The Heart, Part 1 – Under Pressure: Crash Course A&P #25


Your heart, that throbbing, beating muscle,
is probably the most iconic organ in your body. No other organ gets its own holiday, or as
much radio play. And you’re not likely to get a love note decorated with a kidney or a spleen,
or even a brain, which is really what rules the emotions. Don’t get me wrong, the heart does some
great things — namely, it powers the entire circulatory system, transporting nutrients,
oxygen, waste, heat, hormones, and immune cells throughout the body, over and over. But in the end, the heart does not make you
love. It doesn’t break apart if you get dumped by your boo. And it’s not a lonely
hunter. The truth is, the heart is really just a pump
— a big, wet, muscley brute of a pump. And it doesn’t care about poetry or chocolate,
or why you’re crying. The heart only has one concern: maintaining
pressure. If you’ve ever squeezed the trigger on a
squirt gun or opened up a shaken can of soda, you’ve seen how fluids flow from areas of
high pressure — like inside the gun or the can — to areas of low pressure, like outside. The heart’s entire purpose is to maintain that same
kind of pressure gradient, by generating high hydrostatic pressure to pump blood out of the heart,
while also creating low pressure to bring it back in. That gradient of force is what we mean when
we talk about blood pressure. It’s basically a measure of the amount of
strain your arteries feel as your heart moves your blood around — more than five liters
of it — at about 60 beats per minute. That’s about 100,000 beats a day, 35 million
a year, 2 to 3 billion heart beats in a lifetime, the basic physiology of which you can
easily feel, just by taking your own pulse. I don’t have a watch Now, that might not inspire a lot of poetry,
but it turns out, it’s still a a pretty good story. Let us begin with a little anatomy. Unless you happen to be of the Grinch persuasion,
the average adult human heart is about the size of two fists clasped together — one
of the few bits of trivia you often hear about human anatomy that is actually true. The heart is hollow, vaguely cone-shaped,
and only weighs about 250 to 350 grams — a pretty modest size for your body’s greatest
workhorse. And although Americans tend to put their right
hand over their left breast while pledging allegiance, the heart is actually situated
pretty much in the center of your chest, snuggled in the mediastinum cavity between your lungs. It sits at an angle, though, with one end
pointing inferiorly toward the left hip, and the other toward the right shoulder. So most of its
mass rests just a little bit left of the midsternal line. The heart is nestled in a double-walled sac
called the pericardium. The tough outer layer, or fibrous pericardium,
is made of dense connective tissue and helps protect the heart while anchoring it to some
of the surrounding structures, so it doesn’t like bounce all over the place while beating. Meanwhile, the inner serous pericardium consists
of an inner visceral layer, or epicardium — which is actually part of the heart wall
— and an outer parietal layer. These two layers are separated by a thick
film of fluid that acts like a natural lubricant, providing a slippery environment for the heart to move
around in so it doesn’t create friction as it beats. The wall of the heart itself is made of yet
more layers, three of them: that epicardium on the outside; the myocardium in the middle,
which is mainly composed of cardiac muscle tissue that does all the work of contracting;
and the innermost endocardium, a thin white layer of squamous epithelial tissue. Deeper inside, the heart has a whole lot of
moving pieces that I’m not going to pick apart here, because the really big thing to
understand is how the general system of chambers, and valves, veins, and arteries all work together
to circulate blood around your body. Of course fluid likes to move from areas of
high pressure to areas of low pressure, and the heart creates those pressures. Form once again following function. Your heart is divided laterally into two sides
by a thin inner partition called the septum. This division creates four chambers — two
superior atria, which are the low pressure areas, and two inferior ventricles that produce
the high pressures. Each chamber has a corresponding valve, which
acts like — like a bouncer at a club at closing time — like he’ll let you out, but not
back in. When a valve opens, blood flows in one direction
into the next chamber. And when it closes, that’s it — no blood can just flow back
into the chamber it just left. So if you put your ear against someone’s
chest — and yeah, ask for permission first — you’ll hear a “lub-DUB, lub-DUB”. What you’re really hearing there are the
person’s heart valves opening and closing. It’s a relatively simple, but quite elegant
set up, really. Functionally, those atria are the receiving
chambers for the blood coming back to the heart after circulating through the body. The ventricles, meanwhile, are the discharging
chambers that push the blood back out of the heart. As a result, the atria are pretty thin-walled,
because the blood flows back into the heart under low pressure, and all those atria have to do is push it
down into the relaxed ventricles, which doesn’t take a whole lot of effort. The ventricles are beastly by comparison.
They’re the true pumps of the heart, and they need big strong walls to shoot blood
back out of the heart with every contraction. And the whole thing is connected to the rest
of your circulatory system by way of arteries and veins. We’ll go into a whole lot more
detail about these later, but the thing to remember first, if you don’t already remember
it, is that arteries carry blood away from the heart, and veins carry it back toward
the heart. To differentiate the two, anatomy diagrams
typically depict arteries in red, while veins are drawn in blue, which, incidentally, is
part of what has led to the common misconception that your blood is actually blue at some point. But, it isn’t. It is always red. It’s just a
brighter red when there’s oxygen in it. So let’s look at how this all comes together, starting
with a big burst of blood flowing out of your heart. The right ventricle pumps blood through the
pulmonary semilunar valve into the pulmonary trunk, which is just a big vessel that splits
to form the left and right pulmonary arteries. From there — and this is the only time in
your body where deoxygenated blood goes through an artery — the blood goes straight through
the pulmonary artery into the lungs, where it can pick up oxygen. It finds its way into very small, thin-walled
capillaries, which allow materials to move in and out of the blood stream. In the case of the lungs,
oxygen moves in, and carbon dioxide moves out. The blood then circles back to the heart by
way of four pulmonary veins, where it keeps moving to the area of lowest pressure — because
that is what fluids do — and in this case that’s inside the relaxed left atrium. Then the atrium contracts, which increases
the pressure, so the blood passes down through the mitral valve into the left ventricle. So the thing that just happened here, where
a wave of blood was pumped from the right ventricle to the lungs and then followed the
lowest pressure back to the left atrium? There is a name for that, it is the pulmonary
circulation loop. It’s how your blood unloads its burden of
carbon dioxide into the lungs, and trades it in for a batch of fresh oxygen. It’s
short, it’s simple — at least in the way I have time to describe it — and it’s just
delightfully effective. Of all of the substances you need to continue
existing, oxygen is the most urgent — the one without which you will die in minutes
instead of hours, or days, or weeks. But it’s pretty useless unless the oxygen can actually
reach your cells. And that hasn’t happened yet. For that, your newly oxygenated blood
needs to travel through the rest of your organ systems and share the wealth. And that fantastic journey — known as the
systemic loop — begins in the left ventricle, when it flexes to increase pressure. Now the
blood would like to flow into the nice low pressure left atrium where it just came from,
but the mitral valve slams shut, forcing it through the aortic semilunar valve into your
body’s largest artery — nearly as big around as a garden hose — the aorta, which sends
it to the rest of your body. And after all your various greedy muscles, and neurons,
and organs, and the heart itself have had their oxygen feast at the capillary-bed buffet,
that now-oxygen-poor blood loops back to the heart, entering through the big superior and inferior
vena cava veins, straight into the right atrium. And when the right atrium contracts, the blood
passes through the tricuspid valve, into the relaxed right ventricle, and right back to
where we started. This whole double-loop cycle plays out like
a giant figure eight — heart to lung to heart to body to heart again — and runs off that
constant high-pressure, low-pressure gradient exchange regulated by the heart valves. So the first “lub” that you hear in that
lub-DUB is made by the mitral and tricuspid valves closing. And they do that because your
ventricles contract to build up pressure and pump blood out of the heart. This high pressure
caused by ventricular contraction is called systole. Now, the “DUB” sound — and, just to be
clear, I am not talking about dubstep sounds — that’s the aortic and pulmonary semilunar
valves closing at the start of diastole. That’s when the ventricles relax, to receive the
next volume of blood from the atria. When those valves close, the high-pressure
blood that’s leaving the heart tries to rush back in, but runs into the valves. So you know when you get your blood pressure
measured, and the nurse gives you two numbers, like, 120 over 80? The first number is your systolic blood pressure
— essentially the peak pressure, produced by the contracting ventricles that push blood
out to all of your tissues. The second reading is your diastolic blood
pressure, which is the pressure in your arteries when the ventricles are relaxed. These two numbers give your nurse a sense of
how your arteries and ventricles are doing, when they’re experiencing both high pressure
— the systolic — and low pressure — the diastolic. So if your systolic blood pressure is too
low, that could mean that, say, the volume of your blood is too low — like, maybe because
you’ve lost a lot of blood, or you’re dehydrated. And if your diastolic is too high, that
could mean that your blood pressure is high, even when it’s supposed to be lower. Considering how much we’ve talked about
the importance of homeostasis, it should come as no surprise that blood pressure that’s
too high or too low, or anything that affects your blood’s ability to move oxygen around
can be dangerous. Prolonged high blood pressure can damage arterial
walls, mess with your circulation and ultimately endanger your heart, your lungs, brain, kidneys,
and nearly every part of you. So I guess you could say the best way to break
a heart is to mess with its pressure. But good luck trying to write a song about
that. Today you learned how the heart’s ventricles,
atria, and valves create a pump that maintains both high and low pressure to circulate blood
from the heart to the body through your arteries, and bring it back to the heart through your
veins. We also talked about what systolic and diastolic blood pressure are, and how
they’re measured. Thanks to our Headmaster of Learning, Thomas
Frank, and to all of our Patreon patrons who help make Crash Course possible, for free,
through their monthly contributions. If you like Crash Course and you want to help us
keep making these videos and also maybe want to get some cool stuff, you can check out
patreon.com/crashcourse. Crash Course is filmed in the Doctor Cheryl
C. Kinney Crash Course Studio. This episode was written by Kathleen Yale, edited by Blake
de Pastino, and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins;
the script supervisor and editor is Nicole Sweeney; our sound designer is Michael Aranda,
and the Graphics team is Thought Cafe.

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