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3 - Resistances to airflow


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Give the basic equation for resistance.
R = change in P/flow
What two factors complicate the calculation of resistance in the lungs?
1. Bifurcation of airways
2. Flow varies between turbulent and tolerant.
What is the calculation for pressure in laminar flow? Turbulent flow?
1. Laminar: P = VR
2. Turbulent: P = VR^2
Why is a greater pressure required to drive turbulent air at a given velocity? Where in the respiratory tree is the greatest pressure required?
*eddies and vortices dissipate driving force
*the trachea
What type of flow exists in the bronchioles - laminar or turbulent?
Why does resistance decrease with successive airway generations?
The aggregate cross-sectional area increases.
What happens to flow with successive airway generations? Of what benefit is this? What is a drawback of this?
*Flow decreases
*low flow facilitates diffusion in the alveoli
*low flow may complicate diagnosis of obstructive disease in the small airways
Describe the relationship between resistance and lung volume. What is resistance like during expiration? During inspiration?
*resistance is inversely related to lung volume
*during inspiration, when volume is at its highest resistance is lowest
*during expiration when volume is at its lowest, resistance is at its highest
Why does resistance change with lung volume?
As lung volume increases, airway diameter increases thus reducing resistance.
What are two strategies used by asthmatics to increase flow?
1. Breathing through pursed lips to create back pressure that holds airways open
2. Breathing off the top of the lungs to maintain high lung volume
What powers expiration during the effort-independent phase?
Elastic recoil
At end inspiration, what keeps the alveoli from collapsing?
Transmural pressure created by alveolar pressure of 0 and IPP of -8.
What marks the transition between effort-dependent and effort-independent expiration?
The collapse of small airways.
What increases IPP during forced expiration? What effect does this have on pressure within the lungs?
Muscular compression - abdominals and internal intercostals. This creates a pressure gradient between the alveoli and atmosphere allowing air to flow out.
What is the Z point? Where is it located during effort-dependent expiration?
*the point where transmural pressure is zero
*in the cartilagenous airways
What happens when the Z point moves distally into the smaller (non-cartilagenous) airways?
Negative transmural pressure results in the collapse of these airways.
What keeps some small airways and alveoli open during expiration?
Structural interdependence.
Describe the appearance of the flow-volume curve for patients with obstructive lung disease?
*lower peak flow
*higher peak volume
*scooped appearance
What offsets the large resistance created by turbulent air flow in the large airways?
Large pressure drop.
What is one factor that makes resistance in small airways dynamic?
Dynamic compression.
What is FEV-1? What is normal for this value?
The volume of air expired over the first second of a forced expiration. Normal is 4L or 80% of FVC.
What is FEF(25-75%)? What is normal for this value?
*the flow between 75 and 25% of the FVC.
*normal is 3.5L/sec
Describe the changes in FVC, FEV-1, and FEV-1/FVC in patients with obstructive disease.
*all values decrease as less air is able to be expired
Describe the appearance of the flow-volume curve for patients with restrictive lung disease.
*decreased peak flow
*decreased peak volume

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