Ventilation: Static Forces

1. Ventilation: Static Forces Lung Compliance Elastic Recoil Surface Tension

2. Lung Compliance • Air flow in lungs: no muscles in the alveoli; air passively moves in/out of the lungs in response to pressure gradients. • Forces are present that resist the opening of the lungs, I.e., the alveoli • The natural tendency of the lungs to recoil or collapse • The surface tension in alveoli

3. Pneumo-nuggets • What keeps the alveoli (lungs) expanded are: • Negative intra-pleural pressure • The space between the two pleural layers is always negative or sub-atmospheric • This tends to suck the lungs outward

4. Pneumo-nuggets Alveolar pressure pressure within the alveoli themselves tend to keep the lungs inflated Reduced surface tension within the alveoli (more about this later)

5. Why does an inflated lung want to recoil inward? • Lung tissue has elastic properties • Lung parenchyma contains both elastin and collagen fibers, proteins • Smooth muscles are found down to level of alveolar ducts • Both favor lung collapse • The fluid layer lining the alveoli produces surface tension (tendency to shrink)

6. Lung Compliance • Technically: change in volume per unit of pressure change: • CL = ∆V (liters)/∆P (cmH2O) • Ease with which volume can be changed (expanded) • Distensiblity: how easily the lungs can be distended (stretched)

7. Changes in transpulmonary pressure opposing the elastic (static) forces of lung tissue. Lung CL

8. Compliance Curve

9. Pressure changes & volume

10. Compliance terminology • A highly compliant lung is one that is easier to expand • A low compliance lung is one that is harder to expand, more “stiff” • Pneumo-nugget: this concept will play a critical role in understanding lung diseases, e.g., COPD (high compliant) and pneumonia (low compliance)

11. Elastance? • Elastance is the natural ability of matter to return to its original shape after some external force which has acted on it is gone or removed. • Which is more elastic? A rubber band or a piece of string?

12. Elastance and Compliance? • Elastance is the opposite of compliance (reciprocal) = ∆P/∆V • The more elastic something is, the more it wants to return to its original shape • The more compliance something is, the less it wants to return to its original shape.

13. So, why does an inflated lung want to recoill inward? • Answer: through surface tension • Specifically: • Through the physical principles of LaPlace’s Law • Surface tension • Size of the alveoli (bubble)

14. What is Surface Tension?

15. Surface tension • A molecular, cohesive (binding) force found at liquid-gas interfaces. • Expressed in dynes/cm • A liquid film that coats the interior of alveoli, causing air-liquid interfaces to assume a spherical shape

16. Surface tension develops at every liquid-air interface it takes a certain inflation pressure to maintain or support a liquid bubble LaPlace’s Law & Surface Tension

17. P = 2ST/r P = pressure ST = surface tension r = radius Pressure is needed to keep an bubble (I.e., alveoli) inflated This pressure is directly affected by the surface tension of the bubble Inversely affected by the size of the bubble How does it affect lung expansion?

18. As the surface tension of a liquid bubble increases, distending pressure needed to hold the bubble open increases The higher the surface tension, the more pressure required to inflate the bubble The effects of increasing surface tension on pressure

19. As radius of a bubble increases, distending pressure needed to hold bubble open decreases As radius decreases, pressure to hold bubble open increases Surface tension and bubble size

20. Laplace’s Law & Alveoli • The higher the surface tension, the more pressure required to inflate alveolus • The lower the radius (size) of the alveolus, the more pressure required to inflate alveolus

21. Surface tension and the Alveolar Fluid Lining • Because of surface tension, the alveolus • Resists stretching • Recoils after stretching • Favors reduced surface area (to shrink into a sphere)

22. Pulmonary Surfactant • Small alveoli tend to be unstable and collapse • Surface tension of alveolar lining layer is greatly reduced by presence of pulmonary surfactant, offsets the natural tendency of the alveoli to collapse

23. How does it offset surface tension? • Pulmonary surfactant is a lipid produced by alveolar Type II cells. • One end if hydrophobic and the other is hydrophilic • Found within the alveolar fluid layer, surfactant interrupts this layer and alters alveolar surface tension

24. Surfactant to Surface Area Ratio • Surface tension is low in small alveoli (exhalation) because the ratio of surfactant to alveolar surface is high • During inhalation, as alveolar enlarge, surface tension increases because surfactant to alveolar surface decreases

25. Physiological importance of surfactant • Increases lung compliance because surface forces are reduced. • Promotes alveolar stability and prevents collapsed at low volumes--small alveoli are prevented from getting smaller. • Promotes “dry” alveoli--collapsed alveoli tend to draw fluid from pulmonary capillaries (edema-movement of fluid into alveoli)

26. So What? • Are these concepts important regarding lung disease? • Lung compliance changes dramatically in chronic obstructive lung diseases (COPD), loss of surfactant is critical in atelectasis, near-drowning, infant respiratory distress syndrome, etc.