Anatomy & physiology of the circulatory system (heart)

[Osmosis sound logo] The circulatory system is also called the cardiovascular system, where cardio- refers to the heart and vascular refers to the blood vessels. So these are two key parts: the heart –which pumps blood– and the blood vessels –which carry blood to the body and return it back to the heart again. Ultimately, this is how nutrients, like O2 –or oxygen– get pushed out to the organs and tissues that need it; and how wastes, like CO2 –or carbon dioxide– which is the main by-product of cellular respiration, gets removed. The heart itself is about the size of a person’s fist, which makes sense: a bigger person has a bigger fist and, therefore, a bigger heart. And it’s shaped like a cone and sits slightly shifted over to the left side in the mediastinum, which is in the middle of the chest cavity, or thorax. It sits on top of the diaphragm –which is the main muscle that helps with breathing, behind the sternum –or breastbone, in front of the vertebral column, squished in-between two lungs, and is protected by the ribs. Now, if you look a little more closely, you can see that the heart sits inside a sac of fluid that has two walls –called the serous pericardium. The outer layer is called the parietal layer, and it gets stuck tightly to another layer called the fibrous pericardium –which is made of tough, dense connective tissue– which holds the heart in place and prevents it from over-filling with blood. The inner layer is called the visceral layer, and it gets stuck tightly to the heart itself, forming the epicardium –or the outer layer of the heart. The cells of the serous pericardium, both the parietal and visceral layer, secrete a protein-rich fluid that fills the space between those layers and serves as a lubricant for the heart, allowing it to move around a bit with each heartbeat without feeling too much friction. So, moving from the outside of the heart to the inside of the heart: after the epicardium, there’s the myocardium –which is the muscular middle layer. This forms the bulk of the heart tissue, because those cardiac muscle cells contract and pump blood. In addition to cardiac muscle cells, there are crisscrossing connective tissue fibers, which are made of collagen, that –together– form the fibrous cardiac skeleton, which helps support the muscle tissue. The myocardium also has dedicated blood vessels called coronary vessels, which lay on the outside of the heart and then penetrate into the myocardium to bring blood to that layer –because it needs a lot of energy to pump blood. Finally, there’s the innermost layer of the heart called the endocardium, which is made of a relatively thin layer of endothelium –which is the same layer of cells that line the blood vessels. This endocardium lines the heart chambers as well as the heart valves. All right. So, looking at the right side of the heart: deoxygenated blood enters either through the top –through a blood vessel called the superior vena cava– or the bottom –through another blood vessel called the inferior vena cava– in the right atrium, where atrium means “entry way.” Both vena cavas [sic] are veins, which bring blood towards the heart. There’s also a tiny third opening into the right atrium called the coronary sinus, which collects blood from coronary vessels returning from the myocardium. Now, all that blood then goes through the first of two atrioventricular valves that separate the atria from the ventricles. This one is called the tricuspid valve, and it allows blood into the right ventricle. The tricuspid valve has three little flaps, or cusps. And each cusp looks kind of like a parachute, because it has tiny little strings –called chordae tendineae– coming off of it that tether the cusps to a small muscle called the papillary muscle. When the heart contracts, that papillary muscle keeps the chordae tendineae taut; and both of these help to prevent regurgitation of blood back into the atrium, allowing it to only flow out the next valve. That being said, that contraction pumps the blood out the pulmonary valve, which –just like the tricuspid valve– has three cusps and also prevents blood from going backward. But, unlike the tricuspid valve, the pulmonary valve doesn’t have any of those chordae tendineae. Once it’s passed the pulmonary valve, the blood goes into the pulmonary arteries, which carry blood away from the heart –to the left and right lung. Just remember that arteries start with an “A” and also carry blood “away” from the heart. So that blood goes from the pulmonary artery into a pulmonary arteriole –which is a little bit smaller– and, finally, into a capillary –which is the smallest. In the lungs, the capillary lines up alongside a small sack of air called an alveolus. (And when you have a lot of them , they’re called alveoli.) Up until now, that blood has been loaded up with carbon dioxide –which makes the blood look dark red, rather than blue which Is how it’s usually drawn. (And this Is how we’ll still draw it to stay consistent.) Now, at this point in the journey, the carbon dioxide moves from the capillary to the alveolus; and oxygen moves from the alveolus to the capillary –giving the blood that nice, bright red color. Now, in the blood, each red blood cell has millions of hemoglobin proteins. And each of those hemoglobins can bind to four oxygen molecules. So, each red blood cell can carry millions of oxygen molecules when it’s fully loaded. The oxygen-rich blood moves into a venule and then, eventually, into a pulmonary vein that dumps the blood into the left atrium. This trip –from the right ventricle of the heart, through the pulmonary artery to the lungs, and back to the left atrium of the heart– is calaled the pulmonary circulation. After entering the left atrium, the blood goes through the second atrioventricular valve –called the mitral valve– into the left ventricle. The mitral valve only has two cusps –or leaflets– one in front, called the anterior leaflet (that’s a little smaller), and one behind it, called the posterior leaflet. Both of these have chordae tendineae coming off of them that tether the valve to the papillary muscles in the left ventricle. Similar to the right side of the heart: when it contracts, this prevents blood from going backwards. Finally, blood in the left ventricle gets pumped out through the aortic valve –which normally has three cusps– out to the aorta, the largest artery in the body. Just like in the lungs, the aorta branches into arterioles –which are smaller arteries– and, finally, into capillaries –which are the smallest. And at that point they’re at the organs and tissues. In the organs, the red blood cells line up alongside tissue cells and drop off oxygen, as well as pick up carbon dioxide –basically the reverse of what happens with the alveolus in the lung. Loaded up with carbon dioxide, the blood turns that dark red color again (shown as blue) and starts the return journey to the heart by going into small venules and then larger veins. Now, the lower half of the body drains into the inferior vena cava and the upper half drains into the superior vena cava –both of which dump blood back into the right atrium. So this trip –from the left ventricle of the heart, to the body, and back to the right atrium of the heart– is called the systemic circulation. Relative to the pulmonary circulation, the systemic circulation has a lot more blood vessels, which means there’s about a five times greater resistance to blood flow. Which essentially means that it’s a lot harder to pump blood through, even though it’s the same amount of blood being pumped as the pulmonary side. Because of this difference, the left ventricle needs to be stronger; and so the muscular layer of the left ventricular wall –or it’s myocardium– is three times thicker than the right ventricle’s myocardium. Okay, so let’s talk a little bit about pumping. Each heart beat sounds something like “lub-dub”, lub-dub”, lub-dub.” That first heart sound, “lub”, is called S1, and the noise comes from the tricuspid and mitral valves snapping shut when the left and right ventricles contract, which happens at about the same time. Right after the S1 sound, the aortic and pulmonic valve open up –allowing blood to get pushed out to the body– and this period of time is called systole. The second heart sound, “dub”, is called S2, and the noise comes from the aortic and pulmonic valves snapping shut to prevent blood from flowing backwards after it leaves the ventricles, –effectively ending systole. Right after the S2 sound, the tricuspid and mitral valves open back up –allowing blood to fill up the ventricles again– and this period of time is called diastole. And that’s it! Each heartbeat can be broken into systole and diastole. So systolic blood pressure is the pressure in the arteries when the ventricles are squeezing out blood under high pressure; and diastolic blood pressure is when the ventricles are filling up with more blood, so it’s going to be slightly lower pressure. All right, so if we simplify this guy a little bit, then the amount of blood that’s pumped out by either of the ventricles over a period of time is called the cardiac output. And the rate at which veins return blood back to the atria is called venus return, which is pretty easy to remember, right? Since the circulatory system is a closed loop, cardiac output and venous return are equal. Okay, so let’s use some numbers to make this a little more concrete. Let’s say that about 70 milliliters is ejected per squeeze. With a heart rate of 70 beats per minute –or 70 squeezes per minute– then 70 times 70 is 49 hundred milliliters per minute, which in liters is 4.9 liters per minute. Meaning that this heart’s pumping about 4.9 liters per minute. Now, in an average adult, there’s about five liters of blood total in the body, which –just to be clear– is different from the cardiac output we just found, which was amount pumped per minute. Now taking this total volume: 10% of that –or about 0.5 liters– is in the pulmonary arteries, capillaries, and veins, which make up the pulmonic circulation; and 5% –about 0.25 liters– is in one of the four chambers of the heart itself. Another 15% –or 0.75 liters– is in the systemic arteries, traveling away from the heart; 5% –or 0.25 liters– is in the systemic capillaries; and the remaining 65% –or 3.25 liters– is in the systemic veins, returning to the heart. Now, in terms of where all that systemic arterial blood goes: about 15% goes to the brain, 5% nourishes the heart itself, 25% goes to the kidneys, another 25% goes to the gastrointestinal organs, yet another 25% goes to the skeletal muscles, and the last 5% goes to the skin. These numbers can obviously change, for example during exercise, but this does give you a general sense of things. All right, so that was the systemic arterial blood. But you’ll notice that there’s a lot more blood in the systemic veins. With that being said, arteries are generally lower volume, while also being under much higher pressure. And in contrast, veins are usually high volume, low pressure vessels. And this explains why arteries and veins have different structures. For example, veins often have valves to help fight gravity and keep blood flowing in one direction –back to the heart; whereas, arteries don’t need these valves, because they’re under a higher pressure. If we take a closer look at blood vessels, they have three layers –also called tunics or coverings– that surround the vessel lumen, which is the hollow part of the vessel that holds blood. The innermost tunic is called the tunica intima, which includes the endothelial cells –which create a slick surface that minimizes friction for blood moving through it. Next there’s the tunica media –or middle tunic– which is mostly made of smooth muscle cells and sheets of elastin protein. Both the cells of the tunica intima and tunica media generally get the nutrients they need from the blood in the lumen. Finally, there’s the tunica externa –or outside tunic– which is made up of loosely woven fibers of collagen protein that protect and reinforce the blood vessel and anchor it in place. The tunica externa also has nerve fibers, lymphatic vessels, and –in the biggest vessels– the protein elastin. To make things really wild, some huge vessels have a tunica externa that’s so thick that it needs its own blood supply. So there are tiny blood vessels, called the vasa vasorum, –which means vessels of the vessels– that creep along the tunica externa to bring nutrients to specifically that layer of the blood vessel wall. So the largest arteries, closest to the heart, –which are the aorta, its main branches, and the pulmonary arteries– have so much elastin in their tunica externa and tunica media that they’re called elastic arteries. These guys are really stretchy –kind of like Spandex– and this is what allows these arteries to keep their shape, as well as absorb and even-out the systolic and diastolic pressures. These arteries eventually branch into the arterioles –the smallest of the arteries. In the arterioles, the tunica media is bulky and can contract in response to hormones in the autonomic nervous system –which is called vasoconstriction, where the lumen gets a lot smaller– which decreases the blood to a capillary bed and ultimately the organ or tissue it supplies. Also, though, these vessels can relax –or vasodilate– and the lumen’s diameter increases, which allows more blood flow. This process can also help with temperature control –or thermoregulation– because blood carries a lot of heat with it and, when it gets close to the surface of the skin, more of that heat gets lost. So with vasodilation of the arterioles, more heat gets lost –which, therefore, helps to lower body temperature. Likewise, if you wanted to raise body temperature, vasoconstriction reduces blood flow, which means less heat gets lost through the surface of the skin. Finally, the capillary walls are generally only one cell thick –just the tunica intima layer –with some larger vessels that also have a sub-endothelium basement membrane layer, which is a layer of protein just outside of the endothelial cells that provides extra support. In addition to allowing oxygen and carbon dioxide to flow back and forth, capillaries are also where nutrients, like glucose, can get delivered; and where fluid can move out of the blood vessel and into the interstitial space –which is the space between the blood vessels and cells. Water-soluble substances, like ions, cross the capillary wall through either water-filled spaces –called clefts– between the endothelial cells, or through large pores in the walls of fenestrated capillaries. Meanwhile, lipid- or fat-soluble molecules, like oxygen and carbon dioxide, can dissolve and then diffuse across the endothelial cell membranes. On the other end of the capillary bed there’s a venule, and the arteriole and venule are usually directly connected by a vessel called the metarteriole All right, as a quick recap: The pulmonary circulation starts with the right ventricle; which delivers blood to the lungs; which then sends fresh, oxygenated blood to the left atrium; at which point it enters the systemic circulation, starting with the left ventricle; which pumps blood to the body; and then deoxygenated blood comes back to the right atrium; and it all starts over again. [Osmosis closing sound logo] What is happening, guys? Thanks for watching. That was an absolutely monstrous video. So, hopefully, you made it through and learned a thing or two along the way. Huge, huge props are going to go to everyone involved in making it. Andrea wrote the script and Rishi edited the script. And Yifan went through my hours and hours of illustrating and turned it into a coherent video. So thanks guys! I think it turned out really well. And for you viewers, if you want to do a deeper dive Into this content, you should definitely head over to We have a ton of quiz questions and the ability to make and look at flashcards, as well. So if you’re interested in that, you should definitely head over to and make an account. Other than that, you can always help support us by donating on Patreon and, you know, subscribing to our channel and following us on social media. All right, guys. Till next time. See you later.


  1. thank you so much, you make everything easy to understand and this was a good review and refresher before my test!

  2. Wow! Absolutely amazing video, my son and I watched this from beginning to end and bookmarked it so we can watch it again later. 😍👍🙏

  3. could you guys do an anesthesiology video and just a couple videos on anesthesiologist/anesthesiology in general? I was going to tell my friend about it since your videos are so helpful and she wants to be an anesthesiologist. so it would be nice for her to find a few videos about anesthesiology <3

  4. Great video. Love u guys at osmosis. Great speaking voice!!! And I love all the illustrations, amazing , u guys are great. Keep it up!!

  5. KEEP IT UP guys
    Absolutely appreciated JOB
    U people r doing GREAT
    Thanks on behalf of MY all students
    Thanks osmosis TEAM
    LOVE U all

  6. Who’s here because they watch greys anatomy and they want to know what Christina’s saying?😂🤣

  7. That was amazing you guys. Basics are really necessary for medicos. Thanks for explaining so well

  8. You explained it very very well… And it is more easy to understand aside from all of the other videos on youtube … Thanks a lot.

  9. Thanks to the team for sharing this video. Help me a lot for my exam, easy to understand and well explained.
    Awesome team!

  10. I am considering a career in cardio perfusion and this was so helpful in understanding the physiology and anatomy of the heart. Thank you!

  11. Is this video unavailable for anyone else?? I could play it fine yesterday now it says “This video is unavailable on this device”

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