The volume of blood in our body is limited, but it has to perform unlimited work continuously. This leads to one conclusion that, the same quantity of blood must be used over and over again. In other words, blood must circulate.
Heart of mammal is conical in structure, present in between two lungs inside the thoracic cavity. It is covered by an outer love covering called pericardium. The broad end of heart is called the base and the conical end is called the apex on heart. Mammalian heart is four chambered, having two auricles and two ventricles.
A normal human being's body contains 4-5 litres of blood. The blood is the means of transport by which oxygen and nutrients reach parts of the body and waste materials are carried away. This system consisting of the heart and blood vessels that makes the circulation of blood possible is called the circulatory system.