Hank Green:
    Why is everybody so worried about high blood pressure?
    I mean, of all 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 US has high blood pressure; that's two in six; that's three in nine; and often there are no symptoms until it's too late! Just like your favorite song turned up to eleven or an unlimited supply of nacho cheese, even your own life-giving blood can become too much of a good thing. 
 
(Crash Course Anat/Phys Intro)

    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 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 is 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 blood 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 up 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 your 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 muscly heart might 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 blood 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 arterial 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 the ways that chronic high blood pressure can kill you. 
    If you like Crash Course and wanna help us keep making videos like this one, you can go to patreon.com/crashcourse. This episode was filmed in the Dr. Cheryl C. Kinney Crash Course Studio, 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 Café.

(0:27): Blood Pressure: The heart is the pump, the arteries are pressure reservoirs, the arterioles control distribution, the capillaries are the exchange sites, and the veins are blood reservoirs. 

(1:03): Homeostasis isn't just about keeping us at 120/80. It's that sometimes we need to be at 160/120, and sometimes 100/60 might be OK. In other words, it's about trying to keep the balance in any given physiological situation or event. 

(2:13): Cardiac output= Stroke volume (mL/beat) x Heart rate (beats/min)

(2:33): Blood viscosity is usually constant, though an excessive amount of blood cells can thicken blood, just as a very low red blood cell count, like in anemia, can decrease viscosity. 

(5:39): A class of drugs called beta-blockers are used to reduce the risk of heart disease by blocking the beta receptors for epinephrine and norepinephrine. When these hormones bind to beta receptors on smooth muscle it causes vasoconstriction, and on the heart it causes more forceful and faster contractions. Therefore blocking beta receptors leads to more dilated blood vessels and a slower, softer heart even under stress.