Alveoli collapse

Q7 If the embolus is quite large and obstructs a significant area of the pulmonary circulation, the affected area of lung will be underperfused or non-perfused. The area may continue to be ventilated for some time, causing a ventilation-perfusion mismatch, which leads to poor gas exchange and abnormal blood gas tensions. The lung volume in the affected area decreases, and this decrease in size can sometimes be seen on a chest X-ray. After some hours, surfactant production declines in the non-perfused area of lung and the alveoli collapse. 

Respiratory distress syndrome (RDS) is a common problem in premature infants. The immature lung fails to produce dipalmitoylleci-thin, which is a surfactant. RDS occurs when the alveoli collapse inwards after expiration and adhere under the prevailing surface tension (atelectasis). The function of dipahnitoyllecithin is to reduce the surface tension and permit expansion of the alveoli on inflation. Assessment of the maturity of foetal lung function can be made by measuring the ratio of lecithin to sphingomyelin (the L/S ratio) in amniotic fluid. 

Without sufficient surfactant in the lungs of a premature infant, the alveoli collapse, which decreases pulmonary function. 

The second effect of surface tension is that it causes the alveolus to become as small as possible. As the water molecules pull toward each other, the alveolus forms a sphere, which is the smallest surface area for a given volume. This generates a pressure directed inward on the alveolus, or a collapsing pressure. The magnitude of this pressure is determined by the Law of LaPlace ... 

The collapsing pressure (P) is proportional to the alveolar surface tension (ST) and inversely proportional to the radius (r) of the alveolus. In other words, the greater the surface tension and the smaller the radius, the greater the collapsing pressure. 

Figure 17.2 Effects of surface tension and surfactant on alveolar stability, (a) Effect of surface tension. According to the law of LaPlace (P = 1ST/r), if two alveoli have the same surface tension (ST), the alveolus with the smaller radius (r), and therefore a greater collapsing pressure (P), would tend to empty into the alveolus with the larger radius, (b) Effect of surfactant. Surfactant decreases the surface tension and thus the collapsing pressure in smaller alveoli to a greater extent than it does in larger alveoli. As a result, the collapsing pressures in all alveoli are equal. This prevents alveolar collapse and promotes alveolar stability.

Another important factor in maintaining alveolar stability is interdependence. Each alveolus in the lungs is surrounded by other alveoli (see Figure 17.3, panel a) and all of these alveoli are interconnected with each other by connective tissue. Because of these interconnections, any tendency for an alveolus to collapse is opposed by the surrounding alveoli. As the central... 

RDS is attributed primarily to insufficient formation and differentiation of type II pneumocytes with consequent impaired production and release of surfactant. Pulmonary surfactant contains phospholipids that function at the air-liquid interface in the alveolus to lower surface tension, thus preventing alveolar collapse. In the face... 

FIGURE 16.3 Pulmonary surfactant plays a vital stabilizing role in pulmonary mechanics, (a) In the absence of pulmonary surfactant, airflow is directed into the larger alveolus where pressures are lowest (Pj < P ). resulting in collapse and overdistension of the smaller and larger alveoli, respectively, (b) In the healthy lung with functional pulmonary surfactant, the increased interfacial density of pulmonary surfactant molecules in the compressed smaller alveolus reduces the surface tension. This reduces the insufflation pressure and stabilizes the lung. 

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