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Your heart gets a lot of attention from poets, songwriters, and storytellers, but today Hank's gonna tell you how it really works. 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. You'll also learn what your blood pressure measurements mean when we talk about systolic and diastolic blood pressure.

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Introduction: The Heart 00:00
Structure of the Heart 1:41
The Heart's Ventricles, Atria, and Valves 3:25
Arteries & Veins 4:35
Pulmonary Circulation Loop 5:04
Systemic Loop 6:14
Systolic and Diastolic Blood Pressure 7:58
Review 8:59
Credits 9:29


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Hank Green:
   Your heart, that throbbing, beating muscle -- it's probably the most iconic organ in your body. No other organ gets its own holiday, or as much radio play, and you're unlikely 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, muscly brute of a pump, and it doesn't care about poetry or chocolate or why you're crying. The heart has only one concern: maintaining pressure.
    If you've ever squeezed the trigger on a squirt gun or opened a can of shaken 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 5 liters of it, at about 60 beats per minute. That's about 100 thousand beats a day, 35 million a year, 2 to 3 billion heartbeats 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 so much poetry, but it turns out, it's still a pretty good story.

 Anatomy of the Heart (1:30)

(Crash Course Anat/Phys Intro)

    Let us begin with a little anatomy. Unless you happen to be of the Grinch persuasion, the average 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 weighs only 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 towards 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 the fibrous pericardium, is made of dense connective tissue and helps protect the heart while anchoring it to some of the surrounding structures, so that it doesn't, like, bounce all over the place while it's beating. Meanwhile, the inner serous pericardium consists of an inner visceral layer, or epicardium, which is actually part of the heart wall, and the 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 up 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 the general system of chambers and valves and veins and arteries all work together to circulate blood around your body.
    Of course, fluids like to move from areas of high pressure to areas of low pressure, and the heart creates those pressures, form once again following function.

  Maintains Both High and Low Pressure (3:25

    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 in 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 hear that lub-dub, lub-dub. What you're really hearing are the person's heart valves opening and closing.
    It's a relatively simple but quite elegant setup, 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 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.

 Pulmonary Circulation Loop (5:03)

    So let's look at how all this 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 in the very small, thin-walled capillaries, which allow materials to move in and out of the bloodstream. 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's what fluids do. In this case, it's the inside of 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's 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.

 Systemic Loop (6:13)

    Of all 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 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.

  Systolic and Diastolic Blood Pressure (7:25

    So that 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. And now for that "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 you 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 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, lungs, brain, kidneys, and nearly every part of you. So I guess you could say that the best way to break a heart is to mess with its pressure. But good luck trying to write a song about that.

 Conclusion and Credits (8:58)

    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 CrashCourse possible for free through their monthly contributions. If you like CrashCourse and you want to help us keep making these videos and also maybe want to get some cool stuff, you can check out
    CrashCourse was filmed in the Dr. Cheryl C. Kinney CrashCourse 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.



(1:25): Because small mammals have faster heart rates but shorter life spans, and large mammals have slower heart rates but longer life spans, most mammals have a surprisingly similar total number of heart beats in their lifetime.

(1:50): A child's heart is about as big as one of their fists.

(2:42): Pericarditis is an inflammation of the pericardium that roughens those serous membranes, so when the heart beats it rubs against the sac, making a creaky noise, and generally it hurts a lot. Over time that can lead to adhesions and impede heart activity. 

(3:20): Air also moves from high to low pressure areas, which we'll talk about in respiratory system unit!

(5:01): It's always red, but the shade of red varies. Arterial blood looks bright red because it's full of oxygen, whereas vein blood looks dark red because of its lack of oxygen.
(5:40): The pulmonary arteries and veins are the big exception to the artery/vein, oxygen/no oxygen rule

(6:55): Congestive heart failure can occur if the left ventricle can't pump blood out, and it backs up. And since the left ventricle receives blood from the lungs, that blood backs up into the lungs, where plasma leaks out and fills the lungs. Hence, a person is "congested."
(7:59): The units used for blood pressure are millimeters of mercury, or mm Hg. 

(8:16): A normal systolic blood pressure is around 120 mm Hg. A normal diastolic blood pressure is around 80 mm Hg.

(8:36): High blood pressure combined with any of the following really increases your risk of serious health issues:
  • Age
  • Heredity (including race)
  • Overweight or obesity
  • Gender (male)
  • Diabetes
  • Physical inactivity
  • Smoking
  • High cholesterol