In contrast with the other organ systems the lungs begin their function as a gas exchange apparatus only at birth. During the pregnancy, however, they have an important function as an amniotic fluid-producing organ.
Prenatally, the air spaces are thus filled with fluid that, with the first breath, must abruptly be replaced by air. The perfusion relationships also change with birth in a dramatic fashion.
The following developmental steps are of importance for the future task of the lungs as gas-exchanging organ:
- a widely branched respiratory tree with a mucociliar cleaning mechanism
- a complex gas-exchange region with a short diffusion distance
- a thick net of capillaries that stands in close contact with the air spaces (blood-air barrier)
- a surface film (surfactant) that reduces the surface tension of the alveoli and thereby reduces their tendency to collapse.
Morphologically one subdivides the lungs into two sections:
- air-conduction part (air conducting respiratory passages)
- gas-exchanging part (pulmonary parenchyma)
As a gas-exchange organ the adult lung has a surface area of ca. 140 m2 (= a tennis court) which can only be achieved with an enormous differentiation of the embryonic tracheal tube into complex air spaces. The bases for this are the asymmetrical dichotomous divisions, on average 16 generations purely air-conducting respiratory passages that end in the terminal bronchioli. The following 7 generations of dichotomous divisions serve the gas-exchanging parenchyma. Through the differentiation of the parenchyma arises a very thin diffusion barrier between the air and the blood (blood-air barrier) that can be only 0.05-0.25mm thick. Functionally just as important is the differentiation - at the right time - of the type II alveolar epithelial cells that are responsible for the production of the surfactant.