The circulatory system is a system in the body responsible for getting blood throughout the body. In blood, there are four cells, (thus the name: "The Four Cells"), are red blood cells, white blood cells, platelets, and plasma. Red blood cells are responsible for carrying oxygen to the lungs. The arteries and veins carry blood all around the body. The arteries carry blood away from the heart and the veins carry blood to the heart. The heart pumps blood all around the body. It consists of the left atrium, right atrium, left ventricle, and right ventricle.
The Cells
Cells are crucial to human and animal survival. The four main circulatory cells as specified earlier all play a key role to basic bodily function. The white blood cells help with defense. They are part of the immune system as well which helps protect the body from germs, viruses, bad bacteria, and other bad things that might enter the body. White blood cells essentially just fight for the body. Red blood cells help with oxygen. They carry oxygen to the lungs by traveling in the blood. Think of it like this: the blood vessels such as arteries and veins are just a big track and the blood is the train that carries all the cells to where they need to go. Platelets help the blood clot in times of injury. Say you hurt your leg. It starts healing. The reason that it starts to scab up and heal is because the platelets rush the the scene and all start sticking together, (literally), and forming a clot. And last but not least: plasma. Plasma cells just build up plasma which it what makes the blood liquid and gives it all the things it needs to let the other cells travel.
The Heart
The heart is a tirelessly working organ constructed of cardiac muscle that pumps blood throughout the body. It is made up of four columns: the left and right atriums; left and right ventricles. The simplified version of the process goes like this: right atrium, right ventricle, left atrium, left ventricle. This process does not include all the valves, aorta, etc.