neonatal physiology

Terminology

Preterm: a birth that occurs before 37 completed weeks of gestation
Term: a birth between 37 weeks and 42 weeks of gestation
Post term: a birth that occurs after 42 weeks of gestation
'Normal' weight 2.5kg-4.0kg
- Over 4.0kg - large for gestational age (LGA)
- Under 2.5kg - small for gestational age (SGA)

Surface tension is the attraction of the molecules in a liquid to each other, pulling them together and minimising surface area
Alveoli are the small, fluid-lined sacs where gas collects and diffuses into the blood during inhalation
- The molecules of this fluid pull together due to surface tension, in turn pulling the walls of the alveoli towards each-other, attempting to collapse the space in the alveoli
Surfactant is a fluid produced by type II alveolar cells; it keeps the alveoli inflated and maximises the surface area of the alveoli by reducing the surface tension of the fluid in the lungs
- Essentially provides a barrier that reduces the water molecules tendency to pull towards each other
The result is that surfactant keeps the alveoli inflated and maximises the surface area of the alveoli - increases lung compliance
Surfactant also promotes equal expansion of all alveoli during inspiration
- As an alveolus expands, the surfactant becomes more thinly spread and therefore the surface tension increases, making it more difficult to expand that alveolus further
- This stops one alveolus expanding massively whilst another alveolus only expands a little
Type II alveolar cells become mature enough to start producing surfactant between 24 and 34 weeks gestation - pre-term babies have problems associated with reduced pulmonary surfactant.