Info

FIGURE 5 (A) A Wilhelmy balance can be used to measure surface tension on a film of water as the surface area changes. Decreasing area, by moving the barrier to the left, increases surface tension on the film. This pulls down the platinum strip, so surface tension can be measured as the force on a string supporting the strip. (B) The surface tension of surfactant depends on surface area and shows hysteresis, unlike water or detergent. The insets show how molecular organization of surfactant changes with different surface areas. (After Clements and Tierney, Chap. 69 in Fenn and Rahn, eds., Handbook of Physiology, Section 3, Respiration. Bethesda, MD: American Physiological Society, 1964.)

FIGURE 5 (A) A Wilhelmy balance can be used to measure surface tension on a film of water as the surface area changes. Decreasing area, by moving the barrier to the left, increases surface tension on the film. This pulls down the platinum strip, so surface tension can be measured as the force on a string supporting the strip. (B) The surface tension of surfactant depends on surface area and shows hysteresis, unlike water or detergent. The insets show how molecular organization of surfactant changes with different surface areas. (After Clements and Tierney, Chap. 69 in Fenn and Rahn, eds., Handbook of Physiology, Section 3, Respiration. Bethesda, MD: American Physiological Society, 1964.)

monomolecular firm on fluid surfaces. This behavior is called adsorption. The hydrophobic fatty acid ends of DPPC molecules face the surface and the polar phos-pholipid heads contact the fluid (see Fig. 5). When the surface film is compressed, this regular alignment of surfactant molecules squeezes out water molecules from the surface and effectively lowers surface tension.

The apolipoproteins in surfactant include large collagen-like glycoproteins. Their function is not completely understood, but they appear to be involved in tubular myelin formation and surfactant removal and recycling. Surfactant-associated proteins with antioxi-dant and anti-inflammatory properties have also been isolated (e.g., SP-A and SP-D). Surfactant is stored in 100-A-thick stacks inside lamellar bodies, and it can be transformed to tubular myelin during secretion from type II cells onto the alveolar surface. Tubular myelin is probably the direct precursor for the surface active film lining the alveoli. Surfactant may be recycled into type II cells or removed from the alveoli by macrophages.

The hysteresis for surfactant explains the hysteresis in lung compliance curves. The plot of surface tension versus area in Fig. 5 is analogous to the plot of the pressure-volume curve in Fig. 3 except the axes are reversed; surface area is proportional to volume, and surface tension is proportional to pressure. As alveolar surface area decreases during expiration, alveolar surface tension is lower than at an equivalent area during inspiration. Hence, less pressure is necessary to maintain a given lung volume during expiration than during inspiration and this helps prevent the lung from collapsing to very low volumes during normal breathing. The work of breathing is minimized because inspiratory muscles do not have to generate large critical opening pressures on every breath, which would be necessary to expand the lung from minimal volume.

Surfactant is also important for fluid balance in the lung. Surface forces tending to collapse the alveoli will lower interstitial pressure around capillaries in the alveolar walls, and tend to draw fluid out of capillaries (see Starling's law, Chapter 18). Therefore, the effect of surfactant to reduce alveolar surface tension helps to prevent edema.

Back Pain Revealed

Back Pain Revealed

Tired Having Back Pains All The Time, But You Choose To Ignore It? Every year millions of people see their lives and favorite activities limited by back pain. They forego activities they once loved because of it and in some cases may not even be able to perform their job as well as they once could due to back pain.

Get My Free Ebook


Post a comment