The circulatory system is made up of the heart, blood and blood vessels known as arteries, capillaries and veins. The heart pumps blood throughout your body through the blood vessels. Blood delivers oxygen and nutrients to cells and carries away carbon dioxide and other waste materials.
The heart looks like an upside-down pear. It is about the size of your closed fist. It is almost in the middle of your chest. The heart is made of muscle divided into four parts called chambers. The chambers are hollow inside. The two chambers on top are called atria. The chambers on the bottom are called ventricles. The heart also has four valves that let blood in and out of the chambers.
The blood vessels are the arteries, veins and capillaries. Arteries are blood vessels that convey blood from the heart to the tissues of the body. Two arteries have direct connection with the heart: (1) the aorta, which, with its branches, conveys oxygenated blood from the left ventricle to every part of the body; and (2) the pulmonary artery, which conveys blood from the right ventricle to the lungs, whence it is returned bearing oxygen to the left side of the heart.
The arteries expand and then constrict with each beat of the heart, a rhythmic movement that may be felt as the pulse. Veins, on the other hand, returns blood to the heart from other parts of the body. This false-color electron micrograph shows red blood cells packed into a capillary, the smallest type of blood vessel. Blood flows from the capillaries into veins after oxygen has been exchanged. Lastly, the Capillary forms the connection between the arteries and the veins. These tiny vessels vary in diameter from 0.0127 to about 0.2032 mm (0.0005 to about 0.008 in)...