An organism's circulatory system pumps blood, while its respiratory system regulates its gas exchange ( carbon dioxide and oxygen). The circulatory system consists of the blood, heart, blood arteries, and lymph nodes, while the nervous system consists of the brain, spinal cord, and nerves. Like the respiratory system, the circulatory system is made up of a network of blood vessels that transport oxygen and carbon dioxide throughout the body. Taking in oxygen and exhaling carbon dioxide are the primary functions of the circulatory system and the respiratory system, respectively. After entering the heart, deoxygenated blood travels to the lungs, where it is re-oxygenated. Oxygen is necessary for both systems to function.