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Flashcards in Pulmonary Mechanics Deck (36)
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methods of measuring intrapleural pressure

placing a small catheter connected to a pressure measuring device in the intrapleural region

placing a balloon in the thoracic esophagus and measuring the intraesophageal pressure


typical intrapleural pressure at the (passive) functional residual capacity

-3 to -6 cm H2O

this pressure expands the lung and collapses the rib cage by a corresponding amount


transpulmonary pressure

the pressure acting to inflate the lungs

usually negative with respect to the alveolar pressure and thus acts to expand the lungs

PA - Pip


transthoracic pressure

the force acting on the thoracic wall

Pip - Pb

since pleural presssure is generally negative, it "sucks" the chest wall inward


transrespiratory pressure

the potential pressure graident for flow into or out of the alveoli

the difference between alveolar and atmospheric pressure measured with the glottis closed and with respiratory muscles relaxed


if the transrespiratory pressure is negative, gas will flow into the alveoli

if transrespiratory pressure is positive, gas will flow out of the alveoli



the change in volume with respect to change in pressure



the inverse of compliance in the lungs


causes of decreased lung compliance

respiratory distress syndrom ( decrease)

edema (decrease)

atelectasis (decrease)

fibrosis (decrease)


causes of increased compliance

age (increase)

emphysema (increase)

increasing body size (increases)


Law of Laplace

P = 2T/R

P is the pressure within a spherical object

T is the tension int he wall of the object

R is its radius of curvature

a  smaller bubble would have a greater internal pressure than the larger bubble

constant varies with the geometry of the object, for a sphere it would be 4 instead of 2


implications of Law of Laplace

the smaller the radius of curvature, the stronger the inward force resulting from surface tension

bubbles (alveoli) with smaller radii have larger internal pressures

small alveoli will have a tendency to collapse (instability of alveoli)

surface forces tend to pull intersitial fluid into the alveolus


regulation of lung fluid balance

because of the additional surface tension, there is more hydrostatic pressures in the capillaries in the alveoli, so this favors fluid movement into the interstitium 

surfactant reduces alveolar surface tension, and this helps prevent edema



the different relationship between pressure and volume during inflation compared to that during deflation


What is the main effect of introducing saline into the lung?

it eliminates the very large (>70m2) fluid-air interface in the alveoli

this abolishes any effects due to surface tension of the fludis lining the alveoli

this makes the lung much more compliant, almost completely abolishing hysteresis


roles of surfactant in the lungs

reduces alveolar surface tension and varies it with breathing

preserves alveolar integrity

prevents continuous transudate (edema) from pulmonary capillaries to alveoli

reduces work of breathing


composition of surfactant

lipoprotein complex that is approximately 30% protein and 70% phospholipid (mostly dipalmitoyl phosphatidyl choline)

synthesize dnad released from lamellar bodies in type II alveolar epithelial cells


dipalmitoyl lecithin or dipalmitoyl phosphatidyl choline

principle active ingredient of surfactant

hydrophilic head, which aligns with the water surface, and a hydrophobic tail that is more compatible with air

released into the alveoli as a result of lung distention or stimulation of beta adrenergic receptors

quickly spreads in a monolayer along the interior lining of the alveoli at the surface of a thin aqueous lining of the alveolus known as the hypophase


How does surfactant contribute to hysteresis?

during inflation, the surfactant is broken up and micelles beneath the surface help fill in gaps

this reduces the effect of the surfactant and thus increases surface tension

upon deflation, the expanded layer of surfactant is compressed to a surface solid state, which provides the maximal amoutn of action, reducing the surface tension to almost zero


the role of surfactant in the first few breaths

the first breath is very difficult to take as a lot of pressure must build up before the lungs open

once the first few breaths are taken, surfactant is spread across the surfaces, which then makes inspiration much easier

babies with RDS never reach that stage of easy breathing as the lung collapses every time they expire


What is the main difference between the complaince of the lung and chest wall?

at normal lung volumes (up to about 60-80% of vital capacity) the chest wall is attempting to expand

reducing the pressure gradient across the chest wall increases thoracic volume


describe the relationship between the compliance of the lung and the chest wall

the two systems are in series

1/Cresp = 1/Clung + 1/Cchest

at FRC, the total compliance is less than the individual compliances


What happens to the FRC when the chest wall is less compliant?

the FRC is larger with a normal chest wall

Pip is more negative


What happens ot the FRC when the lungs are less compliant?

FRC decreases - stiff lungs pull chest to a smaller volume

Pip is more negative


describe the relaxation pressure-volume curve of the lung and chest wall

the compliance curves for the lungs and chest wall are considered together when determining the total compliance of the system


Where is ventilation the lowest in the lung and why?

lowest in the apices where the negative intrapleural pressure is the greatest

lung is in the upper portions of the compliance curve, making lung complicance low

therefore, for a given change in intrapleural pressure produced by an inspiratory effory, the volume increase will be small


Describe the ventilation of the lung at FRC and at Residual Volume.

at FRC, the base of the lung is open and ventilation is higher there

at RV, the base of the lung is compressed and closed, so ventilation becomes greater at the apex


ventilation at the apex

intrapleural pressure more negative

greater transmural pressure gradient

alveoli are larger and less compliant

less ventialtion


perfusion at the apex

lower intravascular pressures

less recruitment


higher resistance

less blood flow


ventilation at the base

intrapleural pressure less negative

smaller transmural pressure gradient

alveoli smaller, more compliant

more ventilation


perfusion at the base

greater vascular pressures

more recruitment


lower resistance

greater blood flow