In organisms where such functions as gas exchange, nutrient absorption, and elimination of wastes take place in specialized organs, the circulatory system is needed to transport gases, nutrients, and wastes to or from individual cells to those distant, specialized organs. The blood of the circulatory system exchanges carbon dioxide for oxygen in the lungs, picks up nutrients in the intestine and liver, releases nitrogenous wastes in the kidneys, and picks up hormones from the endocrine glands. Transported by the circulatory system to all the tissues of the body, the blood, then, releases oxygen, nutrients, and hormones to the cells and picks up their carbon dioxide and nitrogenous wastes. The circulatory system also plays a major role in the body's ability to thermoregulate itself. Since radiation of heat from the blood in the superficial capillaries of the skin is a major source of heat loss, the body can regulate the amount of heat lost to the environment through the dilation or constriction of the superficial capillaries.
The heart is the mechanism which pumps the blood through the circulatory system. Arteries are the vessels that carry blood away from the heart. Veins are the vessels that carry blood toward the heart. Capillaries are the tiny vessels that connect the arteries to the veins. The capillaries run from the smallest arteries, called arterioles, to the smallest veins, called venules. Substances transported by the blood are exchanged between blood and tissue in the capillaries.
