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from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark short note on Surfactant-function ; note on RDS

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Surfactant – Function (Guyton & Hall, 4th SA Edition)

Source: Secreted by type II alveolar epithelial cells (~10% of alveolar surface area), which contain lipid inclusions released into the alveolar lining fluid.
Composition: A complex mixture of phospholipids, proteins, and ions. The key active component is dipalmitoyl phosphatidylcholine, along with other surfactant apoproteins and calcium ions.
Mechanism of action: Surfactant is a surface-active agent that lowers the surface tension of the fluid lining the alveoli. Its phospholipid molecules do not dissolve uniformly in the alveolar fluid; instead, part of the molecule dissolves while the rest spreads over the fluid surface, greatly reducing the cohesive attraction between water molecules there.
Quantitative effect on surface tension:
  • Pure water: 72 dynes/cm
  • Alveolar fluid without surfactant: 50 dynes/cm
  • Alveolar fluid with normal surfactant: 5-30 dynes/cm (a 2- to 10-fold reduction)
Functional importance (Law of Laplace): Pressure tending to collapse an alveolus = 2 x Surface tension / Radius of alveolus
  • With normal surfactant, a normal-sized alveolus (radius ~100 µm) generates only about 4 cm H2O collapsing pressure.
  • Without surfactant (pure water lining), this pressure rises to about 18 cm H2O, roughly 4.5 times normal.
Thus surfactant:
  1. Reduces the work of breathing by lowering the elastic (surface tension) resistance of the lungs.
  2. Prevents alveolar collapse (atelectasis), especially in small alveoli, by proportionally lowering surface tension more as alveolar radius decreases (stabilizing alveoli of different sizes).
  3. Helps keep alveoli dry by opposing the tendency of surface tension to pull fluid from capillaries into the alveoli, thereby preventing pulmonary edema.
  • Guyton and Hall Textbook of Medical Physiology, p. 494-495

Short Note: Respiratory Distress Syndrome (RDS) / Hyaline Membrane Disease

Definition: A condition, most common in premature infants (also seen in infants of diabetic mothers), in which severe respiratory distress develops within hours to days of birth, sometimes causing death from suffocation.
Cause: Type II alveolar epithelial cells do not begin secreting adequate surfactant until the last 1-3 months of gestation. Premature (and occasionally some full-term) infants are therefore born with a deficiency of pulmonary surfactant.
Pathophysiology:
  • Lack of surfactant causes surface tension of the alveolar fluid to rise several-fold above normal.
  • This produces a strong tendency for alveoli to collapse (atelectasis) and promotes leakage of proteinaceous fluid and plasma from the pulmonary capillaries into the alveoli, causing pulmonary edema.
  • At autopsy, alveoli contain large amounts of proteinaceous fluid and desquamated epithelial cells; on microscopy this material resembles a "hyaline membrane," giving the disease its alternate name, hyaline membrane disease.
Clinical consequence: Widespread alveolar collapse and fluid filling severely impair ventilation, leading to hypoxia and, if untreated, death from suffocation. Severity correlates with degree of prematurity - the more premature the infant, the more severe the RDS.
Management (per textbook): Outcomes have improved greatly with surfactant replacement therapy, assisted ventilation, and antenatal corticosteroids (to accelerate fetal lung/surfactant maturation), although hyaline membrane disease remains an important cause of morbidity and mortality in preterm infants.
  • Guyton and Hall Textbook of Medical Physiology, p. 546, 1050-1051
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