resistance in Poiseuille flow
L= tube length
n = viscosity
r = tube radius
2 effects of airway branching
- Velocity of air movement decreases as the diameter of individual airways decreases (thus favorign laminar flow)
- Total cross-sectional area of the airways increases = reduce resistance
Effect of tube arrangement
Tubes in parallel = lower airway resistance
Describe the respiratory cycle curves of volume, pleural pressure, flow, and alveolar pressure
3 factors upon which maximum expiratory flow depends
- Airways resistance
- Elastic recoil of the lungs
- Expiratory muscles
4 reasons for expiratory flow limitation
- Airways are compressible tubes subject to narrowing by increased pleural pressure = choke points
- Driving pressure diminished as lung volume decreases and elastic recoil falls
- P dissipates in the airways as E is lost, overcoming R
- Raw increases at lower lung volumes as parenchymal tethering effect on airways diminishes and airway caliber decreases
How can forced expiratory flow-volume curves detect diseases
Elastic recoil or airway resistance diseases can be detected since elastic recoil pressure and airway resistance between the alveolus and the “choke point” determine flow in the effort-independent portion of the curve
