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 of 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 over for 100,000 kilometers. That's like, (if you... and then... and 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 end at the heart.

That means that all 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 the corner of a 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 that shriek 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."

(Intro)

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 called the tunica intima, which should be pretty easy to remember, because you know, it, like, has 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, which is 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. And that right there should tell you that the tunic 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 it and reinforces the whole blood vessel.

Now the ratio of the thicknesses of these 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 is been called the "miniature golf of martial arts." How does blood move through your system, its circulatory loop, to get from the 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 blood 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 balloons, they're pressure reservoirs, able to expand and recoil with every heartbeat. If they were rigid like pipes, they would eventually leak or burst after being battered by so many waves of pressure. 

So that blood leaves your aorta, and since it's heading to your thumb, it travels along the elastic subclavian artery, which gives away 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 tunic 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 happen. Capillary walls are made of just a single layer of epithelial tissue, which form only the tunica intima, so they're able to deliver 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 we'll 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. Beside 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 your 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 beds 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 twelfth 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 backwards. If those valve leaks or experiences too much pressure, the back-flow 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 send to the lungs, where it gets oxygenated, and comes back 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 hard-working circulatory system moves about seventy-five hundred liters of blood through your heart every day. Just in the time that you've been sitting there listening to me, probably about fifty-two 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 & Physiology.

This episode of Crash Course was filmed in the Dr. 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 Café.