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 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.
