The circulatory system consists of the cardiovascular system and lymphatic system. The cardiovascular system includes the heart and blood vessels responsible for circulating blood throughout the body. The heart is a hollow muscular organ located in the chest that acts as a pump. It has four chambers - two atria and two ventricles - and pumps deoxygenated blood to the lungs and oxygenated blood to the tissues and organs. Key measurements of heart function include end diastolic volume, end systolic volume, stroke volume, cardiac output, and ejection fraction. Together these measurements assess how efficiently the heart circulates blood to meet the body's demands.