Respiratory Physiology Lecture 3 Part 16: Lung Compliance and Surface Tension

Question 1

How much does surface tension account for elastic recoil of the lungs?

Answer 1

The surface tension at the air-water interface accounts for about 2/3 of the elastic recoil of the lungs

Question 2

Surface tension _____ lung compliance

Answer 2

decreases

Question 3

What is the alveolar surface tension?

Answer 3

Water molecules at the surface of a liquid-gas interface are attracted strongly to the water molecules within the liquid mass

Question 4

Surface tension

Answer 4

a measure of the attracting forces acting to pull a liquid’s surface molecules together at an air-liquid interface

Question 5

Difference between molecules deep in fluid vs. molecules at surface

Answer 5

• Molecule deep in fluid: Interaction with other molecules in all directions creating strong bonds
• Molecules at surface: Do not have similar bonds from all sides since they are in contact with gas molecules from air as well. They will have little interaction with the gas molecules of the air but will creat stronger bonds with the water molecules close to the surface. Creates strong attractive force at interface between the molecules of water.

Question 6

What does surface tension make possible?

Answer 6

Question 7

How does surface tension arise?

Answer 7

Surface tension is seen at all air- fluid boundaries and arises as a result of hydrogen bonding of water molecules

Question 8

What is the role of surface tension in the lungs?

Answer 8

• The importance of surface tension is clear in P-V curve in which surface tension is eliminated with saline-filled lung
• The effect of surface tension is to “cause” the surface to maintain as small an area as possible
• No hysteresis and much lower inflation pressures

Question 9

How does alveolar surface tension work?

Answer 9

1. Air entering the lungs is humidified and saturated with water vapour at body temperature
2. Water molecules cover the alveolar surface
3. Surface water molecules create substantial surface tension; want to interact as much as possible with each other. The water molecules have strong bonds between each other creating strong surface tension.
4. Creates inward recoil which leads to alveolar collapse
5. Decreases the volume of compressible gas inside the alveoli and increases its pressure
6. At equilibrium, the tendency of increased pressure to expand the alveolus balances the tendency of surface tension to collapse

Question 10

What describes the equilbrium for alveolar surface tension?

Answer 10

Laplace’s equation describes this equilibrium: P = 2T/r

(P is the dependent variable; the surface tension T is a constant for a particular interface, and the radius r is the independent variable)

Question 11

What does Laplace’s equation describe?

Answer 11

Alveolar surface tension equilbrium: P = 2T/r

(P is the dependent variable; the surface tension T is a constant for a particular interface, and the radius r is the independent variable)

Question 12

The smaller the bubble’s radius, the ____________ needed to keep the bubble inflated

Answer 12

The smaller the bubble’s radius, the greater the pressure needed to keep the bubble inflated

Question 13

How does surface tension change between alveoli?

Answer 13

• Different alveolar sizes in normal healthy lungs
• Alveoli are not isolated entities (Alveolar sacs)
• In bubbles of different sizes (r), T remains constant Thus, the pressure is greater in the smaller bubbles
  
  • Small bubbles collapse into large ones

Question 14

Why do small alveoli sacs not collapse into bigger ones?

Answer 14

role of surfactant