Articles

Blood Vessels, Part 2: Crash Course A&P #28

September 11, 2019


Why is everybody so worried about high blood
pressure? I mean, of all of the millions of things that could go
fatally wrong in your body at any given moment, it doesn’t seem like the biggest threat you face is
that your blood might be pumping a little too hard. A heartbeat that’s too strong? Isn’t that
like an awesome song that’s played too loud? Or nachos with too much cheese?! Well, no.
The fact is: You’re more likely to die from diseases related to
your cardiovascular system than anything else. And probably you would like to know why. Well, we know that blood pressure is the circulatory
system’s way of getting your five liters of blood flowing throughout your body, so that your
tissues can get the oxygen and nutrients they need. But chronic high blood pressure, or hypertension,
can cause serious damage to both the heart that creates the high pressure, and to the blood
vessels that have to withstand that extra pressure. Over time, the increased force of blood against
the arterial walls can make them stiffen, leak, or rupture, while the heart itself may simply wear out
from all the extra work it’s doing to keep blood moving. Luckily, your nervous and endocrine systems
have some tricks to try and balance blood pressure when it gets too high or too low,
in an effort to create homeostatic balance. But, even with these defenses in place, if
your blood pressure stays out of balance long enough, things will start going really, really
wrong. One in three adults in the U.S. has high blood
pressure — that’s two in six, that’s three in nine — and often there are often
no symptoms until it’s too late. Just like your favorite song turned up to
11, or an unlimited supply of nacho cheese, even your own life-giving blood can become…too
much of a good thing. Part of what might seem confusing about high
blood pressure is the seemingly random list of things that can contribute to it, some
of which are easier to control than others. There’s emotional stress. Physical exertion.
Dehydration. Too much salt on your mashed potatoes. Or too much butter on your bacon.
Why are we buttering bacon? But all these things can in some way affect
how efficiently your blood flows. But in order to understand why your heart hates french fries, you
first have to understand how blood flow works. When we talk about blood flow, what we’re
talking about is the volume of blood flowing through any given vessel, or through the circulatory
system as a whole, per minute. This is also called cardiac output, and it’s
determined by the blood volume pumped during one beat, and the number of beats per minute. Now, if you’ve ever used a garden hose,
or fixed a leaky sink, or had any other hands-on experience with fluids, then you know that
flow can change in response to a number of factors, especially those that affect resistance. Resistance is just anything that hinders flow
or creates friction. In the case of your blood, resistance can
be the result of increased viscosity — the thicker your blood is, the harder it is to
move — or it could be because of increased vessel length, since longer vessels are more
resistant to flow in general. But those factors tend to be pretty constant
in your body over time. Instead, for most people, the biggest factor that affects resistance
has to do with vessel diameter. Changes in diameter can be temporary, like
during vasoconstriction or vasodilation — when the diameter increases or decreases, allowing
more or less blood through. But an excess of low-density lipoprotein,
or LDL, the so-called “bad” cholesterol, in the blood can build up to form a fatty
plaque on the inside of your arteries, permanently increasing the resistance, and hindering blood
flow. So your blood pressure, blood flow, and resistance
are all tied together. In fact, they’re so closely and predictably tied together that we
can even express their relationship mathematically. Among the truths contained in this equation
is the fact that blood flow increases as the difference in blood pressure between two points
increases. Remember: The ventricles of the heart create
very high pressure, while the atria, or the receiving chambers of the heart, have very
little pressure, at least while the body’s at rest. So, the bigger the gap is between the high
pressure in your ventricles and aorta, and the low pressure in your vena cava and atria, the
faster a liter of blood will flow through your system. If you have to back up a few times to review
that, nobody’s gonna judge you. Now, at the same time, blood flow decreases
as resistance increases. So, with a little algebraic rearranging, you can
see that blood pressure equals blood flow, or cardiac output, times resistance. In theory, this means that any change in resistance
or cardiac output would also change blood pressure. But when any one of these variables actually
does change, your body tries its best to compensate for it, in its eternal quest to maintain homeostasis. And it accomplishes this in a few different ways,
mainly using neurons, hormones, and the kidneys. One major short-term fix to wonky blood pressure
comes from your brain, which targets both cardiac output and resistance, by altering
the distribution of the blood flow around the body, or by changing the diameter of certain
blood vessels. This comes in handy when you’re walking
up fourteen flights of stairs in your apartment building with three bags of groceries. The
vessels feeding your digestive organs constrict, which increases resistance there, so more
blood goes to the skeletal muscles in your legs. Most neural responses like this use baroreceptors,
special nerve endings found in the carotid arteries, the aorta, and other larger arteries
in the neck. When blood pressure stretches arterial walls,
that opens mechanically-gated sodium channels in these little receptors. The higher the
blood pressure is, the more frequently they send action potentials to the midbrain informing
it just how much pressure the artery is feeling. When the brain learns what’s happening,
it can do any number of things to correct the situation — like dilate some arterioles
to reduce resistance, or reduce the heart rate to lower cardiac output — and these
things work pretty well for a while. But baroreceptors are not effective for long-term
pressure changes, in part because they end up adapting, essentially reprogramming themselves
to read a high blood pressure as the new normal. Now, other short-term effects on your blood
pressure come from your hormones. We’ve already talked about one classic example:
when your body actually needs a little high blood pressure — like when it has to get
ready to fight or flee — the adrenal medulla starts flooding your blood with epinephrine
and norepinephrine. These hormones raise both the heart rate and the blood
volume, and therefore cardiac output, while also constricting vessels in less essential regions, increasing
the overall resistance, and therefore pressure. And again, just like with neural controls,
these hormonal controls work by changing vessel resistance and cardiac output. But the way to get more long-term control
of blood pressure is to alter the blood volume, and for that, you need the kidneys. Your kidneys cook up hormones like renin and
angiotensin which help regulate levels of sodium and fluids in your body, and also help
expand and constrict blood vessels. And when blood pressure gets too high, your
kidneys will try to reduce the volume of blood, by getting rid of any extra water.
Basically, they make you pee. Blood volume, by the way, is partly why sodium
is the root of all evil in blood pressure treatments: The excess sodium used in processed
foods and salty snacks causes your body to retain water, which creates higher blood volume
and leads to higher blood pressure, which, you know, is a bummer. So, those are some of your body’s automatic
solutions for high blood pressure. But how it compensates for sustained high
blood pressure will almost certainly lead to trouble — either for the heart that has
to work harder to overcome the resistance, or for the vessels that have to take that
extra pressure. An increase in either blood flow or resistance
leaves the heart struggling to do its job, so it might actually build more muscle around that left ventricle
to help generate the force needed to move the blood. Now, a more muscley heart may not sound like
a bad thing, but trust me, it is. Why? Well, because more muscle needs more oxygen,
and your body just can’t create new blood vessels to feed that super-sized ventricle.
So that big muscle is left starving. It’s literally a hungry heart. Plus, if you have cholesterol plaques in your
arteries, then you probably have them in your coronary arteries, which carry oxygen and
nutrients to the heart muscle itself. That increases the resistance, and therefore decreases
blood flow to the heart muscle. So you have this bigger, hungrier heart muscle, but its
nutrient supply is diminished, and eventually those heart muscle cells can slowly die. When that happens, it’s known as heart failure. Or, if you completely block one of those coronary
arteries with plaque, or maybe a blood clot, a whole bunch of heart cells quickly starve
to death. And that can lead to a myocardial infarction,
which is what we also call a heart attack. And then we’ve got the problems on the blood
vessel end, under sustained hypertension, you may see your otherwise elastic arteries
go from being flexible balloons to stiff, hardened pipes, in what’s called arteriosclerosis. Or… the high pressure may make them weak
and bulgy in spots, until they’re stretched too thin and leak or burst. And that is an aneurysm. And if the weak spot ruptures in a smaller
arteriole blood vessel — say, one that leads to an organ like a kidney or eye, it can lead
to organ damage or failure. So although having a huge, strong flow of
blood coursing through your body might sound like some kind of comic book-caliber superpower,
it is not. It’s just too much of a good thing. That’s why you just learned, first and foremost
that you should not butter your bacon — and also what blood flow and resistance are, and
how they relate, in direct proportion, to blood pressure. We also looked at your body’s
short term responses to high blood pressure — including neural, hormonal, and kidney
response — and wrapped up by describing all of the ways that chronic high blood pressure
can kill you. If you like Crash Course and want to
help us keep making videos like this one, you can go to patreon.com/crashcourse. This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written by Kathleen Yale, edited by Blake de Pastino,
and our consultant is Dr. Brandon Jackson. It was directed and edited by Nicole Sweeney,
our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.

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