So the trachea then splits into two. It splits
into a right and a left primary bronchus. So ‘bronchi’ is plural and ‘bronchus’ is singular. So I’ll just remove one of the lungs so we
can take a look at the bronchi. The place where the bronchus enters the lung is known
as the hilum. At the hilum, there’s a couple of other structures which enter the lung. So I’ve just brought the cardiovascular system
and I’m going to rotate the model around posteriorly and you can see the other structures that
enter the lung. So you’ve got the pulmonary veins and the pulmonary arteries which enter
the lung at the hilum. So I’ve just switched to this diagram which
shows the exact same view we’re looking at. We’re looking at the posterior view, so you
can see the trachea coming down here branching off into the left and the right main bronchi.
You can see the pulmonary arteries at the top and the pulmonary veins inferiorly entering
into the lung at the hilum. So you’ve got the pulmonary arteries in blue
carrying deoxygenated blood from the heart, to the lungs, to receive oxygen and you’ve
got the pulmonary veins returning oxygenated blood from the lungs to the heart. So this diagram here just shows the hilum
in a bit more detail. You can see the structures entering this area called the hilum of the
lung. So just coming back to the bronchi, once they’ve
entered the lung, they divide further. They divide into lobar bronchi, which are also
known as secondary bronchi. And the lobar bronchi divide further into segmental bronchi. So you’ve got the trachea dividing into the
primary bronchus. So you’ve got a right and left primary bronchus or main bronchus. Then
the primary bronchi divide into lobar bronchi, which are known as secondary bronchi. And
the secondary bronchi divide into tertiary bronchi, which are segmental bronchi. So the bronchi just keep dividing and they
end up in bronchioles, which are smaller and lack cartilage. And then these bronchioles
eventually form alveolar ducts, which leads to alveolar sacs and form alveoli, which are
responsible for gas exchange and they have a huge surface area for diffusion, have a
rich blood supply and very thing, but we’ll talk about that in another tutorial. So for now, just remember that the trachea
divides into the right and left main bronchi, which divide into secondary bronchi, which
divide into tertiary bronchi. So the secondary bronchi are called lobar
bronchi because they supply the lobes of the lung with air. The right lung, which is this
lung here, has three lobes. You’ve got a superior lobe, a middle lobe and then an inferior lobe.
These lobes are separated by fissures. So on the right lung, you’ve got a fissure,
which separates the superior and middle lobes and this is called the horizontal fissure.
And you’ve also got an oblique fissure because it runs obliquely and this separates the middle
from the inferior lobes and also the superior and inferior lobe at the back. On the other side, the [left] lung, it only
has one fissure. So it’s only got an oblique fissure, which separates a superior and inferior
lobe. So now that we know how many lobes there are
in each lung, we know how many lobar bronchi there must be. There’s only three right lobar
bronchi and there are only two left lobar bronchi because there’s three lobes on that
right and two lobes on the left. So these lobes can actually be further divided
into what is known as bronchopulmonary segments. Each lung has ten bronchopulmonary segments.
These bronchopulmonary segments are supplied by the segmental bronchi, so the tertiary
bronchi. So the lobar bronchi supply the lobes of the
lungs and the segmental bronchi supply the bronchopulmonary segments. So you can see in this diagram the three lobar
bronchi on the right and the two lobar bronchi on the left. So you can see them in different
colors. You’ve got the superior in green; yellow, you’ve got the middle lobar bronchus;
and in blue, you’ve got the inferior lobar bronchus. And you’ve got the superior and
inferior lobar bronchus in green and blue on the left side. So the lungs themselves are surrounded by
a pleural cavity. You can see the cavity here in pink. So the pleura or the pleural cavities
are a serious membrane which line the lungs. There’s a visceral pleura and a parietal pleura.
So the visceral pleura lies very closely to the lung and adheres closely to the lung tissue.
The parietal pleural lines the thorax. Both these pleural layers are continuous at the
hilum. So the visceral pleura and the parietal pleura
is the pleural cavity. This contains pleural fluid. So this space between the visceral
and the parietal pleura is only a potential space. So in normality, these layers lie in
very close contact and you’ve got this thin layer of pleural fluid. So I’ve just switched over to this cross-section
of the lungs. You can see anteriorly up here, posteriorly down here. You’ve got the left
lung and the left lung. We’ll just take a look at the — so you can see the pleura in
this diagram. So you can see at the hilum, these two layers are continuous. So you’ve
got the outer layer, which is the parietal pleura, which lines the thorax. And then the
visceral pleural lies very closely to the lung and it goes into the fissures and everything
like that. And then it is continuous with the parietal pleura at the hilum. So this area, remember, where all these structures
enter — the bronchus, the bronchi, the pulmonary arteries and pulmonary veins entering at the
hilum. So you can see how the viscera and parietal pleura are continuous with one another.
And then you’ve got this space between the two layers, which is the pleural cavity, which
contains pleural fluid. So that’s a very basic and broad overview
of some of the structures in the respiratory system.