Blood Pressure

October 11, 2019 0 By Bertrand Dibbert

– In this video, we’re
talking about blood pressure. Blood pressure’s a very
important consideration in cardiovascular health. It’s what keeps blood flowing
through the vasculature. So your body can’t
let it get too low or blood won’t circulate. But it’s also important that
blood pressure not be too high. Chronic high blood pressure can
damage the arteries as well as the heart. When we’re talking
about blood pressure, we’re talking about the force
exerted on a vessel wall by the contained blood. So if here is your
vessel wall and you have blood flowing
through it, the blood that’s in that blood
vessel kind of pushes out on the vessel wall. And that’s what
blood pressure is. Usually, when we talk
about blood pressure, we’re talking about the systemic
arterial blood pressure. So the arteries of
the systemic circuit, what’s the pressure in those
arteries and usually the larger arteries near the heart? When we measure blood
pressure, we measure it in millimeters mercury. Blood pressure is
related to blood flow and to resistance
in the vessels. Blood flow is the
total volume of blood flowing through a vessel,
an organ, or an organism in a given period. If you’re talking
about total flow, you’re talking about
the volume of blood flowing through the entire
organism, the entire vascular system. So it’s equal to cardiac output. Total flow is essentially
constant at rest. However, if you are
talking about blood flow through a particular
blood vessel or through an
individual organ, you can get huge variations
in blood flow according to the
needs of the organism or of that particular organ. For example, take the skin. When you’re warm and
you need to cool down, blood flow to the skin
increases so you can shed heat. But when it’s cold and you
need to conserve that heat, blood flow to the
skin goes down. When we’re talking about
blood flow to a tissue or to an organ, we use
the term perfusion. So you talk about perfusion
of an organ or perfusion of a tissue. Blood flow is given in units
of milliliters per minute. So it’s a rate, the
amount of blood flowing through a particular
tissue or an organ or through the entire
organism each minute. Now, resistance is
opposition to blood flow. So resistance is
basically the force that blood needs to
overcome in order to flow. And it pretty much has
to do with friction. So if you rub your
hands together, that generates a little bit of
heat because of the friction from your hands
rubbing together. Well, there’s friction when
blood passes through a blood vessel as well. Friction is primarily in the
systemic circulation, so far from the heart. And that’s because as you
get further from the heart as the blood flows, the
vessels get smaller. And the smaller a blood
vessel is, the more resistance it exerts. This friction in the systemic
circulation– resistance in the systemic
circulation– is referred to as peripheral resistance. So what does this have to
do with blood pressure? Well, it’s the heart pumping
that generates blood flow. That’s what gets blood flowing. Blood pressure is what
happens when that flow is opposed by resistance. So if resistance
dropped to zero, then blood pressure
would drop to zero, and you wouldn’t get much flow. So blood resistance
is basically friction of blood along the vessel wall. And there are three things
that affect this friction. Blood viscosity is the
first one, so basically the thickness of blood. This is pretty constant. So your blood viscosity
really doesn’t change unless you have something
going on with your red blood cells. If you have polycythemia,
which is an increased overproduction of
red blood cells, that will increase
your viscosity. But if you have anemia, and
you have decreased production of red blood cells, it will
decrease your blood viscosity. Blood vessel length
is another factor that determines resistance. The longer the vessel, the
more encounters you have between blood and
vessel wall, so the more resistance you have. However, an adult’s blood
vessel length is also constant. If you’re talking about
children who are still growing, yes, their blood vessels get
longer just like their bones and muscles do. So you do see increase
in resistance in children as they grow. But in adults, this is
pretty much unchanging. So these two are constants. This third factor down
here, blood vessel diameter, is highly changeable. So blood vessel
diameter plays a role in resistance and
controlling resistance and thereby controlling
blood pressure because if you have a smaller
blood vessel diameter, you have more resistance. And a bigger blood
vessel diameter gives you less resistance
because there’s less blood coming into
contact with the vessel wall if the vessel wall is expanded,
if the blood vessel is bigger. Blood vessel diameter is
probably the most important of these three
sources of resistance because it’s so changeable. That means that this can
be manipulated by your body in order to regulate
blood pressure. So there’s this relationship
between blood flow, blood pressure, and
resistance that can be summed up by this equation. Blood flow is equal to
the difference in pressure divided by resistance. So what this basically means
is that flow is directly proportional to pressure. So if you have greater
difference in pressure, you have greater flow. But it’s inversely
proportional to resistance. If you have greater
resistance, that actually decreases your blood flow. You can take this
equation, and you can flip it around to say
that blood pressure equals flow times resistance. But now, since flow is basically
cardiac output when you’re talking about the
system as a whole, you could also say that
pressure equals cardiac output times resistance. So now we can see that if
cardiac output goes up, you get an increase
in blood pressure. Let me just say blood
pressure up there too, BP. But also if resistance
goes up, you get an increase
in blood pressure. Your body manipulates resistance
by adjusting blood vessel diameter. And this is an important way
to control blood pressure, to regulate blood pressure,
in the short term. So if we’re talking about
more immediate adjustments to maintain a stable
blood pressure, we’re talking about changes
in blood vessel diameter. If you are talking about longer
term changes in blood pressure, then you have to get
the kidneys involved. And what the kidneys
can do is the kidneys can regulate the amount of water
that is retained in the body or lost to the urine. And so by regulating
water retention, the kidneys can
adjust cardiac output. If you’re holding
onto more water, blood volume is going to
go up and cardiac output is going to go up. If you are losing more
water in your urine, blood volume is going to
go down and cardiac output is going to go down. So changes in resistance
by changing blood diameter, short term regulation. Changes in cardiac output
by regulating blood volume is longer term regulation. I mentioned that when we’re
talking about blood pressure, we’re usually talking about
blood pressure in the arteries. If you’re talking about
pressure in the capillaries or the veins, blood pressure
is significantly lower in both of those types
of blood vessels. This diagram shows you the
changes in blood pressure as blood flows further
away from the heart. So starting at the heart,
blood moves out to the aorta– the largest artery– and then through the arteries,
through the arterioles, through the capillaries,
venules, veins, and eventually into the venae cavae So blood pressure is
highest at the aorta because the aorta’s closest
to the pumping action of the heart. In the aorta, you also
have massive fluctuations in blood pressure as the
heart contracts and relaxes. So your systolic pressure– that’s the top of this
line, this top line– that’s the pressure in
the aorta and arteries when the heart is
contracting, when the ventricles are contracted. The diastolic pressure–
this lower line– is the pressure when the
ventricles are relaxed. The blood isn’t actually
feeling that push from the heart at the moment,
or not feeling as much of a push from the heart, so the
pressure goes down. This fluctuation, this high
systolic, low diastolic, high systolic, low diastolic,
blah, blah, blah, and blah. That’s why you can
feel someone’s pulse when you palpate their
superficial arteries. What you’re actually feeling
is that rhythmic push of systolic pressure
with each heartbeat. Now, as you continue on from the
arteries into the capillaries, blood pressure dissipates
because in the arteries, you have a few vessels, but
those arterioles branch out into capillary beds. So that blood is distributed
among several different vessels, and that
blood pressure is distributed among several
different capillaries as well. So it’s, in essence, dissipated
through the capillaries. And by the time blood
leaves the capillaries, blood pressure is very,
very low in the veins. That’s why the veins
need assistance from the respiratory pump
and from skeletal muscle in order to move
blood through them. And also why you need venous
valves to prevent backflow. So veins essentially are
feeling essentially no push from the heart. And by the time you get
back to the venae cavae at the right atrium, blood
pressure is essentially zero. So you saw what
blood pressure looks like on the graph
of systemic blood pressure on the previous slide. I just want to lay out some
definitions for you here. Systolic blood pressure
is blood pressure at the peak of
ventricular systole. Diastolic blood
pressure, blood pressure at ventricular diastole. Now, when you take
these two pressures, you can calculate
the pulse pressure. The pulse pressure is just the
difference between the two, this difference between
systolic blood pressure and diastolic pressure. The mean arterial
pressure is diastolic plus one third of
the pulse pressure. So it’s your diastolic blood
pressure plus the pulse pressure divided by three. So let’s do a quick calculation
just to see how that works. If you take a typical,
average, normal blood pressure of, say, 120 over 80. 120 is your systolic
blood pressure, 80 is your diastolic
blood pressure. So to calculate the
mean arterial pressure, you take your diastolic
plus the difference between systolic and
diastolic, so 120 minus 80. And then you divide that
difference by three. So you get 80 plus 40 over
three, which works out to about 93 millimeters mercury
as your mean arterial pressure. Notice that your mean
arterial pressure is much closer to the
diastolic pressure than it is to the
systolic pressure. Notice that diastolic
pressure counts fully towards the mean arterial
pressure, and pulse pressure, you’re only taking one
third of the difference between diastolic and systolic. So it’s not a strict average. And the reason why is
because diastole actually lasts longer than systole. If you look over here,
ventricular diastole takes up about 2/3
of the cardiac cycle, and ventricular systole,
about one third. So this is a weighted
average, and it’s weighted towards
the amount of time that the arteries
spend experiencing diastolic pressure versus
experiencing systolic pressure. Mean arterial pressure
is useful clinically because in some cases, it’s
actually a more accurate clinical predictor. So a more accurate
predictor of problems, like if you are trying to
predict a pregnant woman’s risk of developing pre-eclampsia. Pre-eclampsia is dangerously
high blood pressure during pregnancy. Mean arterial
pressure is actually a better predictor of
that than just looking at systolic and diastolic. Mean arterial pressure also
indicates the average pressure that’s pushing blood through the
organs, the average perfusion pressure. So it can be a better
indicator of how well organs are being supplied with blood. If your systolic
blood pressure is lower than about 100
millimeters mercury, you may be diagnosed
with hypotension. Now, there is some variation,
individual variation, in terms of blood pressure. So for some people a
slightly lower blood pressure may be perfectly normal. But if you typically have a
normal blood pressure, and then your blood pressure changes
and now your blood pressure is lower than usual, it
may indicate that you have another problem going on. Perhaps something nutritionally
or some endocrine problem affecting your blood pressure. So having a slightly
lower-than-normal blood pressure is not
usually a problem, except for what it might
indicate about your health otherwise. However, if you have an acute
drop in blood pressure– an acute severe drop
in blood pressure is a condition called shock. And this can be a
problem because all of a sudden your organs aren’t
getting the blood that they need. There are a number of different
things that can cause shock. A severe drop in blood
volume is referred to as circulatory shock or
hypovolemic shock, low blood volume shock. And this can happen
during dehydration, during severe burns, blood loss. Hemorrhage or severe,
severe bleeding will cause a drop
in blood volume, which drops your cardiac output
and drops your blood pressure. Damage to the heart can
cause shock as well. This is referred to
as cardiogenic shock. So if you have an
arrhythmia or if you have a myocardial
infarction or valvular disease, it can lead to
cardiogenic shock. If you have blockage
of blood flow to either the heart
or lungs, it’s referred to as
obstructive shock. And this most commonly comes
from a pulmonary embolism, which is a loosened blood
clot that blocks capillaries in the lungs. Lastly, if you have
widespread systemic peripheral vasodilation, that can lead to
shock as well because remember, as the blood vessels
dilate, as they get bigger, resistance goes down so
blood pressure goes down. And widespread vasodilation
can be neruogenic, which means that it’s
resulting from damage to the brain or the
sympathetic nervous system. So there are vasomotor
centers in the brain that control vasoconstriction
and dilation, that control the size of
your blood vessels, the diameter of
your blood vessels. And if you have
damage to the brain, those vasomotor centers
can stop doing their job. Spinal cord injury can
also keep those signals from getting to
the blood vessels. You can also have this
widespread peripheral vasodilation in response to
inflammatory stimulators. So you can have septic shock. Septic shock is where you
have a bacteria in your body that are releasing
endotoxins that stimulate widespread inflammation. The inflammatory molecule
histamine is a vasodilator. So if you have too
much histamine release, you have too much vasodilation,
and you go into septic shock. You can also get histamine
released in response to allergies. And if you know anyone
with a severe peanut allergy or a severe
allergy to bees, you may be familiar with
the term anaphylactic shock. Anaphylactic shock is where you
have a severe allergic reaction causing this widespread
peripheral vasodilation. High blood pressure is
called hypertension. So hypotension is
low blood pressure, hypertension is
high blood pressure. It’s typically classified as a
blood pressure greater than 140 over 90. High blood pressure is normal
under certain conditions. When you have a fever, your
blood pressure tends to rise. When you’re exercising,
your blood pressure goes up. When you’re upset, when
you’re emotionally activated, your blood pressure
goes up as well. This is all perfectly normal,
but it’s also temporary. When you calm down, your blood
pressure comes back down. When you stop exercising, your
blood pressure comes back down. When your fever is gone, when
your kick that bacteria out of your system, your blood
pressure comes back down. The problem becomes when you
have chronic hypertension, when your blood pressure
doesn’t come back down. Chronic hypertension is
bad for the arteries, and it’s bad for the heart. Think about after – the
hypertension increases the afterload– remember,
afterload is the pressure in the aorta– that the left ventricle
has to overcome in order to eject blood out to the body. High blood pressure
increases the afterload, and the ventricles
have to work harder. What happens as the
ventricles work harder is that the myocardium
gets bigger, so you see an enlargement. It’s trying to
respond to the load, just like if you lift weights,
your muscles will get bigger. If you are constantly pushing
against this high blood pressure, your heart
will get bigger. However, over time that
enlargement gets too big, and the blood flow
to the myocardium– the coronary circulation–
it can’t keep up with that increased tissue mass. And so your myocardium
may be getting bigger, but it’s actually
starved for blood. So it may be bigger,
but it’s getting weaker. Chronic hypertension also
damages the arteries. Remember, blood
pressure is the force of blood pushing on
the wall of the artery. And if you’re
pushing too hard, it can lead to inflammation
and thickening of the arterial wall. And if the arterial wall becomes
thickened, it can’t flex, it can’t expand and
contract the way it needs to with the pulsing of the heart. Chronic hypertension,
there are a number of factors that play into it. Genetics is one. High blood pressure tends
to run in the family. High salt or high fat intake can
lead to chronic hypertension, although it’s thought
that not everyone responds to salt in the same way. Some people have blood pressure
that is very sensitive to salt, and some people
it doesn’t really make that much of a difference. Stress, chronic stress can
lead to high blood pressure. Obesity can increase
blood pressure. Smoking. There are endocrine
disorders that affect blood pressure as well. If you produce high
levels of aldosterone, you are going to be
holding on to more water and you’re going to have
higher blood pressure. I want to take a
couple of minutes and step away from talking
about systemic arterial blood pressure to talk about some
special concerns with blood pressure in the
capillaries for a moment. Blood pressure
drops significantly in the capillaries as the
blood from an arterial gets diffused into the
networked capillary bed. So pressure in the arteriole is
around 50 millimeters mercury, but in the capillaries it
drops to 15 to 35 millimeters mercury. Blood pressure in
the capillaries needs to be delicately balanced. It has to be high
enough to force fluid out of the bloodstream
into the tissues. But remember, capillaries
have a very thin wall, just the tunica intima. So if blood pressure
gets too high, it’s going to rupture
those capillaries. You need to have blood
pressure low enough so that the capillaries
don’t burst. It’s a delicate balance. That forcing fluid
out of the capillaries is an important component
of bringing nutrients to the tissues. There is a constant exchange
of blood plasma and tissue interstitial fluid called
the bulk fluid flow. And it depends on two
things, hydrostatic pressure and the osmotic pressure
in the capillaries. Hydrostatic pressure
is the pressure of blood against the
walls of the capillary. Basically, it’s the
capillary blood pressure. And the hydrostatic
pressure pushes fluid out of the capillary. So the hydrostatic
pressure in the capillary is a force that pushes
plasma out into the tissues. Osmotic pressure– when you
think osmotic, think osmosis. Osmotic pressure is
sometimes referred to as colloid osmotic
pressure, or sometimes you may hear the term
oncotic pressure. And that’s the osmotic pressure
exerted by proteins, primarily by proteins in the blood. So the proteins are solutes
that cannot diffuse out of the capillary. They’re too big to
leave the capillary. So they sit in the capillary,
they stay in the capillary. But because they’re
solutes, they end up exerting a force
that tends to pull water into the capillary. So you can think of the
proteins in the capillary as kind of sucking
water back in. So hydrostatic pressure
forces plasma out. Oncotic pressure
sucks fluid back in. These are two
counterbalancing forces, they act in opposite directions. Now, in the interstitial
fluid, the interstitial fluid has essentially zero
hydrostatic pressure. It’s a huge space under
normal conditions. The interstitial
fluid also tends to have very few proteins. So it has a little bit
of an osmotic pressure but just a little bit, very low. The hydrostatic pressure in
the capillary and interstitial fluid, and the
osmotic pressure in the capillary and
interstitial fluid, when you kind of take all of
these and put them together, you get the net
filtration pressure. And the net filtration
pressure basically is a way to calculate the
balance of the pressures and determine whether fluid
is going to be pushed out or pulled back in
to the capillary. When you’re calculating
net filtration pressure, you basically want to
look at the difference in hydrostatic pressure
minus the difference in osmotic pressure. Now let’s look at the diagram
to actually work that out. So we’re looking at a very
simplified capillary bed, a single capillary. Blood flows from the
arteriole to the venule, so the direction of
blood flow is like that. At the arteriole end,
capillary hydrostatic pressure is as high as it’s going to be. So at the arteriole end,
capillary hydrostatic pressure is at 35 millimeters
mercury, it’s essentially zero in
the interstitial fluid, in the healthy
interstitial fluid. So the net hydrostatic
pressure is 35 millimeters. Now, the osmotic
pressure in the capillary is about 26 millimeters mercury. And in the interstitial
fluid it is one. So the net osmotic pressure
is 25 millimeters mercury. Now, you take 35 minus 25, and
your net filtration pressure is 10. And notice that it’s positive. That means that the net
hydrostatic pressure is greater than the net osmotic pressure. Positive filtration
pressure is going to push plasma out at the
arteriole end of the capillary. But as fluid leaves
the capillary, the hydrostatic
pressure goes down. If there’s less fluid to
push against the walls of the capillary,
pressure goes down. So when you’re looking
over here at the venule end of the capillary bed,
your hydrostatic pressure in the capillary is down
to 17 millimeters mercury. So that gives you a net
hydrostatic pressure of 17. Net osmotic pressure is still
the same, that doesn’t change. So you have 26 in the
capillary, 1 millimeter mercury in interstitial fluid, and a
net osmotic pressure of 25. But now, if you subtract
these, 17 minus 25, gives you negative 8
millimeters mercury. The osmotic pressure is greater
than the hydrostatic pressure. So at the venule end of the
capillary, some of that fluid is going to be drawn back in. So there’s a balance. You have plasma being
pushed out and some of the interstitial fluid
being taken back in. Anything that interferes
with this balance of flow can lead to edema. Edema is an accumulation
of fluid in the tissues. What you’re looking
at here is a condition called pitting edema,
which is basically where the edema is so severe
that when you depress the skin, it stays pitted even after
you remove your finger. Edema can happen when you have
inflammation or vessel damage. If you’ve ever had a bruise
that was a little bit swollen, you are familiar with edema. It’s also seen
with hypertension. Elevated arterial
pressure means that you have elevated hydrostatic
pressure in the capillaries, and you get more
blood forced out. So an increase in hydrostatic
pressure can lead to edema. You also see edema
when you have impaired synthesis of blood proteins. Remember, it’s the
proteins in plasma that are largely responsible
for the oncotic pressure, particularly albumin, the most
abundant protein in the blood. So if you can’t make
these plasma proteins, fluid is not going to be
sucked into the capillaries. You’re not going to have
as much osmotic pressure in the capillary,
and you’re not going to get fluid taken back
up out of the tissue. In this situation,
you can get edema, but you also get
fluid accumulating in the abdominal
cavity called ascites. And that’s why if you
have ever seen pictures of starving children
with swollen bellies, part of the reason why
their bellies are swollen is because they have
fluid accumulating in their abdominal
cavity because they aren’t eating enough protein. They aren’t taking
in enough protein to be able to make albumin
and other plasma proteins. You also see this swollen belly
fluid accumulation in patients with failing livers
because the liver is the organ that makes albumin
and most of the other plasma proteins. So if your liver isn’t
functioning well, like if you have alcoholic
cirrhosis or some other damage to the liver, if your liver
can’t make plasma proteins, your capillary osmotic
pressure is going to decrease and you’ll get more fluid
staying in the tissues and accumulating in
the abdominal cavity. So after studying
this video, you should be able to talk about
the relationships between blood flow, blood pressure,
and resistance. You should be able to talk
about where resistance comes from, a little bit about
how it’s regulated, though there’s another
video that will go into much more detail on that. Should be able to talk
about blood pressure through the circulation,
differences between arteries and veins and capillaries. You should be able to
recognize these terms and either describe how to
measure or calculate them. Should know the difference
between hypotension and hypertension and the
health problems associated with chronic hypertension. A little bit about shock. And you should be
able to talk about how hydrostatic pressure
and osmotic pressure contribute to exchange of
fluid at the capillaries.