The circulatory system transports nutrients and blood cells to and from cells throughout the body. Blood is oxygenated in the lungs then sent back to the heart to be pumped to the rest of the body in blood vessels. The types of blood vessels include veins, arteries and capillaries. Veins carry blood to the heart while arteries carry blood away from the heart. Oxygen diffuses into the cells directly from the capillaries.
The circulatory system is the major transport system of the body and its task is to transport the oxygen your body needs, the nutrients your body needs, things like hormones, the immune system cells things like antibodies as well as collect all the wastes that are being produced by your cells especially things like carbon dioxide.
Now there's three major parts to the circulatory system; there's the blood, the blood vessels and the heart. The blood contains things like the liquid plasma, that is the non cellular part of the blood, there's the red blood cells, the white blood cells and then these cell parts called platelets that are involved in blood clotting. There's the blood vessels, these are the tubes that the blood is actually going through, there's the arteries, veins and capillaries. Now it's very easy and common for people to get confused about the difference between arteries and veins because a lot of times people say what the arteries have oxygen in them the veins don't and most of the times that's true but not all the time. The proper way to distinguish the two is what direction is the blood going in. In arteries they carry blood away from the heart did you notice the a there while veins carry blood back to the heart so again most of the time yes the arteries are carrying oxygenated blood but if you're sending blood from the heart to the lungs, you don't do that to get that to bring oxygen to the lungs, you get it you send blood away from the heart to the lungs to get the oxygen so that's the one case where arteries will carry deoxygenated blood while the veins that are coming back from the lungs to the heart there are the ones that are carrying lots of oxygen.
Now capillaries are very thin walled and typically one cell thick and that allows the easy exchange of oxygen and carbon dioxide as well as the other nutrients with the cells that capillaries are going through and they are what connect ultimately arteries to veins. You can think of this as arteries are like freeways or highways as are veins, capillaries are residential streets. That is where you can park your car you can get out of your car and people, can climb into the car, you don't try to park in and exchange passengers in the middle of in let's say freeway just would not be a wise idea.
Now the heart pumps the blood and I'm going to go through the structure of the heart more materially but first I want to just mention a couple of concepts. The first is something called, the systemic circuit versus the pulmonary circuit. The systemic circuit, is that circuit of blood from arteries to capillaries to veins and back to the heart that is carrying blood to the body, to your brain to your muscles et cetera and then it after it delivers its nutrients it gathers up waste and returns to the heart. The pulmonary circuit is the arteries, capillaries and veins that go off to the lungs, so if we take a look at this diagram of the heart over here you can see on the diagram what looks to be the left, that's actually remember the patience right, this is something that screwed me up when I first started studying the heart and that was I looked at the diagram and I said right atrium and I said no that's on the left, it's the patients' right so a lot of times when you're looking at this go okay so on the right hand side do you notice how everything is kind of this darkish purply blue, that's a common symbol used in diagrams in textbooks to indicate that this is low oxygen blood so blood is collected from the upper body from something called the superior vena cava and from the lower body by a blood vessel called the inferior vena cava. These are two very large veins that collect blood that is low in oxygen has been used up by the rest of the body doing its normal activities. It comes into this entrance chamber of the heart on the right side, these entrance chambers are called atria so the right atrium collects the blood and squeezes it into this lower chamber, the lower bigger chambers are called ventricles. The right ventricle when it squeezes the blood it pumps the blood out this pulmonary trunk to the pulmonary arteries off to the lungs. In the lungs, oxygen is absorbed from the lungs, carbon dioxide is dropped off and we have our nicely bright red blood that we turns through the pulmonary veins and now it's oxygenated in the these veins, those are the only veins that are oxygenated.
These pulmonary veins deliver the blood to an entrance chamber again it's a vein sorry again it's a atrium but this time it's on the left side, the patients' left, the left atrium pumps the blood into the left ventricle that left ventricle is the most muscular of the 4 chambers that left ventricle when it squeezes, it pushes the blood out the aorta which is the biggest artery in the body squeezing that blood out from the aorta every other artery branches off ultimately some going off to the upper body and then the aorta dips down behind the heart to feed the lower body so the aorta is the beginning of the systemic circuit the vena cavas are the end of the systemic circuit. The pulmonary arteries are the beginning of the pulmonary circuit, the pulmonary veins are the end of the pulmonary circuit.
Just so you know, these weird white things here, those are valves they're one way doors that close to prevent back flow so that when the right and left ventricle squeeze together whoosh they blow open these valves here which are called semilunar valves but as they squeeze, the valves that led backwards, they slam shut and that's the first sound of a sound beat the first sound. When the ventricles are relaxed, the pressure inside here drops the semilunar valves close that's the second sound of the heartbeat so when you're hearing the sound of the heartbeat the first sound is the atrioventicular valves slamming shut the second sound are the semilunar valves slamming shut.I have heard some kids tell me that it is the sound of the heart squeezing, if you've ever wondered what the squeezing of a muscle would sound like put your ear to your biceps and squeeze all you hear is your shirt moving alright? So now that you know the anatomy of the heart and the functions of the circulatory system you're good to go.