Lungs and circulation

Before birth, the lungs are fluid filled. Lung development is divided into several stages:

Embryonic (3-6 weeks)
Primitive bronchial buds form which enlarge to form two primary bronchi. These will eventually develop into the left and right lungs.

Pseudoglandular period (6-16 weeks)
Bronchi and major blood vessels of the lung are formed. Bronchial branching results in the formation of over 20 generations (divisions) of airways.

Canalicular period (16-28 weeks)
The diameter (size) of airways increases and the lung vasculature (blood supply) continues to develop. The structure of a lung becomes increasingly more complex as it grows. Primitive end respiratory units start to form; these consist of a respiratory bronchiole, leading to alveolar ducts and terminal sacs (primitive alveoli) with associated capillaries. At the same time the lung structure increases in complexity the cells which line the terminal sacs begin to differentiate and surfactant production begins at around week 22-28.

Saccular period (28 weeks to term)
During the terminal sac period, numerous terminal sacs surrounded by capillaries develop. Surfactant production starts to increase significantly between 30 and 32 weeks.

Alveolar period (first few years after birth)
Most alveoli are formed in the first and second year of life, whilst between the ages of 3 and 8 years the alveoli increase in size and to a lesser extent in number.