From Wikipedia, the free encyclopedia Blood is a highly specialised circulating tissue consisting of several types of cell suspended in a fluid medium known as plasma. The cellular constituents are: red blood cells, which carry respiratory gases and give it its red color, white blood cells (leucocytes), which fight disease, and platelets, cell fragments which play an important part in the clotting of the blood. Medical terms related to blood often begin with hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood." Anatomically, blood is considered a connective tissue from both its origin in the bones and its function.