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Flashcards in L5 Mechanics of Ventilation Deck (16):

Mechanics of Inspiration

Alveolar pressure less than barometric pressure
-increase volume of thorax via exercising voluntary muscles
-decent of diaphragm predominates at rest
-elevation of thorax during exercise
V. propn (Pb-Pa)
-flow is proportional to the difference in pressure
--Pa decrease by increasing vol of lungs
--Pb increase at mouth
V.1 ---> PA



the UNIQUE lung volume
Ballon diagram: Residual volume --> FRC --> Tidal Volume --> Total Lung Capacity
Sit at FRC at rest unconciously prior to tidal volume breath
-Residual Volume= 1L
-air comes through dead space each time


Balance of Forces at FRC

Intrapleural space is inside the thorax = potential space with Intrapleural pressure
-intrapleural pressure determined by extent of separation b/w vp and pp
(Pip lung --> pp --> cw
FChestWall= also elastic body recoiling afterwards
FRC= occurs when inward forces of collapsing lung are equal an opposite to outward forces of collapsing chest wall.



-intrapleural pressure determined by extent of separation b/w vp and pp
-intrapleural pressure is commonly -ve negative (below barometric pressure)
-subatmospheric (standard)



Imbalance of Forces
Air in the thorax
enter through a wound
Broken adherence b/w the 2x pleural surfaces, allowing lung to collapse and chest wall will recoil outwards


Pleural and lung models illustrate:

1. Persistence of FRC (return back to FRC after every breath, regardless of its magnitude) &
2. the Response to a Pneumothorax
-demonstrates that both the Lungs and the Thorax are ELASTIC BODIES


Elastance of the Lung

Hooke's Law
1D elastic body (rubber band): F propn changex
-the force used by the rubberband was proportional to its change in length
3D: changeV volume propn to changeP pressure
deltaV = C x deltaP
Elastance = C-1
-elastance is the inverse of compliance
-elastance is the 3D equivilant of "k" hooke's constant in F=kx (hooke's Law)
(1D and 3D equivilants of one another)
Lung= extremely compliant and highly elastic


Components of Lung compliance

1. Tissue compliance (conferred primarily by elastic fibres)
-inherent stiffness of elastic fibres embedded in lung tissue (cause lung to collapse)
2. Air water Surface Tension
3. Atelectasis. (collapse of individual alveoli where surface touch eachother due to air-water surface tension)


Compliance of the Lung equation

Change V / Change P
(L) / (cmH2O)
gravity of Hg mercury is 13.6
mercury is 13.6x more dense than water
compliance of lung is so low that if measured in mmHg wouldnt be able to see on a dile
mmHg : 13.6cm H2O


Compliance of the Lung

Isolated lung in airtight glass chamber(thorax)
Pump extracts air from intrapleural space (reduces pressure Pip)
Decrease Pip = Lung expands
Spirometer shows increase in Volume
-lung is responding to difference in intrapleural pressure (reduced pressure around lung cmH2O)
Open to atmosphere lung will collapse to relative volume
Release pressure = Lung volume decrease via different Loop
Hysteresis Loop


Contribution of Air-Water Surface Tension to Lung compliance

Excised lung
Relative lung volume
Pressure= mmHg and positive pressure
Positive pressure= Increasing pressure by Filling lung. not reducing pressure around lung.
Saline filled lung= progressively filled. progressively released same path.
*Saline-filled lungs are more compliant than air-filled lungs due to absence of air-water surface tension.
(only 5mmHg to fill completely)
Air filed lung= Air water surface tension not removed. Hysteresis Loop
(require atleast 3x amount of pressure to fill to same amount)



Lowest point on Excised and Air filled Lung graph
-alveoli have shrunk down w/o air (empty balloons) with all sides touching
-large amount of force/pressure required to change volume of atelectic alveoli
-pressure must be as high or larger than achieved in the saline-filled lung to have any increase in volume


Method of measuring surface tension

Tray filled with fluid contaianing pulmonary surfactant (surface active agent)
-could use soap
Change area exposed to force transducer by moving barrier
Narrow plywood= water will adhere/stick to it
=pull on plywood, and measure how much force is required to lift its out of the film of fluid (break the air-water surface tension)
Amount of force will depend on length of the slab (longer = more surface tension)
Surface tension measured as function of area of surface exposed (dynes/cm) (tiny. sufficient to allow mosquitos to land on water and not go through)
=proof that lungs contain a substance that can be washed out or sprinkled ontop of layer of fluid and produce hysteresis loop
Expansion= larger area. Compression= smaller area
Tension= force/length (not area)


Laplace's Law

form a bubble and can maintain shape
Pressure inside soap bubble propn (Tension in wall(force per unit length) / r
pressure inversely dependant on radius (small bubbles, must have larger pressure to maintain the volume)
Connect bubbles= small bubble will empty into large bubble (large bubble will expand)
Small bubble shrinks until radius of curvature is identical as pressures will be equal. No more flow)


Why dont small alveoli empty into large ones, ending with a single gigantic alveolus?

a paradox (according to La Places Law)
septa= Due to thinned flattened cells being tethered together by tin fibres of collagen
"Tethering" among alveoli within acinus
-prevents from collapsing
(if do collapse and become apaleptic, then huge pressure required to reinflate/change volume)


Contributors to Pulmonary compliance

Pulmonary = entire system
Human able to fill lung to any desired volume, turn tap off so no air can enter and relax muscles of chest wall