Chapter 61. Development of The Lower Respiratory System: Surfactant and Respiratory Movements - Review of Medical Embryology Book

Respiratory movements occur before birth, causing aspiration of amniotic fluid into the lungs

AT BIRTH, THE DEVELOPING LUNGS are half-inflated with fluid from the lungs themselves, the amniotic cavity, and tracheal glands. Therefore, aeration is the replacement of intra-alveolar fluid with air

During and after birth: one-third of the fluid expelled by the lungs is via the mouth and nose; one-third enters the pulmonary capillaries; and one-third passes into the lymphatic system
In the adult, the average surface area of the alveolar-capillary membrane is between 70 and 80 square meters, favoring the daily loss of about 800 ml of water in the expired air
The alveolar-capillary membrane, in the adult, consists of a thinned-out cell wall plus the cytoplasm of a type I cell with its basement membrane and the thinned-out cell wall and cytoplasm of a capillary endothelial cell with its basement membran Where they meet, the two basement membranes fuse

In the mature, fully distended alveolus, the thickness of the alveolar-capillary membrane varies from 0.2 mm to 5 mm. In the newborn, this membrane is 0.4 mm in thickness
The extreme thinness of the alveolar-capillary membrane favors diffusion of oxygen and carbon dioxide

TYPE II ALVEOLAR EPITHELIAL CELLS (cuboidal cells of the terminal sacs) secrete surfactant by weeks 23 to 24

The cells contain osmophilic inclusions or granules, said to be either surfactant or a precursor of surfactant
Surfactant is capable of lowering surface tension at the air-alveolar surface, forming a monomolecular layer at the interface between the air in the alveoli and the fluid layer covering the alveolar cells. This helps maintain the patency of the alveoli and prevent atelectasis (lung collapse)

Surfactant is a mixture of lipoproteins rich in phospholipids, especially dipalmityl lecithin, and has a half-life of 14-24 hours. It is present in lungs of all airbreathing vertebrates

BY WEEKS 25-28, THE AMOUNT OF SURFACTANT is enough to prevent alveolar collapse when breathing begins
THE ABSENCE OR DEFICIENCY OF SURFACTANT is a major cause of hyaline membrane disease and is one major cause of respiratory stress syndrome in newborns

The lungs are underinflated, and the alveoli contain fluid high in protein content
The membrane is derived from a combination of substances in the circulation and injured pulmonary epithelium

THYROXINE is known to be a potent stimulator of surfactant production
SUPRARENAL CORTICAL HORMONE, cortisol, controls both maturation of type II cells during uterine life and the production of surfactant
PROLONGED INTRAUTERINE ASPHYXIA may produce irreversible changes in the alveolar cells, making them incapable of producing surfactant. However, there are undoubtedly many causes for the absence or deficiency of surfactant in both premature and full-term infants