Surfactant is a phospholipid based substance secreted by the alveolar type II cells, which lines the alveoli and acts by markedly reducing surface tension. This action has the following effects:
• Reduction of surface tension, which helps to even out the distribution of compliance and hence ventilation, since if without surfactant, alveoli with low resting volumes are significantly more difficult to expand, than those with larger resting volumes. This effect is important in neonates, in whom deficiency of surfactant is associated with infant respiratory distress syndrome
• Stabilization of small alveoli. In a bubble wall, surface tension acts to shrink or collapse the bubble. A similar effect is seen in an alveolus. The smaller the alveolus the greater the tendency to collapse. Because of this, small alveoli tend to collapse by forcing their gas into larger communicating alveoli. The reduction of surface tension by surfactant decreases this effect
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Figure RR.10 Inspiration-expiration loop showing hysteresis
• Reduction of the energy expended as heat during each inspiratory-expiratory cycle, i.e. the hysteresis area of the pressure-volume loop is decreased
• Surfactant also keeps the alveoli dry by reducing the "suction' effect created by surface tension as it tries to collapse alveoli. Surface tension creates negative interstitial pressures as it tries to shrink alveoli, thus drawing fluid from capillaries into the air spaces
When inspiration occurs, the fresh gas entering the lung is not distributed evenly but the majority of the fresh gas passes to the most dependent regions of the lung.
The mechanism underlying this uneven distribution of ventilation lies in the variation of intrapleural pressure from top to base of the lung. Gravitational effects cause the lung in the thoracic cage to behave as a fluid volume, producing a hydrostatic pressure gradient due to the blood perfusing the lung. Thus, in the upright position intrapleural pressure at the base of the lung is greater than at the apex. In the upright adult there is a difference in intrapleural pressure of about 0.7 kPa between the apex and the base of the lung. This variation of intrapleural pressure means that the alveoli also vary in degree of distension, and hence position on the pressure-volume curve of the lung (Figure RR.11). The more dependent alveoli are less distended, and situated on the linear part of the pressure-volume curve, compared with the apical alveoli which are over distended and situated on the top part of the pressure-volume curve. Thus, at the base of the lung the alveolar compliance (gradient of curve) is greater than at the apex. Consequently during inspiration, greater expansion of alveoli in the lower areas of the lung occurs, and ventilation is preferentially directed to the base. This preferential distribution of ventilation matches the relatively high pulmonary blood flow to the dependent parts of the lung, since perfusion is gravitationally determined. In addition, there is lower airway resistance to the dependent areas of the lung that enhances gas flow to these regions.
This distribution of ventilation during spontaneous breathing contrasts with the situation during artificial ventilation, when a reduced FRC tends to reverse the distribution of alveolar compliances, and gives preferential ventilation of non dependent areas (Figure RR.12).
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Figure RR. 12 Compliance during mechanical ventilation
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