Heart

The heart is the organ that pushes blood so it reaches every part of the body. It doesn't work alone: it's the center of the circulatory system, meaning the heart together with the vessels, the tubes the blood travels through. Picture a house with a water pump and a network of pipes: the pump moves the water, the pipes deliver it, and neither one is any use without the other.

Inside, it isn't one hollow bag but four spaces called chambers, and each pair has its own job. The two on top are the atria, and their job is to take in the blood that arrives; the two on the bottom are the ventricles, and their job is to push it back out. The wall around all of that isn't a single thing either: on the outside is the epicardium, in the middle is the myocardium, which is the muscle layer, and on the inside is the endocardium, which lines the inner surface the way a coating lines the inside of a container.