USMLE Step 1 Respiratory Physiology Review 60 11 Compliance, Elastance, and Resistance

USMLE Step 1 Respiratory Physiology Review 60 11 Compliance, Elastance, and Resistance

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Begin 60 11 Compliance, Elastance, and Resistance Transcription

Now let’s talk about compliance, elastance, and resistance. Lung tissue is very distensible or stretchable.

What is the measure of distensibility of a structure or substance?

  • Compliance is a measure of distensibility of a structure or substance.

How is lung compliance expressed in terms of volume and pressure?

  • Lung compliance is the change in lung volume per unit change in pressure.

What is the mathematical formula for compliance?

  • Compliance= change in volume / change in pressure

Lung volume depends on body size. Therefore, absolute values of compliance cannot be used to compare lung compliances of different sized individuals.
What is the term for compliance that has been adjusted to facilitate comparison of different lung volumes. It starts with the letter S.

  •  Specific compliance

Express specific compliance in terms of volume and pressure.

  • Specific compliance = change in volume per unit change in pressure/ initial volume

Student doctor please pause the tape and summarize what we’ve discussed about compliance.

  • Lung tissue is very distensible or stretchable. Compliance is a measure of the distensibility of a structure or substance.  Lung compliance is the change in lung volume per unit change in pressure. Compliance equals the change in volume/ the change in pressure. Lung volume depends on body size. Therefore, absolute values of compliance cannot be used to compare lung compliance of different sized individuals. Compliance that has been adjusted for different lung volumes is called specific compliance. Specific compliance is change in volume per unit change in pressure / initial volume.

What is the reciprocal of compliance? In other words, what is the tendency of material to return to its original length or shape after being distended. It starts with an E.

  •       Elasticity.

Express elasticity in terms of volume and pressure.

  • Elasticity is the change in pressure per unit change in volume or the change in pressure/ the change in volume.

The lungs are very elastic. Both tissue forces and surface forces contribute to the total recoil forces of the lungs. Two connective tissue fibers found in lung tissue help generate the elastic or retractive forces of the lungs. One of them is thought to prevent lung tissues from being over distended at high lung volumes. This tissue fiber begins with the letter C. What is it?

  • Collagen.

What other fibers in lung tissue help generate recoil forces? These fibers begin with the letter E.

  •   Elastin fibers found in lung tissue help generate the recoil forces of the lungs.

At a gas liquid interphase, what force holds the molecules of a liquid together at the surface?

  •  Surface tension.

Surface tension contributes to the total elastance of the lungs. Where in the lungs does surface tension occur?

  • Surface tension occurs at the interpahse between alveolar gas and the watery fluid which lines the alveoli.

Whose law predicts that because of surface tension, alveoli with smaller radii should collapse into alveoli with larger radii?

  •  The law of Laplace.

This does not occur, however, because of fluid secreted inside the alveoli decreases surface tension.
What is the secretion called? It starts with an S.

  •  Surfactant, or pulmonary surfactant, decreases surface tension in the alveoli.

Student doctor please pause the tape and summarize what we’ve discussed about elasticity, tissue forces, surface tension and surfactant.

  • The reciprocal of compliance is elasticity. It is the tendency of a material to return to its original length or shape after being distended. Elasticity can be expressed as the change in pressure per unit change in volume or dP/dV. The lungs are very elastic. Both tissue forces and surface forces contribute to the total recoil forces of the lungs. Collagen and elastin are two connective tissue fibers found in lung tissue that help generate the elastic or retractive forces of the lungs. Collagen is thought to prevent lung tissue from being over distended at high lung volumes. At a gas liquid interphase, surface tension is the force that holds the molecules of a liquid together at the surface. In the lungs, surface tension occurs at the interface between alveolar gas and the watery fluid which lines the alveoli. Surface tension contributes to the total elastance of the lungs. The law of Laplace predicts that because of surface tension, alveoli with smaller radii should collapse into alveoli with larger radii. This does not occur, however, because a fluid called pulmonary surfactant is secreted inside the alveoli. The presence of surfactant decreases surface tension inside the alveoli and they don’t collapse.

When lung volume changes, resistance forces are created. What are the two resistance forces created by changing lung volume? The first one is caused by friction between gas molecules and air way walls. It begins with an A.

  •    Air way resistance.

The second kind of resistance begins with the letter T. What is it?

  •   Tissue resistance.

What percent of total resistance is due to airway resistance?

  •  80 to 90% of total resistance is due to airway resistance.

What is the general term for lung diseases which decrease the rate of air flow by increasing resistance?

  • Obstructive diseases increase air flow resistance thereby decreasing rate of air flow.

Name the three main obstructive lung diseases. I’ll cue you with the first letter of each.

First,  E

  • Emphysema. 

Second, CB

  • Chronic bronchitis.

And third, A

  • Asthma.

Student doctor please pause the tape and summarize what has been discussed about resistance.

  • When lung volume changes, resistance forces are created. One of the resistance forces created by changing lung volumes is air way resistance which is caused by friction between gas molecules and air way walls. Tissue resistance is also created by changing lung volumes. 80 to 90% of total resistance is due to air way resistance. Lung diseases which decrease the rate of air flow by increasing resistance are called obstructive diseases. The three main obstructive lung diseases are emphysema, chronic bronchitis, and asthma.

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