Why is blood classified as a tissue?

According to School of Medicine at Southern Illinois University Carbondale, blood is classified as a type of connective tissue because it has the same mesodermal origin as other connective tissues. As About.com describes, all connective tissues, including blood, consist of cells dispersed in the extracellular matrix.

School of Medicine at SIUC describes the principal role of connective tissues in the body as providing mechanical support and cohesion to the organs while acting as a connective element between the body's vital systems. Blood is a specialized connective tissue with free-flowing extracellular matrix (plasma) that provides the medium for oxygen and nutrient transportation throughout the body.