The circulatory system is also called the cardiovascular system, where “cardi” refers to the heart, and “vascular” refers to the blood vessels. So, these are the 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 waste like CO2, or carbon dioxide, which is the main byproduct of cellular respiration, gets removed.
The heart 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 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 the two lungs, and protected by the ribs.
If you look 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. 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 overfilling 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 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 supports 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 and heart valves.
All right, so on 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 “entryway.”. Both vena cavas 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 of 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 tendinae coming off of it that tether the cusp to a small muscle called a papillary muscle. When the heart contracts, that papillary muscle keeps the chordae tendinae taut, and both of these help to prevent regurgitation of blood back into the atrium, allowing it to only flow out next valve.
That being said, that contraction pumps the blood out the pulmonary valve which like the tricuspid valve has three cusps and also prevents blood from going backwards - but unlike the tricuspid valve, the pulmonary valve doesn’t have any of those chordae tendinae. Once it’s past the pulmonary valve, the blood goes into the pulmonary arteries which carry the blood away from the heart to the left and right lung. Just remember that arteries start with “a” and carry blood “away” from the heart.
The blood goes from the pulmonary artery into a pulmonary arteriole, which is a 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 the blood has been loaded with carbon dioxide, which makes the blood look dark red rather than blue, which is how it’s usually drawn, and 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 these hemoglobins can bind to four oxygen molecules, so each red blood cell can carry millions of oxygen molecules when 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 called 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 has only 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 tendinae coming off of them that tether the valve to 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 and pick up carbon dioxide, basically the reverse of what happened 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. Now, relative to the pulmonary circulation, the systemic has a lot more blood vessels, which means there’s about a 5 times greater resistance to blood flow, which essentially meaning 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 ventricle wall - or its myocardium - is three times thicker than the right ventricle’s myocardium.
Okay so let’s talk a little bit about that pumping. Every heartbeat, sounds something like, “lub dub, lub dub, lub dub.” So the 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 valve 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. That’s it, each heartbeat can be broken into systole and diastole. So a 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.