LaPlace's Law Flashcards
Get crunk up in LaPlace!
What is the Law of LaPlace?
Describes the relationship between wall tension, pressure, and radius in cylers and spheres
Cylinders: T=Pr
Spheres: 2T=Pr
How does the law apply to cylinders (vessels)?
Wall tension is proportional to radius, thus as one increases, the other shall follow.
How does the law apply to spheres (alveoli)?
Pressure and wall stress (tension) are related to the radius over wall thickness
Why do alveoli tend to empty into larger alveoli in the absence of surfactant?
P=2T/r (Pressure, surface tension, and radius)
With a constant surface tension, smaller alveoli generate a higher pressure than larger alveoli, and air moves along the pressure gradient. Surfactant equalizes this by decreasing surface tension as alveoli decrease in radius.
How does surfactant prevent small alveoli collapse?
Again, by decreasing surface tension as radius decreases. Surface tension is the force that pulls alveoli in on themselves to collapse. Ergo, if this force is decreased, the alveolus will be less likely to collapse.
Which has greater surface tension in a normal lung, small or large alveoli?
Small alveoli
IN ARDS OR HYALINE MEMBRANE DISEASE: which alveolus has the greater surface tension, large or small?
The large alveoli (as there is no surfactant production in these states)
What is surfactant?
Lipoproteinous substance produced by type II alveolar cells; composed of dipalmityl phosphatidylcholoine and lecithin and spingomeylin
How do it do work?
Coats the air-liquid interface in alveoli, interrupts intramolecular forces between the liquid molecules that promote collapse (Van der Waal’s forces), eases off the surface tension.
What is surface tension?
Cohesive forces among liquid molecules create surface tension. These forces attempt to pull the liquid into the shape with the lowest surface-to-volume ratio: a sphere! This is typically accomplished with hydrogen bonding (Van der Waal’s)
Which is more likely to burst: 3 cm aortic aneurysm or 8 cm aortic aneurysm?
8 cm
because at the larger radius, the wall tension is greater!
In a normal lung on spontaneous inspiration, which is easier to fill: a small alveolus or a large alveolus?
THEY SHOULD BE EQUAL!! Because surfactant is the great equalizer!!!
What is the PaO2 assuming normal V/Q mismatch when 75% O2 is given?
PaO2 = FiO2 x (Patm-Ph2o)-PaCO2/RQ PaO2 = 484.75
What is normal V/Q match?
0.8 (0.2 mismatch)
Most important thing about surface tension and surfactant?
“Decreasing radius decreases surface tension by increasing surfactant concentration.”
