surfactant. Now let’s look at the development
of the lungs as seen in a transverse section
across the axis of the embryo. In this image,
we can see the neural tube at the top, and
we can see the gut suspended, surrounded by
mesoderm with the heart hanging in the developing
pleuropericardial cavity. And off to the side
of the gut, we can see the lung buds.
So this is a very early stage when only two lung buds
are present. And in this view, they are cut
in section. A little later still, we can see
that lung buds are distinctly separate and
surrounded by their mesoderm. Now looking
at the cavity, we can see a little piece of
the heart, but the most striking thing is
two-folds which are growing in from either side.
These folds are known as the pleuropericardial
folds, and these are dividing the primitive
cavity which is both pleural and pericardial
in nature into the definitive pleural and
pericardial cavities. A little later still,
we can see that the pleuropericardial folds
have met in the midline, and the pleural cavity
and the pericardial cavity are now
completely separate. What will happen now is that the
lungs begin to expand around with their pleural
cavities until they come to encompass the
heart. So when you look at the front view
of an adult, only a small part of the heart
would be directly accessible, otherwise, the
lungs would be covering over where the heart
actually lies. Now, we’ve followed through
the normal process of development, but there
are significant events associated with the
birth itself. While still in the uterus, the
baby is showing breathing movements, and fluid
is moving in and out the mouth and lungs.
This fluid is a combination of amniotic fluid
and fluid secreted by the lungs themselves.
When the baby is born, this fluid will drain
from the lungs, and it may have to be assisted
by suction to allow that to take place completely.
At that point as the baby breathes in, the
surfactant would normally be present in sufficient
amounts to keep the lungs inflated and to
permit respiration to take place.
If there are inadequate amounts of surfactant, then
there’s a risk that the lung might collapse
at the stage when the fluid leaves the lungs.
If there was any fluid left remaining in the
lungs that’s not been drained out, it’s
normally reabsorbed by the capillaries.