No doubt about it, your heart is a champion. It electrifies itself, it maintains your blood
pressure, it keeps your blood moving, and it’s got like a nice shape you can put some
chocolates inside of and give to people you like. But the circulatory system is much, much more
than just that pump. Because the heart also needs a network to
actually send all that blood through, right? Cue the blood vessels. Although it’s easy to think of them as a
glorified plumbing system for your body, that’s not a very good analogy. These aren’t just
passive tubes made merely to carry liquid around, like the pipes behind your walls at
your home. Blood vessels are actually active, dynamic
organs, capable of contracting and expanding as they deliver oxygen and nutrients to cells
throughout the body; carry away waste products; and do their part in maintaining that all
important blood pressure. You already know about the three major types
of blood vessels: the arteries that carry blood away from the heart, the veins that
bring it back, and the little capillaries that act as the transfer station between the
two. But you’ve also got arterioles -- which
are like mini-arteries that branch out into those capillaries -- and venules, the smallest
vein components that suck blood back out of the capillaries and merge into the larger
veins that head home to the heart. And it’s quite an incredible journey, really. If all your blood vessels could be strung
together in a single line, they’d stretch out for 100,000 kilometers! That’s like...if you...and then...carry the two --
that’s like two and a half times around the Earth. And together this extensive network forms a
closed system that begins and ends in the heart. That means that all of the five or so liters
of blood in your body are contained within it at all times, unless you’re bleeding,
which I hope you’re not. If you prick a finger and watch a drop of
blood pop up, you know that you’ve nicked a blood vessel, and that blood is leaking
out of its closed system. Likewise, if you slam your shin against a
corner of the coffee table on your way to the bathroom, and an hour later you see a
big nasty bruise forming, then you know you’ve damaged your blood vessels again, because bruising
is internal bleeding, usually into loose connective tissue. And, if you’re embarrassed about the piercing
shriek that you let out when you bumped your leg, and you start to blush. Well, that’s your
blood vessels, too, expanding just to say hello. Blood vessels are another great example of
how anatomy and physiology go together like peanut butter and jelly. How they look and
what they do go hand in hand. Most of your blood vessels share a similar
structure consisting of three layers of tissue surrounding the open space, or lumen, that
actually holds the blood. Anatomists call these layers “tunics,”
and the innermost section is the tunica intima -- which should be pretty easy to remember
because, you know, it has like intimate contact with the lumen. It’s like your circulatory
underpants. The cool thing about this layer is that it
contains the endothelium, which you may remember is made up of simple squamous epithelium tissue
and is continuous with the lining of the heart. These cells form a slick surface that helps
the blood move without friction. Surrounding the tunica intima is the middle
layer, the tunica media, made of smooth muscle cells and sheets of the protein elastin. That smooth muscle tissue is regulated in
part by the nerve fibers of the autonomic nervous system, which can decrease the diameter
of the lumen by contracting this middle layer during vasoconstriction, or expand it by relaxing
during vasodilation. That right there should tell you that the
tunica media plays a key role in blood flow and blood pressure, because the smaller the
diameter of the blood vessel, the harder it is for blood to move through it -- kinda like trying to
drink milk through a cocktail straw versus a soda straw. And finally, the outermost layer of your blood
vessels is the tunica externa. It’s like an overcoat, if that coat were
made mostly of loosely woven collagen-fiber. Actually, if your coat happens to be made of leather,
it is made of collagen. And like a coat, this outer layer is what protects and reinforces
the whole blood vessel. Now the ratio of the thicknesses of three
layers varies between blood vessels of different types -- because, guess what?! Yes, form follows function! Let’s take a
look. Say you’re gearing up for a big tournament
of thumb wrestling, or what has been called the “miniature golf of martial arts.” How does blood move through your systemic
circulatory loop, to get from your heart to your champion right thumb-flexing muscle,
the flexor pollicis brevis, and back again? Well, you will remember from our lessons on
the heart that blood leaves the left ventricle through the aorta -- the biggest and toughest artery
in your body, roughly the diameter of a garden hose. The aorta and its major branches are elastic
arteries -- they contain more elastin than any other blood vessel type, so they can absorb the
large pressure fluctuations as blood leaves the heart. What’s more, that elasticity actually dampens
that pressure so that big surges don’t reach the smaller vessels, where they could cause
damage. This is really where that whole pipe analogy
falls apart. These arteries are really more like a balloons
-- they’re pressure reservoirs, able to expand and recoil with every heartbeat. If
they were rigid like pipes, they’d eventually leak or burst after being battered by so many
waves of pressure. So that blood leaves your aorta, and since
it’s headed to your thumb, it travels along the elastic subclavian artery, which gives
way to a series of muscular arteries -- in this case, the brachial artery in your upper
arm, and the radial artery in the lower arm. Muscular arteries distribute blood to specific
body parts, and account for most of your named arteries. They’re less elastic and more
muscular. These arteries invest in additional smooth
muscle tissue, and proportionally, have the thickest tunica media of any blood vessel. This allows
them to contract or relax through vasoconstriction and vasodilation, which we’ve talked about a lot in
terms of the nervous system’s stress response. These arteries keep tapering down until they
turn into the nearly microscopic arterioles that feed into the smallest of your blood
vessels, your tiny, extremely thin-walled capillaries which serve as a sort of exchange or
bridge between your arterial and venous systems. They may be little, but your capillaries are where the
big, important exchange of materials actually happens. Capillary walls are made of just a single
layer of epithelial tissue, which form only the tunic intima, so they’re able to deliver
the oxygen and other nutrients in your blood to their cellular destinations through diffusion. The capillaries are also where those cells
can dump their carbon dioxide and other waste back into the blood and send it away, through
the veins to the lungs and kidneys. But I will come back to that in a second. Unlike arteries and veins, capillaries don’t
operate on their own, but rather form interweaving groups called capillary beds. Besides exchanging nutrients and gases, your
capillary beds also help regulate blood pressure, and play a role in thermoregulation. Say you’re in the room where you’re, like,
practicing thumb calisthenics -- which probably isn’t a thing -- but the room is a little
chilly, so the blood feeding your dermis loses a lot of heat to that cold air. Well, smooth muscle forms tiny sphincters
-- yeah, you’ve got sphincters everywhere -- around the vessels that lead to each of
your capillary beds. When they tighten up, they force blood to bypass some of those capillaries,
which means less blood is exposed to the cold, and you lose less heat. If it’s really cold, the smooth muscles
around your larger arterioles and muscular arteries -- like that radial artery in your
lower arm -- will also squeeze, slowing blood flow to your whole hand. Which is no way to
win at thumb wrestling. But it’s one reason why your fingers get
all stiff and numb in the cold -- they’re not getting as much warm blood, because your
blood vessels are trying to conserve heat. Conversely, if your thumb is working really
hard and producing heat from all that exertion, those capillary sphincters relax and open
wide, flooding the capillary bed with blood to help disperse heat -- which is part of
the reason that you might get red-faced when you’re hot or exercising hard. So anyway, now your thumb muscles have just
feasted on a batch of oxygen and glucose served up on a fresh bed of capillary, and they’re
ready to take out the trash. The cells send their CO2 and other junk out
to the venal end of the capillary exchange where the capillaries unite into venules, and then
merge into veins that head back to the heart. Remember that the pressure in these vessels
has to be dropping, since fluids always flow from higher to lower pressure. But since the pressure is so low in your veins
-- it’s like one 12th of the pressure in your arteries -- there isn’t much pressure
gradient left to push the blood back to your heart. So veins require some extra adaptations
to keep the blood moving in the right direction. That’s why some of them -- especially veins
in the arms and legs that have to work against gravity -- have venous valves that help keep
the blood from flowing backward. If those valves leak, or a vein experiences
too much pressure, the backflow of blood can stretch and twist the vein, leaving you with
varicose veins, or if this happens in another part of the body, hemorrhoids. But, anyway, we’ve gotten pretty far from your
thumb at this point. We’ve got a loop to finish here! From the capillaries and venules in your thumb,
that low-pressure blood flows from the radial vein to the brachial vein to the subclavian
vein, where it dumps into the superior vena cava and settles for a second in the right
atria, before dropping into the right ventricle. From there it’s sent to the lungs, where
it gets oxygenated, and then comes back into into the left atria, before sliding down into
the left ventricle, where it builds up pressure again, and spurts back out into your aorta. It takes about a minute for all the blood
in your body to complete that circuit, which means, even if you’re mostly at rest, your
hardworking circulatory system moves about 7,500 liters of blood through your heart every
day. Just in the time that you’ve been sitting there listening
to me, probably about 52 liters has coursed through. So yes. Much like the Internet, your blood
vessels are more than just “a series of tubes.” During the time that you’ve been circulating
all that blood, you learned about the basic three-layer structure of your blood vessels;
how those structures differ slightly in different types of vessels; and you followed the flow
of blood from your heart to capillaries in your right thumb, and all the way back to
your heart again. If you like Crash Course and you want to help
us keep making videos like this, you can go to patreon.com/crashcourse. Also, a big thank
you to Matthew Pierce for co-sponsoring this episode of Crash Course Anatomy and Physiology. This episode of Crash Course 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 by Nicholas
Jenkins; the editor and script supervisor is Nicole Sweeney; our sound designer is Michael
Aranda, and the Graphics team is Thought Cafe.
