Let's continue our discussion of the respiratory system
by talking about the lower respiratory tract.
We mentioned last time that the lower respiratory tract
picks up after the larynx and begins with the trachea.
The larynx, which is made of cartilage connects to the trachea,
which is also made of cartilage.
Although this cartilage doesn't go all the way around the airway,
it's actually more of a C-shape,
with cartilage missing at the posterior portions.
It then terminates at something called the carina
where it becomes the right main bronchus and the left main bronchus.
And you see that the left main bronchus
branches at a different angle
than the right main bronchus.
And that's because of the presence of the heart
in the left side of the chest.
Here we see how the lungs sit in relationship to things like
the trachea and the heart.
We have the trachea, again, coming down from the larynx,
dividing into bronchi that supply the lungs,
one on the right, and one on the left.
The right lung can be further divided into lobes.
We have a superior or upper lobe,
a middle lobe and an inferior or lower lobe.
On the left, however, we typically only have two.
We have a superior upper lobe, and an inferior or lower lobe.
Here we can see the right and left main bronchus
entering the right and lung respectively.
We can also see by removing some of the substance of the lungs,
just how much branching there is occurring
in association with the right and left pulmonary arteries.
And that's happening both at a macroscopic and microscopic level
in order to help achieve gas exchange.
And we'll see that a little bit later.
We also find veins. The right and left pulmonary veins
are merging into larger and larger ones
until eventually there's a pair on each side emerging from the lung.
And where these things emerge or enter
is an opening in the lung called the hilum.
And hilum is a term you see in a lot of organs.
A hilum is an area where vessels
and other things in this case, the bronchus
enter along that otherwise doesn't have any other connections.
So here we see the right hilum of the lung,
where the right pulmonary artery is entering.
And we also see the pulmonary veins.
And here we have the left hilum with the left pulmonary artery in veins.
The space between lungs, something called the mediastinum
and contains a lot of important structures.
Most posteriorly, we have the esophagus,
which is really just carrying food through the thorax
down to the abdomen where it's going to empty into the stomach.
Just anterior to the esophagus, we have the trachea.
We also have coming off of the heart which has been removed here,
the aortic arch, which ultimately becomes the descending aorta
and will eventually pass down into the abdomen as well.
Here we see the left lung from a lateral point of view.
And we see that it vaguely has a pointed area
towards the top called the apex.
And at the bottom or the inferior end, it's much broader,
and we call that portion of the lung, the base.
Again, we have an upper or superior lobe
and a lower or inferior lobe.
And those two lobes are separated by something called an oblique fissure.
And this fissure is just what the name sounds like
it's oblique or diagonal.
There's also a little bit of the upper lobe
that sits directly over the heart and it's a very thin portion.
And that thin portion somewhat resembles a tongue
hence the term lingula
because lingula refers to the tongue.
If we were to look from a medial point of view,
we would see a little bit of a gap in the lung called again the hilum,
where structures can enter an exit.
And the case of the left lung, we have the pulmonary artery sitting
just superior to the bronchus,
and then most inferiorly we have the pulmonary veins.
Here we go back to the lateral view,
but this time we look at the right lung.
Again, the pointed area at the top is called the apex
and the broad area at the bottom is called the base.
And we still have an upper lobe and a lower lobe.
And they're also separated by an oblique fissure,
just as in the left,
but on the right, we have an extra load called the middle lobe.
And it's separated from the upper lobe
by a more horizontally oriented fissure
called the horizontal fissure.
And this makes sense that we have room for an extra lobe on the right,
because the heart is occupying space on the left side of the chest.
Again, if we switch back to a medial view,
we find the hilum where things enter and exit.
And this case on the right side,
the pulmonary artery sits anterior to the bronchus.
But again, the veins still sit inferior to both.
So this brings us to the concept of pulmonary circulation.
It's very different than systemic circulation,
which is essentially all the rest of our circulation.
And pulmonary circulation starts
with the pumping action of the right ventricle.
That's going to pump blood out through the pulmonary arteries,
where it can get oxygenated in the lungs
and come back as the pulmonary vein into the left atrium.
And from there, that reservoir of oxygenated blood
can be used for systemic circulation.
Now, this may be a little confusing because
some of you probably have always thought
that arteries carry oxygenated blood
and veins carry deoxygenated blood.
And that's because we typically think of circulation as systemic
in which that is the case.
But arteries and veins aren't defined by
whether they carry oxygenated or deoxygenated blood.
They're defined by whether they're going away from the heart,
or coming toward the heart.
So an artery is an artery as long as it's going away from the heart.
In this case, the pulmonary arteries are going away from the heart,
but they are carrying deoxygenated blood.