The main function of the circulatory system is to move blood and lymph through the body. Doing this transports nutrients and oxygen to the cells of the body and removes waste products such as carbon dioxide. The circulatory system is necessary to regulate temperature and pH balance and protect the body from diseases.
One primary function of the circulatory system is the transport of oxygen from the lungs to the cells. This is why the system has two basic parts: the pulmonary circulation, which takes blood to the lungs to pick up oxygen, and systemic circulation, which takes the oxygenated blood throughout the body. Along the way, the blood picks up nutrients, attacks diseases and gathers waste for eventual elimination.
Though some scientists separate the cardiovascular and the lymphatic systems, since both systems operate using the same biological pathways, many scientists categorize them together as the circulatory system. Although all vertebrates, such as mammals, birds and reptiles, as well as annelids and cephalopods, have closed circulatory systems, in which the blood travels through vessels to organs and back to the heart, other animals, such as mollusks, have open circulatory systems in which the blood bathes the organs directly. However, in both types of circulatory systems, the purpose is the same: to get nutrients to the cells and ferry waste away from the cells.