The cardiovascular system is a body system (composed of the heart, blood vessels, and blood) that pumps and transports blood throughout the body, delivering oxygen, nutrients, and hormones while removing carbon dioxide and other metabolic wastes. This process includes delivering blood to and from the lungs, where red blood cells absorb oxygen and carry it to all of the body's tissues. Bood vessels are spread throughout the entire body to ensure that all of the body's cells are properly oxygenated. If all of an adult human's blood vessels were laid out end to end, they would stretch close to 100,000 miles. Humans possess a closed circulatory system, in which blood is contained within a closed cycle of blood vessels that constantly recycle blood through the lungs and tissues. Blood vessels are generally placed into one of three categories based on their function: arteries, veins, and capillaries. Arteries carry blood away from the heart. Veins carry blood toward the heart. Capillaries are small vessels that allow diffusion of oxygen and nutrients between the cardiovascular system and the tissues in which they are located. These capillaries also connect the blood flow of arteries to veins, where they return to the heart and lungs.The pumping action of the heart carries oxygenated blood through the circulatory system. When a person is upright the contraction of muscles provides the needed pressure on the veins for deoxygenated blood to complete the return trip to the heart. The human heart is composed of four chambers. The top two chambers are called the atria (singular atrium), and the bottom two chambers are called the ventricles. The right atrium receives deoxygenated blood from the body, which is pumped into the right ventricle before being sent to the lungs via the pulmonary artery. The pulmonary artery is the only artery in the body through which deoxygenated blood travels. In the capillaries of the lungs, red blood cells release carbon dioxide and collect oxygen. Oxygenated blood is returned by the pulmonary veins to the left atrium before passing to the left ventricle, which pumps it out to the body. The pulmonary veins are the only veins in the body that carry oxygenated blood. As blood travels through capillaries it transfers oxygen, carbon dioxide, sugar, wastes, hormones, and other compounds with the tissues of the body.