BIOC34H3 Lecture Notes - Lecture 3: Pulmonary Artery, Pulmonary Vein, Pulmonary Circulation

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3 Aug 2016
Slide 3
General anatomy of the blood vessel. Most of the things we are already aware of; vasculature is divided
into 3 parts: arterial system, capillaries, and venous system. By definition the arterial system: series of
blood vessels that is taking blood away from the heart whereas venous system takes the blood to the
heart and capillaries are there for exchange system.
Arterial system is in red because arteries coming from the heart are carrying oxygenated blood. The
blood that is being brought back to the heart is deoxygenated which is blue. One generalized exception
to this trend is the pulmonary artery and pulmonary vein. Pulmonary artery so named because it is
taking blood from the heart to the lungs (carrying deoxygenated blood) and pulmonary vein is doing the
opposite, carrying oxygenated blood.
Elastic arteries (conducting arteries) – they have a large amount of fibres on their wall. Their function is
to take blood from the heart then conduct it to distant arteries. They are elastic; they do have some
capacity to expand as blood enters them, and recoil when blood leaves them. This would represent
things like pulmonary trunk, aorta (those 2 major arteries are very elastic).
Muscular arteries (distributing arteries): rather than being elastic, they are very smooth. They are
carrying blood to distant parts of the body. They are not doing the similar job as elastic arteries with
expanding and recoiling though.
Muscular arteries give rise to these smaller diameter structures which are called arterioles. Arterioles
are similar to muscular arteries (they too have smooth muscle surround them), they tend to contract or
relax the smooth muscle that’s in their wall so they can change their diameter. The arterioles connecting
the muscular arteries to capillaries beds… they control just how well perfuse just any capillary bed is. For
example, if these arterioles were to constrict, as smooth muscle contracts, the blood flow would be
reduced and therefore reduced the perfusion of that capillary bed. This means the tissue that this
capillary bed is supposed to be in would now undergo less gas exchange and fewer nutrients is brought
to it.
Another layer of control are these sphincters: these act as secondary act of control. In the example
mentioned earlier, you would want some blood flow in the arterioles to occur; you wouldn’t want to
entirely constrict it; perhaps leave a bit dilated. You use these sphincters to decide where precisely in
the capillary bed is the blood going to flow. In one case you see here on the very bottom, this arterial
here got blood flowing across but majority of layers are regulated to control precisely which capillary
bed would meet the circulatory demands for those tissues.
Vena cava is supposed to be labelled where the blood is flowing into the heart.
Arteriovenous Anastomosis: direct connection between an artery and a vein. This is more or less serves
as a shunt to skip all those capillary beds. This might completely look like a silly thing to exist, but it just
adds another layer of control to perfusion of capillary beds. There are some bodies, where you see this
common such as your skin.
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For example: your plasma is carrying heat. Now let’s say if I am producing heat trying to keep my body
warm, and I decide to go out in this weather, I would want to minimize the heat loss to the
environment. So the one way I could do is by minimizing how much I can perfuse the dermis of my skin.
So, the more I perfuse to skin with blood, the closer it is to my body, and therefore the closer to the cold
environment I am bringing that blood. So if I want to minimize the heat loss and perfusion of the skin, I
would use these arteriovenous anastomosis is to more less bypass the perfusion of the capillary beds of
the skin. I am keeping that blood further away, making it hard for me to lose heat. Conversely, if I want
to lose heat, in that case, we would want to reduce the flow from these anastomosis... direct more
blood through these arterioles to the capillary beds, bring the blood to the surface and help dissipate
that heat in the environment.
Another place that these are commonly found is erectile tissue, so for example, erection of penis. You
want to do that via filling those things with blood. You don’t want to always erect penis, so you
sometimes want to direct blood flow away from those structures to refrain from erecting when it’s not
The other structure labelled here is the lymphatic system. At the level of capillaries, you not only have
the exchange for nutrients but there’s usually net loss of fluid from capillaries into the tissues. That
means there is a buildup of fluid in the tissues, loss of fluid in the circulatory system, unless there was
some mechanism to return the fluid back to the circulatory system. The lymphatic capillaries are found
near cardiovascular system, that will take any fluid that leaks out of these capillaries, and turn it back to
the circulatory system close to the level of the heart. Lymphoid tissues got the opportunity for its role to
take place
Slide 4
When we think of blood vessels, they are comprised in 3 layers. Tunica intima; endothelium example.
The name suggests, it is the inner most layer of the blood vessel, which is the endothelium (shown in
pink), which is the single layer of cell that makes up the inner most lining of any blood vessel. It’s the
only cellular layer that makes up the capillary. Another part of this tunica intima is subendothelial layer.
It is a layer of collagen fibres that lie underneath the endothelium. So when endothelium is damaged,
this layer is exposed and some of its components play a big role such as platelets plug etc. Internal
elastic lamina: is the outermost layer. It is the thin elastic structure which underlies the subendothelial
layer which gives more elasticity.
Tunica media: smooth muscles and/or elastic fibres are found here. They are the outer pink layer. They
are interwoven between smooth muscle fibres to make them more elastic. If you look at the veins, the
tunica media is very THIN whereas in artery it’s quite THICK.
Tunica externa: layer of connective tissue. They are collagen fibres that surround the entire vessel. They
serve as the outer structural support for the entire blood vessel.
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Last thing to note is valves that are found in the veins but in the arteries. The venous part of the
vasculature is very low pressure system; it’s very difficult for the blood to move back to the heart. So
these valves help move the blood to prevent any back flowing.
Slide 5
This is an artery on the left. The tunica externa and adventitia is the same thing. It’s the outer layer of
connective tissue. The structure is quite thinner in the vein. Then there is tunica media of the artery,
which is pretty thick. There is tunica interna and these valves that are caught in this structure (veins).
Slide 6
The function of the blood vessels is to bring blood to the tissues. So Q is the blood flow. The ultimate
question with a blood vessel is: How fast is the blood flowing through the vessel because if we got a
tissue that got a high demand for oxygen, nutrient, etc. we would want the flow to be fast to meet the
metabolic demands. The pressure difference: the blood always flows from the regions of high pressure
to low pressure. So in other words, there must be a pressure in the vasculature that is necessary.
Resistance (R): There are factors that work to slow blood flow down; things like friction (so when the
blood interacts with the wall of the vessel, it will impede the flow of blood). The equation tells us that
the blood flow will occur as long as there is pressure difference between say arteries and veins, that is
sufficient enough to overcome all the factors that are working to oppose the flow of blood (like viscosity,
or friction).
This equation has been known for a long time. Poiseuille’s Law that relates these three variables. Story
time: this law may be referred to as Hagen Poiseuille’s Law or Poiseuille’s Hagen law. Both of these guys
came up with the law independently. It’s interesting; there was a paper that addressed this controversy,
to what this law be called. We cannot put all the names for just one law… their work was depended on
people who came up with foundations. Who had the greatest contribution? Though Hagen came up
with this law first, but it was Poiseuille who took great conviction. The name of this law reflects
particular significance for Poiseuille to have confidence in this law to give prominence in his writing.
Slide 7
Pressure difference: talking about blood pressure
Is it pressure representing force another way? When we talk about blood pressure, we are talking about
different individuals of forces that are acting upon the blood. There are three forces, thereby contribute
to blood pressure. The first one is inertial forces: things in motion stay in motion unless there is
something that opposes this motion. For example, blood is flowing; therefore the blood had some kind
of kinetic energy, this inertial force. However there may be other forces to slow it down (fraction).
Another component to blood pressure are the lateral forces: the blood would push against the wall of
its container; then the other part of the question becomes, how much does the container does it push
back against the blood. So imagine, there is blood that is pushing against the walls of the artery. If the
walls don’t push at all and they just give up, there won’t be any pressure in that system. But as long as
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