Porth's Essentials of Pathophysiology, 4e

523

Control of Respiratory Function

C h a p t e r 2 1

the collagen fibers resist stretching and make lung infla- tion more difficult. In lung diseases such as interstitial lung disease and pulmonary fibrosis, the lungs become stiff and noncompliant as the elastin fibers are replaced with the collagen fibers of scar tissue. Pulmonary con- gestion and edema produce a reversible decrease in pul- monary compliance by increasing the water content of the lung. Elastic recoil describes the ability of the elastic com- ponents of the stretched or inflated lung to return to their original position after having been stretched. Overstretching lung tissues, as occurs with emphysema, causes the elastic components of the lung to lose their recoil, making the lung more compliant and easier to inflate but more difficult to deflate because of its inabil- ity to recoil. Surface Tension. An important factor in lung compli- ance is the surface tension in the alveoli. The alveoli are lined with a thin film of liquid, and it is at the interface between this liquid film and the alveolar air that surface tension develops. This can be explained by the fact that the forces that hold the liquid molecules together are stronger than those that hold the air molecules together. In the alveoli, excess surface tension causes the liquid film to contract, making lung inflation more difficult. The relationship between the pressure within a sphere such as an alveolus and the tension in the wall can be described using the law of Laplace (pressure = 2 × sur- face tension/radius). If the surface tension were equal throughout the lungs, the alveoli with the smallest radii would have the greatest pressure, and this would cause them to empty into the larger alveoli (Fig. 21-12A). The reason this does not occur is because of the surface ten- sion-lowering molecules, called surfactant , that line the inner surface of the alveoli. Pulmonary surfactants, particularly surfactant B, exert several important effects on lung inflation. They decrease alveolar surface tension, thereby increasing lung compli- ance and ease of inflation. Without this, lung inflation would be extremely difficult. In addition, surfactant helps to keep the alveoli dry and prevents the development of pulmonary edema. This is because water is pulled out of the pulmonary capillaries into the alveoli when increased surface tension causes the alveoli to recoil. Surfactants also stabilize alveolar inflation by chang- ing their density in relation to alveolar size, with the sur- factant molecules becoming more tightly compressed in the small alveoli with their higher surface tension and less compressed in the larger alveoli with their lower surface tension (Fig. 21-12B). At low lung volumes, the molecules become tightly packed, and at higher lung vol- umes they spread out to cover the alveolar surface. In surgical patients and bed-ridden persons, shallow and quiet breathing often impairs the spreading of surfactant. Encouraging these persons to cough and deep breathe enhances the spreading of surfactant, allowing for a more even distribution of ventilation and prevention of atelec- tasis (incomplete expansion of a portion of the lung). The type II alveolar cells that produce surfactant do not begin to mature until the 26th to 27th week

P = 2 T/r

Alveolus

Alveolus

Airways

Radius = 1 Surface tension = T

A

Radius = 2 Surface tension = T

Surfactant molecule

Hydrophobic (nonpolar) part

Palmitate

Air Liquid

Glycerol Phosphate Choline

Hydrophilic (polar) part

Air Liquid

B

Surfactant film (thicker in small alveolus; thinner in larger alveolus)

C

FIGURE 21-12. (A) The effect of the surface tension (forces generated at the fluid-air interface) and radius on the pressure and movement of gases in the alveolar structures. According to the law of Laplace (P = 2T/r, P = pressure, T = tension, r = radius), the pressure generated within the sphere is inversely proportional to the radius. Air moves from the alveolus with a small radius and higher pressure to the alveolus with the larger radius and lower pressure. (B) The surfactant molecules with their hydrophilic heads (that attach to the fluid lining of the alveolus) and their hydrophobic tails (that are oriented toward the air interface. (C) The surfactant molecules form a monolayer (shaded in blue) that disrupts the intermolecular forces and lowers the surface tension more in the smaller alveolus with its higher concentration of surfactant than in the larger alveolus with its lower concentration of surfactant.