Surfactant and the Respiratory Distress Syndrome

Alveolar fluid contains a phospholipid known as dipalmitoyl lecithin (probably attached to a protein) that functions to lower surface tension. This compound is called surfactant—a contraction of the term surface active agent. The surfactant molecules become interspersed between water molecules at the water-air interface of the alveoli, thereby reducing the attractive forces (hydrogen bonds, described in chapter 2) between water molecules that produce the surface tension. Thus, because of

Respiratory Physiology

Basal lamina


Basal lamina


■ Figure 16.12 The production of pulmonary surfactant. Produced by type II alveolar cells, surfactant appears to be composed of a derivative of lecithin combined with protein.

surfactant, the surface tension in the alveoli is reduced. Further, the ability of surfactant to lower surface tension improves as the alveoli get smaller during expiration. This may be because the surfactant molecules become more concentrated as the alveoli get smaller. Surfactant thus prevents the alveoli from collapsing during expiration, as would be predicted from the law of Laplace. Even after a forceful expiration, the alveoli remain open and a residual volume of air remains in the lungs. Since the alveoli do not collapse, less surface tension has to be overcome to inflate them at the next inspiration.

Surfactant is produced by type II alveolar cells (fig. 16.12) in late fetal life. Premature babies are sometimes born with lungs that lack sufficient surfactant, and their alveoli are collapsed as a result. This condition is called respiratory distress syndrome (RDS). Considering that a full-term pregnancy lasts 37 to 42 weeks, RDS occurs in about 60% of babies born at less than 28 weeks, 30% of babies born at 28 to 34 weeks, and less than 5% of babies born after 34 weeks of gestation. The risk of RDS can be assessed by analysis of amniotic fluid (surrounding the fetus), and mothers can be given exogenous corticosteroids to accelerate the maturation of their fetus's lungs.

People with septic shock (a fall in blood pressure due to widespread vasodilation, which occurs as a result of a systemic infection) may develop a condition called acute respiratory distress syndrome (ARDS). In this condition, inflammation causes increased capillary and alveolar permeability that lead to the accumulation of a protein-rich fluid in the lungs. This decreases lung compliance and is accompanied by a reduced surfactant, which further lowers compliance. The blood leaving the lungs, as a result, has an abnormally low oxygen concentration (a condition called hypoxemia).

Even under normal conditions, the first breath of life is a difficult one because the newborn must overcome great surface tension forces in order to inflate its partially collapsed alveoli. The transpulmonary pressure required for the first breath is fifteen to twenty times that required for subsequent breaths, and an infant with respiratory distress syndrome must duplicate this effort with every breath. Fortunately, many babies with this condition can be saved by mechanical ventilators and by exogenous surfactant delivered to the baby's lungs by means of an endotracheal tube. The exogenous surfactant may be a synthetic mixture of phospholipids, or it may be surfactant obtained from bovine lungs. The mechanical ventilator and exogenous surfactant help to keep the baby alive long enough for its lungs to mature, so that it can manufacture sufficient surfactant on its own.

Test Yourself Before You Continue

1. Describe the changes in the intrapulmonary and intrapleural pressures that occur during inspiration and use Boyle's law to explain the reasons for these changes.

2. Explain how the compliance and elasticity of the lungs affect inspiration and expiration.

3. Describe pulmonary surfactant and discuss its significance.

■ Figure 16.13 Changes in lung volume during breathing. A change in lung volume, as shown by radiographs (a) during expiration and (b) during inspiration. The increase in lung volume during full inspiration is shown by comparison with the lung volume in full expiration (dashed lines).

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