The Circulatory System includes the heart (pump) arteries (outflow pipes) and veins (inflow pipes). The heart acts like a pump to push oxygenated blood around the body through arteries to give the cells food and oxygen, and to pump deoxygenated blood back to the heart through the veins. So let's see how it works.
Let's say you're an oxygenated red blood cell and you're starting your journey in the Aortic arch you will go out to the lower body in one of the three arteries. As you go through the body the arteries will start to branch off and get smaller. You will go past lots of cells and tissues giving them your oxygen and they will give to you their carbon dioxide. You will then go into a vein and make the journey back as a deoxygenated red blood cell. Eventually you will come to the Inferior Vena Cava (if you had gone to the upper body will have come to the heart through the Superior Vena Cava) from there you will go into the Right Atrium (this is from the person who owns the heart's perspective) and when the Tricuspid valve opens (the valves opening and shutting make the lub dub (S1 S2) sound you hear) you will be pumped into the Right Ventricle. From there the Pulmonary Semilunar valve will open letting you be pumped into the Pulmonary trunk (anything with the word Pulmonary means it has some thing to do with the lungs). From the Pulmonary trunk you will branch off into either the right or left Pulmonary arteries, You will then go into the lungs. Passing by the alvioli you will get oxygenated and give the lungs the carbon dioxide which it will exhale out. You will then be an oxygenated red blood cell again!
From the lungs you will be pumped back into the heart via the Pulmonary veins. From there you will make your way into the left atrium. When the Mitral valve (here the Bicuspid valve) opens you will go into the Left Ventricle and finally when the Aortic valve opens you will be back inside the Aortic arch ready to go back in to the body many more times (unless your owner is a crazy one and cuts him/herself and you find youself outside the body but lets hope that doesn't happen).
Well that is the Circulatory system for you and I hope you go back just that little bit more smarter.
By the Tiger