The heart is "a hollow muscular organ that pumps blood through the circulatory system by rhythmic contraction and dilation. In vertebrates there may be up to four chambers (as in humans)."
There are four chambers to the heart: the right and left atrium and the right and left ventricle. The right atrium is responsible for receiving oxygen poor blood from the lungs and sending it to the right ventricle. Once received by the right ventricle, the oxygen poor blood is pumped back to the lungs. If we jump over to the left atrium, here is where oxygen rich blood is received from the lungs. Which means....the blood from the left atrium is passed to the left ventricle where this oxygen rich blood then gets passed on to the rest of the body. Let's review that before we get into more detail...
Right atrium Recieves oxygen poor blood and sends it to the Right ventricle which also Recieves oxygen poor. Once the blood is re oxygenated by the Lungs it heads over to the Left atrium. This oxygen rich blood is then sent over to the Left ventricle where it is then sent on to the rest of the body. So right atrium right ventricle lungs left atrium left ventricle body.
Blood is vital for humans to sustanin life and ensure the proper function of all the bodys organs. Blood carries pure or "fresh oxygen" from the lungs (see pulmonolgy) as well as nutrients and waste products like carbon dioxide.