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.