# Law of Laplace

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### What is tension?

Tension may be defined as the internal force generated by a structure

### What does Law of Laplace state for cylindrical shaped structures? ?

Wall tension is equal to pressure of a liquid in a cylinder times the radius; T= Pr
T= wall tension
P =Pressure of a liquid in a cylinder

### What happens to wall tension as the radius of a structure increases?

As a structure expands it's radius, the wall tension increases.

### How can wall tension and radius be applied to the blood vessels??

-Aortic aneurysm is more likely to rupture than a normal segment of the aorta because the radius is greater - increasing wall tension to breaking point.
- Capillary will withstand a pressure of 100mmHg better than a vein because capillary has a tiny radius compared to a vein.

### What does Law of Laplace state for spherical structures? ?

T=Pr/2 or 2T=Pr (2T=Pr/H)

T=wall tension, P=pressure of liquid in cylinder, r=radius, H=wall thickness

### In law of Laplace for spheres, tension is a factor of what?

Tension is a factor of two due to spherical area

### What happens to the tension in the wall structure a sphere as the radius expands?

As a sphere expands its radius, the tension in the wall structure increases

### What is a clinical application of Law of Laplace for spheres?

-For the left ventricle, the greater the
filling pressure, the greater the tension in the ventricular wall.
-This works well while you are on Starlings curve. After a critical stretch this tension is not compensated by greater contraction and risks ventricular failure and/ or rupture in weakened hearts.

### Spheres: As the pressure increases, what happens to the tension?

The tension increases.
P=2T/r

### If the law of laplace states 2T= Pr or P= 2T/r, then what are two things that is needed to expand an alveolus?

Pressure needed to expand an alveolus is :
1) directly proportional to thesurface tension
2) inversely surface tension a

### Why are alveoli unique spheres?

They are unique because the have a fluid (H20) covered thin membrane

### Does water have surface tension?

Yes, water has surface tension

True

### Alveoli act like bubbles and bubbles tend to pull toward a smaller radius and collapse. What drives this collapse?

The collapse is driven by surface tension of water

...

### What is surface tension?

Surface tension is the cumulative effects of cohesive intermolecular forces on the surface of a fluid at a liquid/gas or liquid/liquid interface

Wall tension

...

### Does Law of Laplace explain what would happen in the alveoli if there was not surfactant? How so?

Yes, law of laplace explains what would happen in the alveoli if not for surfactant.
-Smaller alveoli would empty into Smaller alveoli would empty into larger alveoli
-Small alveolar spheres would be more difficult to expand than large more difficult to expand than large alveoli
↑2T =↑P↓r

### What is surfactant composed of?

composed of dipalmityl phosphatidylcholine and phospholipids (lecithin and spingomyelin)

### What secretes surfactant?

Secreted by the alveolar epithelium's greater alveolar (Type 2) cells.

### What syndrome in newborn causes a lack of surfactant? What is it characterized by? What happens to the bronchiloles and alveolars?

-RDS Respiratory Distress Syndrome (formerly Hyalin membrane disease) in premature newborns
-Characterized by glassy appearing terminal bronchioles and alveoli due to lack of surfactant.
-Terminal bronchiole constriction and alveolar collapse ensues.

### How do you estimate PaO2 from FiO2?

FiO2<50% multiply FiO2 by 5
FiO2 >50% multiply FiO2 by 6
Calculated product is PaO2

10

### What is the alveolar gas equation??

Relates PaO2 to FiO2:
PAO2=FiO2x(P atm -P H20 )-PaCO2/RQ
PAO2=Alveolar oxygen tension
FFiO2= Inspired oxygen concentration
P atm=760mm Hg
P H2O=47 mm Hg, saturated H2O vapor press. at body temp.
PPCO2= 35-45 mm Hg
RQ= Respiratory quotient,nml 0.8

### What should the alveolar normal ventilatory gas flow to perfusion ration be? What happens with inadequate gas flows?

-0.8
-V/Q mismatch occurs with inadequate gas flows

### What does atelectasis occur in pts with ARDS?

-T=Pr
-Deficient surfactant in the alveoli
- If r decreases, T decreases with P requirements
-Alveoli with small radi have higher pressure inflation requirements and will empty into alveoli with lower pressures
→Atelectasis

### In a normal lung, why doesn't a small alveoli empty into a large alveoli?

T= Pr
- pressure in the alveoli doesn't change
- Surfactant is the greater equalizer

True

### What are the small alveoli are easier to expand?

Small alveoli have more concentrated surfactant verses expanded (larger) alveoli

### What happens to the surface concentration if a decrease in radius leads to a decrease in surface tension?

The surfactant concentration increases

WHAT?!

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