The cardiovascular system consists of the heart and blood vessels. The heart has four chambers - two atria that receive blood and two ventricles that pump blood out. The right side receives deoxygenated blood and pumps it to the lungs, while the left side receives oxygenated blood and pumps it to the body. The left ventricle wall is thicker since it must pump against higher pressure. The heart contracts automatically and regularly, alternating between systole and diastole. Each side functions as a separate pump - the right side is a volume pump for low pressure circulation to the lungs, while the left side is a pressure pump for high pressure circulation to the body.