USMLE Step 1 Physiology Review 52 07 Tension

# USMLE Step 1 Physiology Review 52 07 Tension

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### Begin 52 07 Tension Transcription

These next few questions are on tension.  In a sphere formed by an elastic membrane (for example, a soap bubble), the tension in the wall increases with the volume, and the pressure required to maintain the distention is directly proportional to the tension and inversely proportional to the radii of the curvature of the sphere.  This is known as the Law of LaPlace.

What is the formula that describes the tension in the wall of the sphere?

• Tension equals pressure multiplied by the radius of the sphere, divided by two.

Although the Law of LaPlace is strictly correct only for very thin membranes like soap bubbles, the principle can describe the relationship between volume and wall tension in at least two organs or structures in the body.  What are these structures or organs?

• First is a structure in the lungs.  Alveoli.  The second is an organ in the circulatory system.  The heart.

The principle of the law also applies to cylinders like blood vessels, but since there is only one radius of curvature, the formula for tension in the cylinders wall is different; what is it?

• Tension equals pressure multiplied by the radius of the blood vessel.

The cohesive force among water molecules is an important factor in the expansion of what?

• In the expansion of the lungs.

Student Doctor, please pause the tape and summarize what we have learned about tension.

• In a sphere formed by an elastic membrane, for example a soap bubble, the tension in the wall increases with the volume, and the pressure required to maintain the distention is directly proportional to the tension and inversely proportional to the radii of the curvature of the sphere.  This is known as the Law of LaPlace.  The formula that describes the tension of the wall of the sphere is the following: Tension equals pressure, multiplied by the radius of the sphere, divided by two.  Although the law of LaPlace is strictly correct only for very thin membranes like soap bubbles, the principle can describe the relationship between volume and wall tension in the heart and the alveoli of the lungs.  The principle of the Law also applies to cylinders like blood vessels, but since there is only one radius of curvature, the formula for tension in the cylinders wall is different.  It is: tension equals pressure multiplied by the radius of the cylinder or blood vessel.  The cohesive force among water molecules is an important factor in the expansion of the lungs.

Okay, now for some questions about compliance and elasticity.

What is compliance?

• It is the tendency of a material to be extended or stretched by a force.

Is it the elasticity or the compliance of the cardiac ventricles that is very important to the filling of the heart?

• The compliance.

Elasticity is the reciprocal of compliance.  Elasticity is the tendency of a material to do what?

• To return to its unstretched length.

What is the formula for elasticity?

• Change in pressure over the change in volume, or delta P over delta V.

What condition affecting the arteries leads to a loss in both compliance and elasticity?

• Hardening of the arteries or arteriosclerosis.

In emphysema, expiration is difficult because the lungs have lost much of their what?

• Elasticity.

Student Doctor, please pause the tape and summarize what we have learned about compliance and elasticity.

• Compliance is the tendency of a material to be extended or stretched by a force.  The compliance of the cardiac ventricles is very important to the filling of the heart.  Elasticity is the reciprocal of compliance.  Elasticity is the tendency of a material to return to its unstretched length.  The formula for elasticity is the change in pressure over the change in volume, or delta P over delta V.  Hardening of the arteries or arteriosclerosis leads to a loss in both compliance and elasticity.  In emphysema, expiration is difficult because the lungs have lost much of their elasticity.

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