Heart

In a complex organism such as a vertebrate, with multiple cell layers and complex organ systems, blood serves to distribute nourishment and oxygen to the cells and remove waste products. Specialized cells within the blood, such as the red blood cells (erythrocytes) and white blood cells (leukocytes) serve specialized functions. The red cells hold and distribute oxygen to release in the cells and return carbon dioxide to the lungs to be eliminated. The white cells carry out immune functions to destroy invading bacteria and other foreign material. Still other components of the blood are involved in forming clots when a blood vessel is opened. The liquid medium, the plasma, carries vitamins and other nutrients throughout the body.

Animals require a means for the blood to obtain oxygen, whether through gills or lungs, and a means to propel the blood through those structures. The heart is central to that purpose.