The human circulatory system consists of a muscular four-chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated. The heart is mesodermal in origin. It is situated in the thoracic cavity, in between the two lungs. The heart is slightly tilted towards the left side because the right lung is larger than the left lung. This condition provides the heart with enough space to function properly and pump blood efficiently to different parts of the body.