The pulmonary vasculature:
There are two arterial
circulations to the lungs. There's the pulmonary
arteries and veins, which arise from the right
ventricle and then drain into the left atrium.
Those are involved in gas exchange and is
a low-pressure system. The normal pressure
of a pulmonary artery is about 20, 25 mmHg
maximum, whereas for most people, the systemic
circulation is 100, 110 mmHg. Then there's
the bronchial artery circulation. That's a
system circulation arises from the normal
left-sided cardiac circulation and provides
oxygenated blood to the lung tissue. And because
it comes from the left ventricle circulation,
it's a high-pressure system.
So we'll describe the pulmonary arteries in
a bit more detail—the low-pressure system
that arises from the right ventricle. The
pulmonary trunk comes out from the right ventricle.
That divides into a right and into a left
pulmonary artery, and those enter into the
lung at the hila. Those arteries then divide,
and basically, they follow the bronchi. So
every time the bronchus divides, the pulmonary
artery divides, so you get an accompanying
bronchial artery with each bronchus during
the further subdivisions out further into
the lungs, and that supplies blood eventually
to the pulmonary capillary bed around the
So this is an invasive pulmonary angiogram.
Somebody's injected contrast into the right
ventricle, and that contrast is being pumped
out from the right ventricle down the pulmonary
artery. And you can see the branching nature
of the pulmonary arteries that are delineated
by this contrast and how the arteries divide
and get thinner and smaller as they move out
into the distal part of the lung.
The alveoli: The pulmonary arterioles form
the pulmonary capillaries, which we've already
described, form a plexus round the outside
of the alveoli, covering about 70% of the
alveolar surface. And this is a diagram just
showing that in a diagrammatic form.
Those pulmonary capillaries then drain into
pulmonary venules, which then drain into pulmonary
veins, and essentially, the venous—pulmonary
venous—circulation does the reverse of the
pulmonary artery circulation, becoming…
the branches forming together and forming
bigger and bigger blood vessels in exactly
the same pattern as the pulmonary arteries,
but in reverse—eventually ending up, though,
in two veins, leaving each lung: the right
and left inferior and superior pulmonary veins,
and these drain directly into the left atrium.
So two veins coming from the right, two veins
coming from the left.
The clinical relevance of the drainage into
the left atrium has become more recently,
because there's increasing number of patients
with atrial dysrhythmias who are undergoing
atrial procedures where they cause ablation
and… to try and prevent the atrial dysrhythmia.
And those ablations are often around the origins
of where the veins are coming into the left
atrium, and that can cause mechanical problems
with drainage of the blood back into the left
atrium from the lungs.
The bronchial artery is completely different.
It's an important source of blood for the
lungs, and it's important clinically because
it's often the source of blood for major hemoptysis.
Being under higher pressure, it's much more
likely to cause a significant hemoptysis than
the low-pressure pulmonary artery circulation.
They supply… the pulmonary arteries… bronchial
arteries supply blood down to the terminal
bronchioles, and they also supply blood to
the visceral pleurae, the intrapulmonary blood
vessels, the walls of those vessels, and the
lymphatics. They arise from the systemic circulation,
and this… where they come from does vary
quite a lot between people, even in normal
circumstances. So the left bronchial artery,
generally speaking, comes from the aorta but
may not. The right bronchial artery arises
in the 3rd or 4th intercostal artery, but
there are often very abnormal arrangements
where the bronchial artery arises directly
from subclavian arteries, example… for example.
The pulmonary... Sorry, the bronchial veins
drain back into the systemic circulation—
venous circulation—into the azygous and
the hemiazygous veins. Occasion... There is
actually a small amount of blood that goes
back in through the pulmonary capillaries
to the left atrium as well, and that's essentially
an anatomical physiological shunt, where deoxygenated
blood reaches into the pulmonary venous circulation
and reaches the left ventricle but is actually
physiologically not particularly relevant
in most circumstances.
The lymphatic drainage of the thoracic cavity
is important, because this is how lung cancers
and lung infections spread. So there are lymphatics
which drain most of the lung parenchyma, the
airways, and the visceral pleurae. The drainage
pattern is essentially similar to the pulmonary
venous drainage pattern, that they go up through
the circulation to the hila, where there are
hilar lymph nodes, and then from there, they
go to mediastinal lymph nodes, and from there,
they go to the thoracic ducts and back into
the venous circulation.
This is a diagram showing some mediastinal
lymph nodes. The point about this diagram
is that, scattered throughout the mediastinum,
there's a large number of lymph nodes, which
are relatively small normally. But because
of the drainage from the lung via the hilar
nodes to the mediastinal nodes, patients with
lung cancer may present with enlarged hilar
nodes or enlarged mediastinal nodes due to
the metastatic spread of their disease. And
those mediastinal nodes could be in very different
positions within the mediastinum because of
the extensive network of lymphatics that are
The nerves of the thoracic cavity: The most
important nerves is the phrenic nerve, which
supplies the diagram we discussed already
arises from the 3rd, the 4th, and the 5th
cervical roots. It runs through the mediastinum
and over the pericardium to reach the diaphragms
and innervate the diaphragms. The intercostal
nerves we've also already described already.
They arise from the thoracic nerve roots,
and they run under each rib, and they provide
pain sensation to the pleurae and to the overlying
chest tissue. There's also a vagus and sympathetic
nervous system to the bronchial walls, and
that's important, because that generates the
smooth muscle action that can cause bronchoconstriction,
and that can be reversed by treatment, and
it is the main target for the bronchodilator
therapies used for asthma and COPD. The vagus
and sympathetic nerve system also supplies
the mucous glands, as may the vagus nerve,
and stimulates production of mucus.
The important thing about the phrenic nerve
is that it has a long course, and because
it crosses the pericardium, it is actually
quite fragile. And it is quite easy for one
single phrenic nerve to be damaged in some
way by multiple different methods—often
by cardiac procedures, for example. And that
will leave that side's diaphragm paralyzed,
and therefore, it will not contract during
ventilation, and its effect on ventilation
would be lost. And single paralysis of a hemidiaphragm
is fine; a bilateral diaphragmatic paralysis
is fatal without respiratory support.
So just to summarize the main learning points
of this lecture on the lung anatomy:
• The lungs are contained within an expandable
bony cage that comprises the vertebrae, the
sternum, and the ribs.
• Air enters the lungs through the trachea
and is conducted down through the bronchial
tree to reach the alveoli.
• The main divisions of the bronchi on the
right-hand side are the upper, middle, and
lower lobes, and on the left-hand side, just
the upper and lower lobes.
• The alveoli themselves are very thin-walled,
have a huge surface area, and are closely
in contact with pulmonary capillaries. And
this is all important for gas exchange. And
this is an anatomical design to ensure that
maximum oxygen input occurs from the alveoli
into the blood.
• The lungs are supplied by two different
circulations: There's the low-pressure pulmonary
artery circulation, which is required for
oxygen uptake, and then there's the high-pressure
systemic bronchial artery circulation, which
is required for the delivery of oxygen to
the lung tissue.
• The lungs are surrounded by a thin pleural
space. It's a potential space that can get
filled up with pleural fluid or an air in
pathological circumstances, and the lymph
drainage of the lung is first to the hilar
and then to the mediastinal nodes, and that
dictates what happens when patients have cancer
and with the metastatic spread of the disease.
Thank you for listening.