Professor Mark Saltzman:
This week we're going to
talk about cardiovascular
physiology and the
cardiovascular system,
which includes the blood,
the blood vessels,
and the heart.
We're going to focus on several
features.
This is obviously a big topic,
not only understanding the
basic physiology of the
cardiovascular system,
but what can fail in the
cardiovascular system in
disease.
Of course, cardiovascular
disease is the number one or two
cause of deaths in Western
world.
So, it's serious and important
topic for medicine.
It's also something that I
realize you probably all know
something about.
We're going to try to look at
things in a slightly different
way, look at them the way that
an engineer,
a biomedical engineer might
look at the cardiovascular
system, to try to understand the
relationship.
In particular,
between pressure,
blood pressure,
which is the force that moves
blood around through the
circulatory system and flow;
the actual movement of blood,
which is what's essential for
distributing oxygen and
nutrients and everything else
that we move around in our
circulatory system.
I'm going to talk briefly
about the anatomy of the heart
in the circulatory system.
We'll start by thinking about
the whole level.
I'm going to talk in some
detail about how pressure
differences generate flow,
starting with a very simple
example and building up to some
of the complexity that we have
within our cardiovascular
systems.
One of the most amazing things
about the cardiovascular system
is its ability to change the
pattern of blood's flow such
that organs that need more blood
can get more blood.
It's not just that there's a
static system where blood is
flowing throughout the body and
it flows at the same rate
everywhere all the time.
It increases in one region and
decreases in another in response
to physiological need.
We'll talk about the sort
of engineering principle that
the cardiovascular system uses
to accomplish that.
Then next week we'll talk
about--we'll talk in some detail
this Thursday about how the
heart generates the pressure
that's needed to move the blood
and how you can create this flow
with a relatively simple but
durable machine,
the heart.
Next Tuesday we'll talk about
cardiac conduction
electrophysiology,
how the heart regulates it's
beating in order to generate
orderly pattern of pressure
generation.
This is a cartoon of the
cardiovascular system,
not showing the blood except by
the colors here,
but showing the heart.
In the center of this diagram
is the heart and surrounding it
is sort of a schematic diagram
like you might see for plumbing
in a house but certainly not in
anatomical detail;
this drawing isn't of how blood
flows from the heart to all the
other regions of the body.
Let's just--now I know that
you've seen diagrams like this
before, you understand the basic
connections between things,
but let's just go through them
briefly.
You have to start somewhere
because this is a cyclic--it's a
closed system.
You start at one point and you
go around and you come back to
the beginning again.
If we start in the--in this
side of the heart,
this is the left side of the
heart and there are two chambers
in the left side of the heart:
the atrium and the ventricle.
You see that blood blows into
the left side of the heart and
it's come from the lungs.
The blood has come from the
lungs and so it's full of oxygen
and in this diagram it's
indicated as red.
If you follow the arrows here,
the blood flows from the lungs
into the left atrium,
into the left ventricle,
and out through a large vessel
called the aorta.
Fairly simple;
so far two chambers in the
heart, blood flows through these
two chambers,
atrium first then ventricle,
and out through the aorta.
If I was able to measure flow
at this point here in the aorta,
I would measure the total rate
of flow of blood out of the
heart.
You measure that in volume per
minute, in liters per minutes
let's say.
All of the blood flow is coming
through this one vessel.
If I could measure what the
flow is through the aorta over a
period of time,
I would measure what's called
the 'cardiac output' or the rate
of flow that the heart is
producing in its function.
Now, because it's a closed
system the rate of flow has to
be the same anywhere in the
cycle.
If you imagined a simple tube
that connects back to the--to
itself.
If I'm generating five liters
per minute of flow here I have
to have five liters per minute
everywhere in the cycle.
Otherwise what would happen?
Well, otherwise the volume of
the vessels would have to be
expanding or contracting at some
point if the flow rate is
different there.
It would have to be changing
with time if the flow rates
different and it doesn't do
that.
It's a steady state system
where there's a constant flow
all throughout the cycle.
We're going to come back to
this point but it's important to
remember.
Now, immediately after the
blood comes out in the aorta
there's an opportunity for it to
go somewhere else.
There's a branch point and that
first branch point is from the
aorta into this,
what's shown as a fairly large
vessel, but they're pretty small
vessels actually.
These are the coronary
arteries, the arteries that
actually bring blood to the
surface of the heart in order to
nourish it.
You can imagine that the heart
is using a lot of--it's using a
lot of energy.
It's generating work,
that work is being transmitted
into pressure and flow of the
blood.
It gets the energy to do that
from nutrients,
oxygen, glucose,
and so it needs a lot of blood
flow.
It's in the best possible
position to get good blood flow
because it's getting it's blood
right off of the aorta where
flow is a maximum.
We talked about stents last
week that are used to treat
disease in these blood vessels
and we'll talk more of that as
we go through the next couple of
weeks.
What I want you to focus on now
is just this phenomenon that the
flow branches here.
Some of the blood goes further
down the aorta and some of it
goes through the coronary
arteries.
Now, how much blood goes in
which direction is going to be
one of the main things we talk
about in the rest of the lecture
today,
but first let's just continue
in our journey around.
You'll notice that this pattern
repeats that there are points
where the flow branches.
For example,
after the aorta,
some of the flow goes up in
ascending arteries.
This goes up above the heart
into the head,
for example,
and so the carotid arteries
which carry blood flow up to
your brain are descendants of
this branch here.
Those are important;
blood to the brain is pretty
important.
Also, blood goes down through
the abdominal aorta,
so it splits in two directions
down the abdominal aorta,
That blood goes to your legs
and all of your visceral organs
that are below the heart.
This pattern repeats itself.
A pattern of flow,
branch, branch,
branch to ever smaller branches
so that you can serve ever
smaller regions of tissue.
Now, these vessels that
carry oxygenated blood are
called arteries and they have
thick muscular walls.
They're generally large
diameter, a centimeter or so,
there's a table in your book
that gives you the diameters of
different kinds of vessels.
They don't exchange materials
with their outside environment.
You can think of these as
conduits whose function is to
carry blood from the heart to a
particular organ,
but not to exchange nutrients,
not to exchange oxygen,
they're just carrying the
blood.
Eventually, if we looked for
example, at the branch of this
artery that goes to the
digestive tract,
for example,
to the organs of the gut where
digestion takes place,
you have continual branching of
these arteries until they become
very fine.
Those smallest conducting
pathways are called arterioles,
small arteries or arterioles.
Now they have the same basic
anatomy as an artery.
They have a muscular wall and
we'll talk about why that's
important in a minute and they
don't conduct any nutrients
across their surface,
they're just smaller.
Eventually those arterioles
branch into very fine vessels
called capillaries.
It's in these capillary beds,
which are illustrated here by
these little meshes,
where you have the smallest
diameter vessels.
They're feeding very particular
regions of tissue,
not your whole intestine but
only a very small region of the
intestine.
They have thin walls that lack
muscle;
they're not muscular walls.
It's in these capillary beds
that exchange of nutrients takes
place.
It's in these capillary beds
that exchange of oxygen,
carbon dioxide,
sugar, amino acids--that takes
place in capillaries.
Capillaries are not muscular
but they're able to transport
nutrients and molecules.
What's happened as we've
gone from the aorta,
a single vessel,
down to a capillary bed and
there are many,
many capillary beds in your
body.
Well, if I could take a
cross-section at any point--a
cross-section of this--then the
total flow rate into all the
capillaries,
the total flow rate through all
the capillaries has to be the
same as the flow rate through
the aorta.
By this process of branching
where this total flow of 5L/min
gets divided,
and divided,
and divided.
By the time it gets to a
capillary there's a very small
vessel carrying a very low flow
rate,
a very small fraction of this
5L/min, but there's now millions
of these of vessels all over
your body.
The blood is not flowing at a
large flow rate through a big
vessel, it's flowing at a much
smaller flow rate through many,
many vessels but the total has
to add up to the total that goes
in.
Well, you know the rest of
the story;
the capillaries merge after
they leave the tissue area.
They merge into small veins;
veins are collecting tubules
for de-oxygenated blood.
Now, this is blood that has
transported all of its nutrients
into the tissue that it serves
so it doesn't have a lot of
oxygen anymore.
It has a lot of waste products
now, the waste products that are
generated by these tissues
during metabolism.
It has a lot of carbon dioxide
which is the end product of
glucose metabolism,
it has acids that are generated
from metabolism,
it has the end products of
nitrogen metabolism,
which is chiefly urea and
ammonia, and that blood is
indicated here as a blue line.
Now, we usually show these
blue colored vessels,
usually show them as red and
blue, indicating lots of oxygen
red, not very much oxygen blue.
It's not just oxygen that's
changed.
The chemical composition has
changed dramatically,
there's a lot more carbon
dioxide,
there's a lot more urea,
there's a lot more of other
things coming out of
the--through the veins.
These small veins called
venules merge to form larger
veins, eventually very large
veins like the vena cava,
which come back to the right
side of the heart.
The right side of the heart is
anatomically very similar to the
left side.
There's an atrium,
there's a ventricle,
and blood flows out of the
ventricle through the pulmonary
artery to the lungs.
Now, notice that there's
something different here;
that on this side of the
circulatory system the part
where the blood is going out to
the rest of your body,
the artery contains oxygenated
blood.
On this side,
the pulmonary artery is an
artery--it has the same features
as a systemic artery like the
aorta.
It has a muscular wall,
for example,
and doesn't transport
nutrients, so it looks an artery
but it carries de-oxygenated
blood.
It's carrying it only one
place, back to the lungs.
It's collected in
the--collected on the other side
of the lungs now with oxygen and
returns to the point where we
started.
It's not just a closed
system, it's 2 closed systems.
One that we're going to call
the system that's fed by the
left side of the heart where
oxygenated blood goes out to the
whole periphery of your body and
is collected back by the right
side of your heart,
that's one closed system.
The second is the pulmonary
vascular system which is--the
force is generated by the right
side of the heart,
the flow is generated by the
right side of the heart,
flows only to the blood--to the
lungs and oxygenated blood comes
back to the left side,
so it's sort of like a figure 8;
one loop, a second loop.
What's the--if the rate of flow
is 5L/min through the aorta,
what's the flow rate in the
pulmonary artery?
They're two independent
systems, is that right?
What's the rate of flow in the
pulmonary artery?
It has to be 5L/min because
they're not independent.
They're really--I described
them as two closed systems,
really one closed system with a
loop, so the flow has to be the
same everywhere.
Let's talk about blood flow
in these vessels for a moment.
Now you have this picture of
the overall composition of the
circulatory system,
let's think about a very simple
problem that engineers have
thought about for hundreds of
years now.
Turns out to have a lot of
relevance in the cardiovascular
system, but has relevance
everywhere.
It has relevance in terms of
getting water to all the faucets
in this building.
You have to solve the same
problem, and that is generating
enough pressure to allow fluid
to move through a length of
tubing.
We'll think about a typical
length of tubing and we'll make
it as simple as possible.
It's a vessel that has a
constant diameter or radius and
a known length.
Fluid is going to flow in one
end of this vessel and it's
going to flow out the other,
and this is called internal
flow through a tube.
Turns out to be one of the
most well studied problems in
all of engineering;
I'm just going to tell you one
aspect of this.
Over a certain range of flow
rates--so this isn't true all
the time but it's going to be
true most of the time in our
circulatory system.
If I try to generate a flow
through this tube,
and the flow again is going to
be measured in something like
volume per minute,
so let's say 1L/min,
then the pressure that I need
to produce the pressure drop,
meaning, 'What's the pressure
difference from one side of the
tube to the other?'
I don't care about absolute
pressure I only care about a
difference in pressure between
one side and the other.
That difference in pressure is
proportional to the rate of flow
and that's what this equation
says here.
If I knew what the pressure
drop was the flow is
proportional to that and the
constant of proportionality is
this symbol R,
the resistance to flow.
We say that ΔP,
the drop in pressure over the
tubing is equal to R x flow
rate, ΔP = R x Q.
ΔP is a driving force,
pressure difference is a
driving force,
it's what causes the flow.
The flow is what results from
that, so ΔP = RQ.
Does that kind of equation
look familiar to anybody from
their high school physics?
Have you ever seen an equation
like that?
ΔP = RQ--driving force is
equal to some constant times a
flow.
Ever seen ΔV = RI?
Anybody know where that
equation comes from?
Student:
[inaudible]Professor
Mark Saltzman:
Circuits--it's the flow
through an electrical circuit.
What's the potential for flow
through an electrical circuit?
It's a driving force,
it's a voltage driving force,
you hook up a battery to a
length of wire,
for example.
What flows is electrons
current, and how much flows
through wire--how much flows
through a circuit in response to
a voltage change depends on a
property of the wire,
its resistance.
Thicker wires,
more cross-section has less
resistance, you can flow more
current through it,
thinner wires less.
It takes more--for a given
driving force,
for a given voltage less
current flows.
If you haven't seen that before
don't worry about it.
If you have seen it then
hopefully this will be useful in
thinking about this.
Which is the same thing,
that the pressure drop produces
as flow and how much you produce
for a given pressure drop
depends on a property called the
resistance.
Now the resistance is a
property of the geometry of the
vessel.
If I have a very--a vessel with
a very large diameter for a
given pressure drop I can create
a large flow.
For a vessel with a small
diameter, for that same pressure
drop I get less flow.
Imagine your--whatever your
favorite beverage is,
think about a milkshake because
those are difficult to draw
through straws.
If you're going to drink a
milkshake in your favorite
flavor, do you hope you get a
narrow straw or a big,
thick straw?
You would not pick the narrow
straw if you really like the
milkshake because you can only
generate a certain pressure drop
with your mouth.
If you generate that pressure
drop you're not going to get
much milkshake,
you're not going to get much
flow.
If I have a bigger tube,
same pressure drop,
bigger flow,
more milkshake.
Here the resistance,
it turns out,
we can for very simple vessels
like this, the resistance can be
related to the geometry of the
vessel.
That is, if I told you what
shape this vessel was how--what
it's diameter was and how long
it is you can calculate what the
resistance is from this formula
here,
R =
8µL/πr^(4). Now
you know what all these things
are,
you know what π
is, that's 3.14--π
is familiar;
r is the radius;
L is the length.
What's µ?
µ is a property of
the fluid and it's the
viscosity.
Milkshakes have high viscosity,
water has a low viscosity.
So, if you're going to drink
water you're not so worried
about how big your straw is.
You would accept a small straw
not just because milkshakes are
more delicious,
but because you don't need to
generate as much of a pressure
drop to move water as you do to
move a milkshake,
because milkshakes have a
higher viscosity.
The viscosity of the blood is
in general a constant,
it's a constant that is about 3
centipoise.
You can read in the book about
units of viscosity and how
they're defined.
It tells you something about
the force that you need in order
to cause the fluid to flow.
We're going to be dealing
with fluids in here that are
basically constant in their
viscosity.
Blood is about three times as
viscous as water.
Why is blood more viscous than
water, why is it harder to flow?
It's largely water,
it's mostly water,
that's the major component,
why isn't the viscosity of
blood the same as the viscosity
of water?
Student:
[Inaudible]Professor Mark
Saltzman: It has molecules
in it and so that makes a
difference.
It also has something else,
50% of the volume of blood is
cells.
A concentrated solution of
cells, like red blood cells,
is not as easy to flow as water
is, so that's why water is more
viscous.
If you wanted to then think
about how much flow there is
through this vessel,
and I told you what the
dimensions were,
and I told you that blood was
flowing,
you could calculate r
and then you would know
everything about flow through
this vessel.
You would know if I have a
pressure drop of so many units,
this is the flow that would
result.
Justin?Student:
[Inaudible]Professor Mark
Saltzman: The interior
surface of the tube could make a
difference.
Now it turns out for low flows,
surprisingly,
the surface of the tube doesn't
make a difference if the flow
rate is a below a certain level.
You're thinking about,
'What if the surface of the
tube is rough?'
Then, fluid that's flowing past
that surface is going to
experience more friction than if
it was flowing past a very
smooth surface.
At low flow rates the friction
that occurs when a liquid meets
a solid is so high that fluid at
the surface doesn't really move
at all.
For low rates,
fluid that's right at the
surface doesn't move at all,
and so roughness doesn't matter
when you have no flow there.
As the flow rate increases then
roughness becomes more
important.
Two things about it in our
vascular systems,
the flow rates are generally
low enough that you don't have
worry about that.
Our vessels are all really very
similar in terms of their
microscopic geometry,
and that all of the blood
vessels are covered with a
monolayer of cells called
endothelial cells that basically
form a blanket over the entire
surface of the inside of the
circulatory system.
There's very little frictional
resistance to flow.
That's a really good point.
Other questions?
Okay, so this equation sort
of makes sense,
right?
If you go on to study
Biomedical Engineering you'll
learn exactly where this
equation comes from and you'd be
able to derive it yourself.
For now, just accept it and it
is true for almost all the
vessels in our body,
under all the circumstances of
human physiology.
It tells us that resistance
varies with viscosity,
length, and radius and it
varies in kind of the way that
you would expect.
As viscosity of the fluid goes
up resistance goes up,
as the length of the tube goes
up, resistance goes up;
harder to pull through a long
straw than through a short
straw.
As the radius goes up
resistance goes down,
because the radius is in the
denominator here,
so as radius goes up the
resistance goes down.
Remarkably,
the resistance goes with the
fourth power of the radius.
What does that mean?
That means a 2-fold increase in
radius leads to a 16-fold
reduction.
A 16-fold reduction in
resistance;
so resistance is,
of all the parameters,
resistance is most sensitive to
radius.
Caitlin, did you have a
question?Student:
[Inaudible]Professor Mark
Saltzman: Yeah.
So what's the unit here?
Well let's think of it.
What unit would you measure
pressure in?
What's a unit of pressure?
Pascals (Pa) is one;
Newtons per millimeters squared
(N/m^(2)), so its force per area
is pressure.
What's a more commonly--those
are great units--sorry?
Pounds per square inch is
another unit,
PSI.
What's the pressure in the room
here?
1 Atmosphere (atm);
atmosphere is another pressure,
so relative to atmospheric
pressure and if you have a--any
others?
Millimeters of mercury (mmHg);
it turns out that all of those
are proper units for pressure
and you could convert between
one or the other as long as you
knew what the conversion factor
was between the--we usually
atmospheres to describe
Atmospheric Pressure.
Physiologists usually use
millimeters of mercury to
describe pressure in the
circulatory system.
So, 1 atm is 760 mmHg.
What are the blood
pressures that are relevant in
your circulatory system?
How many millimeters of mercury
do you think?
Anybody know what their blood
pressure is?
120/80 what?
They don't tell you that--120
mmHg over 80 mmHg.
We're going to talk about where
those numbers--where those
pressures actually exist in a
few minutes but that's the range
of pressures in your circulatory
system from 0 mmHg up to 120
mmHg;
roughly and we'll see you go
outside that range sometimes.
Now these are pressures
that are inside our circulatory
system so they must be--but
they're only a fraction of 1
atm, how can that be?
An atm is 760 mmHg,
that's the pressure in this
room.
If your blood pressure is
120/80 what does that mean?
Well, pressure is--we only care
about differences in pressure.
It's relative pressure that
matters not the absolute
pressure.
When you say your blood
pressure is 120/80 that means
120 mmHg above Atmospheric;
80 mmHg above Atmospheric.
Otherwise our blood vessels
would collapse because we're
surrounded by air that's 760
mmHg,
that to inflate those vessels
you have to be at pressures
above that.
Since all we care about are
pressure differences then the
pressure that you measure when
you measure blood pressure is
pressure above Atmospheric.
Does that make sense?
ΔP is in mmHg,
Q is in L/min,
so R is in units of
mmHg/(L/min).
It's in units of--if we just
write this out mmHg= R,
and this is L/min,
then R must be in units of
mmHg/(L/min).
This resistance changes,
it changes with geometry,
the most sensitive thing that
changes is the radius.
Imagine if you had a vessel
that could change its radius,
that could become smaller or
could become larger.
Then, that would be a vessel
that could regulate its
resistance.
If that vessel was faced with a
pressure drop,
let's say a pressure drop of 10
mmHg from beginning to end of
this vessel here,
and it could change its radius
then it could change the amount
of blood flow through it.
If it decreased its radius,
the rate of blood flow would go
down even though the pressure
drop stays the same.
If it expanded its radius,
the rate of low would go up
even though the pressure drop
remains the same.
Does that make sense?
I mentioned before that
arteries, arterioles,
small arteries,
all arteries have muscular
walls.
They have a special kind of
muscle called smooth muscle
which is arranged
circumferentially,
sort of wrapped around the
vessel, and that muscle can
contract to constrict the vessel
or relax to dilate the vessel.
One feature of arteries that
makes them important in
physiology is that they can
change their diameter.
When they change their diameter
they change their resistance,
and they change the flow that
goes through them at a fixed
pressure drop.
Why is that important?
Well, let's say you get up and
you start running;
lecture's over,
you're excited to tell your
friends about it,
so you get up and you run out
of class, and you went from
resting to running.
Now, all of a sudden your
muscles need more blood because
they're going to start
generating work.
The blood--the arteries that
feed your muscles are going to
contract--are going to relax.
The diameter will get bigger,
the pressure available is the
same, they'll get more flow.
That flow has to come from
somewhere so simultaneously
other vessels are constricting.
What would those vessels be?
Well, probably the vessels that
go to non-essential functions
when you're starting to run.
Not your brain,
you want to keep blood flow to
your brain so you don't run into
the wall, but you might have
less flow going to your
digestive organs.
You'd stop digestion for some
period or slow it down;
provide less blood flow for
that purpose.
Does that make sense?
Well if we look at how
pressure varies through the
circulatory system,
your blood pressure is
generated by the heart.
Blood pressure ranges between
120/80 mmHg.
We're going to see this in a
few minutes but those numbers
represent the maximum pressure
and the minimum pressure during
a heartbeat.
The maximum pressure occurs
when the heart is actively
contracting or beating.
We'll talk about that more in a
few minutes and we'll talk about
that in great detail on
Thursday.
Then it drops when the heart is
relaxing.
Blood pressure is generated in
a cyclic fashion because of
beating of the heart,
contract, relax,
contract, relax,
120,80, 120,80,
120,80.
That pressure of 120/80 gets
transmitted to the aorta.
The pressure in the aorta is
roughly the same as it is in the
heart.
Now why is that?
Why isn't there a pressure drop
through the aorta?
Well there is a pressure drop
through the aorta but it's very
small.
The reason it's small is that
the aorta is big;
has a large diameter and so
it's resistance is low.
In order to generate 5L/min of
flow you don't need much
pressure.
In order to--even this whopping
flow that goes through the
aorta, because of the large size
of the aorta,
you don't need much pressure
drop to generate that flow.
If I measured pressure drop
over the aorta,
from the beginning to the end
of the aorta,
I'd find that pressure doesn't
really change very much and
that's what's shown in this
diagram here.
As I move from the beginning of
the aorta to the end there must
be some drop in pressure.
Has to be, otherwise blood
wouldn't flow at all but there's
not very much of a drop.
Similarly,
the large arteries would still
have large diameters,
have low resistances.
Doesn't take much pressure to
move a flow rate through those
large arteries,
so over the large arteries
there's not much pressure drop
either.
This would be the branches,
like the main branch that goes
to your arm, the branch that
goes to the carotids that goes
up to your brain,
the branch that goes down to
your leg called the femoral
artery.
Still large arteries,
large diameter,
not much pressure drop needed
to generate a large flow.
As we move to more
branches, smaller and smaller
vessels, it turns out at some
point,
and the point at which this
occurs is the arterioles,
the diameter has gotten small
enough that the resistance
becomes significant compared to
the flow rate.
Now, there's so many branches
that occurred before this that
these individual vessels aren't
getting 5L/min,
they're only getting some small
fraction of that.
There might be 1,000 of these
arterioles, let's say,
and so they're all getting
1/1,000th of the maximum flow.
How could that pressure drop be
more?
How come they need a big
pressure drop,
which is what's illustrated
here.
It takes a substantial drop in
pressure in order to move this
lower flow rate through these
smaller vessels.
How could that be?
Well remember that the
resistance changed with
r^(4).
That means that as the radius
goes down it's not a linear
relationship between the radius
going down and the resistance
going up,
it's a 4^(th)-power
relationship.
As it gets small resistance
starts to become very,
very high.
When you divide a flow between
vessels that's kind of a linear
process, it goes two ways,
it goes four ways,
it goes eight--but this
r^(4) starts to dominate
when you get down to a certain
radius.
It turns out that that's at the
level of the arterioles.
Most of the pressure drop on
this side of your--on this left
side of the cardiovascular
system,
most of the pressure drop
occurs at the level of the
r^(4).
There's a significant pressure
drop through the arteries and
then not much pressure drop at
all through the veins.
The veins are low pressure
vessels, generally large in
diameter, and not much
resistance to flow.
Now why--if you were going
to design a system like this,
is there any advantage to
having the largest pressure drop
occur over the arterioles,
or the largest relative
resistance occurring over the
arterioles?
Well, there's a huge advantage
because wherever the pressure
drop occurs, that's the place
where you have the best
opportunity to adjust the flow
because small changes in radius
are going to lead to large
changes in relative resistance.
It's at the arterioles,
or the very smallest arteries
that are supplying blood to our
tissues, where we have the most
control over flow rate.
That makes sense because
sometimes you're not making big
movements in blood flow,
like saying,
'I want very little blood flow
to my gut and a lot of blood
flow to my muscle'--that's a big
change.
You're making smaller changes
within a tissue;
like, 'I want more blood to go
to the region of my brain that's
involved in talking and less to
the region in my brain that's
involved in reading.'
When you're talking versus
reading you want to regulate
where the blood flows on a more
smaller scale,
on the scale of a tissue,
and its arterioleswhich are
branching within tissues that
have the capacity to do that.
You could do something like
this, let's say this is a very
imaginary situation,
but I drew it just for
illustrative purposes.
Let's say that this was an
artery that was feeding some
tissue and so there was a blood
flow that's coming in here and
it splits five different ways.
So, here are five different
regions of the tissue that are
served by these arterioles.
Now, I could draw this--one of
the nice things if you do know
this analogy between this
equation describing pressure
drop flow relationships and this
equation describing voltage
current relationships,
is that you could draw a
diagram for pressure flow
relationships.
It looks just like a circuit
diagram because the equations
are the same.
That's what I've done here
is taken this anatomical
picture, cartoon,
and converted it into an
equivalent circuit where I'm not
thinking about flow of electrons
now or current,
I'm thinking about flow of
fluid.
I'm drawing a resistance not as
an electrical resistance but as
a flow resistance;
as this kind of a resistance.
Let's say that the blood is
flowing in at a rate of 100
mm/min and that the resistance
here is one unit,
and the resistance here on each
of these smaller segments is
smaller--or higher.
These are smaller--it branches
into smaller vessels so they
have a higher resistance,
and it's five units here and
one unit here and the units are
these.
Well, if all the resistances
are the same then the--of the
blood that flows in here it's
going to flow equally through
each one of these tubes,
and so basically this 100
mm/min of flow gets split five
ways.
You could do this
calculation yourself;
the pressure drop here is just
1 mmHg and if you did this
calculation it should work out
okay,
the total resistance of this
circuit with 1 mmHg gives you
100 mm/min of flow.
What if one of these vessels
changes its diameter?
Let's say it's this one and
let's say that it shrinks its
diameter so this resistance goes
up.
Goes up here from five units to
100 units, so I made a big
change in that,
goes up by a factor of 20 so
that would be about a factor of
two difference in radius,
because of the 4^(th)-power.
If this one goes up to 100 now
there's more resistance through
this particular vessel here and
the same resistance through
these three,
so what happens?
Less flow goes through this
vessel and the same flow goes
through the others,
so that the total flow now is
89 mm/min.
Now why did the total--you
could understand sort of because
that's pretty obvious right why,
when I decreased the radius and
increased the resistance of this
vessel, why less flow went
through it than through its
partners,
that kind of makes sense.
You've got a smaller vessel so
now I've got four large vessels
and one smaller one,
and if I feed them with the
same pressure drop more of the
flow is going to go through the
big vessels because there's
least resistance there.
Fluid takes the path of least
resistance, some goes through
but not as much,
that makes sense.
Why does the overall flow
through the whole circuit
change?
Why does the overall flow
through this whole thing change?
Well, because if any one of the
vessels in here increases its
resistance it increases the
overall resistance of the
aggregate of vessels.
If the pressure that I have
available is constant,
and it is, the pressure I have
available is constant in this
example,
it's 1 mmHg,
then when the whole thing
increases its resistance that
means the flow rate through the
whole circuit has to go down as
well.
There's two things that are
really illustrated here.
One is that you could regulate
in pathways that are competing
with each other for flow then
the relative resistance between
equal members determines how
much flow goes to each vessel;
that's one thing that's
illustrated.
The other is that the
whole--the flow through this
whole piece of the circulatory
system,
this whole part of the network,
depends on the resistance of
every single vessel within it.
One vessel changing its
resistance changes the whole
thing.
That's a feature of our
circulatory system.
If I want more blood to go to
the brain there's two ways to
get more blood to the brain.
Well, there's more than two
ways, but two ways as long as
the pressure--blood pressure
stays the same.
One way is to dilate the
vessels of the brain to create
less resistance there.
The other way is to constrict
the vessels somewhere else to
create more resistance there.
Either one of those is going to
get me more flow to the brain.
Because it's a closed system,
everything is interconnected
and a change in flow in one
place can influence flows
throughout the whole system.
Does this make sense?
I want to go back to this
picture, now,
and talk about what we've
learned in terms of flows.
Now you can think about this in
a little bit different way,
that if there's flows going
through here and here,
how much goes each direction,
depends on the relative
resistance between this vessel
and this vessel.
It does depend on the relative
size between this vessel and
this vessel, but it depends on
the relative resistance of the
whole network after it as well.
It depends on the whole
resistance--even if you have a
big tube here,
if it's feeding lots of very
constricted tubes down here,
the overall resistance that the
fluid experiences at this branch
point is going to be high in
this direction.
The whole--the resistance of
the whole circuit matters.
Think about that.Let's now
think about what's driving this
flow.
That is, what's generating the
pressure and you know that
what's generating the pressure
is the heart.
That's the only place where
pressure gets generated within
this closed system.
The pressure that's generated
by the heart,
which is experienced by the
aorta, is what drives flow
throughout all of these vessels.
The only opportunity that
individual vessels out here have
to adjust how much flow goes to
them is by changing their
diameters, which they do.
The only way to change the
overall rate of flow is to
change the overall pressure drop
which can only be done by the
heart.
How does the heart work?
Well let me just start this and
then we'll talk about this in
much more detail next time.
This is a picture of the heart.
It sits--about the size of your
fist, basically in the center of
your chest, but shifted a little
bit to the left hand side.
You can find out where yours
are by just putting a finger
there and feeling where you can
feel the heartbeat,
it's fairly easy to define.
The apex of the heart,
the tip of it sits downward
like that and the major vessels
are near the center line;
so the tip is down here,
the major vessels are near the
center line.
The heart has muscular walls.
The walls of the heart are
called the myocardium,
muscle heart.
'Myo' is muscle,
'cardium' is heart and there's
muscular walls so it's a big
muscle basically.
There are blood vessels on
the surface and so this is the
aorta coming up here.
Where this arrow is here is the
aorta, this is the aortic arch,
and then goes down to the
abdomen and up to the brain.
These are those coronary
vessels that I mentioned
earlier.
The first branches off of the
aorta are to these coronary
arteries which branch over the
surface of the heart,
so now you can see this
branching pattern more clearly
in one particular tissue.
Here's a coronary artery the
blood flows through,
here's a branch,
one goes more to the left,
one goes more down the center
line, branches again,
branches again,
and by continuously branching
you define smaller arteries
which serve smaller regions of
tissue.
These blood vessels provide
nutrients to the muscle in the
muscular walls of the heart.
There are inflows and
outflows, you know this is the
aorta up here;
this is the vena cava which is
bringing blood back down here.
The heart has a right side and
a left side.
If I divided it into the right
side and the left side I could
put a dotted line here,
where the left ventricle and
left atrium are on this side of
the line and the right ventricle
and the right atrium are on the
other side of the line.
What we'll see when we talk
about this more next time,
is that the left side of the
heart is more muscular and
slightly larger than the right
side of the heart,
even though they're generating
the same flow rate.
The left side of the heart is
larger.
If we reduce this down to a
simpler picture,
this is where I'm going to
start next time and take out the
anatomy and replace it with this
cartoon,
you can see that there's a
difference between the thickness
of the muscular walls.
The muscular walls on the left
side of the heart are slightly
thicker than the muscular walls
on the right side of the heart.
You don't see them in this
diagram, but if you looked at a
real heart you would see that
that's true.
The right and left side of the
heart are divided into two
chambers, atrium,
ventricle,
atrium, ventricle:
left ventricle,
right ventricle,
right atrium,
left atrium.
The inflows and the
outflows are guarded by valves
and there are four valves that
are important here.
There's a valve which is shown
by this sort of floppy structure
here between the left atrium and
the left ventricle.
There's another valve between
the left ventricle and the
aorta.
Same thing on the other side,
there's a valve between the
right atrium and the right
ventricle,
and between the right ventricle
and the pulmonary artery.
What we're going to talk about
next time is how these valves
function in order to convert the
work that's done by the heart in
generating pressure into a
directional flow.
That's what we'll start with
next time.
 
