The circulatory body system, otherwise known as the cardiovascular or vascular system, is composed of four major parts: the heart, arteries, and veins. The vital responsibilities of the said system is to circulate blood throughout the body, transport nutrients, and carry away waste for elimination. Circulation starts when the heart pumps out blood through the main artery aorta, into the lung capillaries for oxygenation, and throughout the body via the arteries. Besides oxygen, nutrients and metabolic products are distributed as blood circulates, waste is processed and soon after eliminated by the kidneys as blood circulates there, and un-oxygenated blood returns to the heart through the superior and inferior vena cava (veins), the whole cycle therefore carrying on.