The heart serves as a pump to move blood through vessel called as arteries and veins. Blood is carried away from the heart in arteries and is brought back to the heart in veins.
In actuality the heart is a dual pump consisting of two chambered pump on both left and right sides. The upper chambers are inputs to the pumps, and are called atria (atrium singular). The lower chambers of the heart are called ventricles, and are the pump outputs.
When blood is circulated through the body it carried oxygen (O2) and nutrients to the organs and tissues, and returns carrying carbon dioxide (CO2) and the various waste products that are to be excreted through the kidneys. The deoxygenated blood is returned to the right side of the heart via the venous system. Blood from the head and arms, as well as the rest of the upper portion of the body, returns to the heart through the superior vena cava, while blood from the lower portion of the body returns through the inferior vena cava. Note that the terms "inferior" and "superior" refer not to some qualitative assessment, but to the respective positions of the two vessels. The "inferior" is generally lower placed in the body than the superior.
Blood leaves the right atrium through the tricuspid valve to enter the right ventricle. From the right ventricle it passes through the pulmonary semilunar valve to the pulmonary artery. This vessel carries blood to the lungs where CO2 is given up and O2 is taken on.
Blood returning from the lungs via the pulmonary vein re-enters the heart through the left atrium. It then passes through the mitral valve to the left ventricle, and then back into the mainstream of the cilculatory system via the aortic valve. The great artery attached to the left ventricle is called the aorta. Blood then circulates through the body to again returns to the right side of the heart via the superior and inferior vena cava.
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