Why is blood considered a connective tissue even though it is a fluid?

Tissues are classified by their origin and structure, not just by being solid. Connective tissue has a common plan: cells scattered within a non-living background substance called the matrix. In bone the matrix is hard, in cartilage it is firm, and in blood the matrix is simply a liquid called plasma. Blood fits the definition because it has cells, the red blood cells, white blood cells and platelets, suspended in this fluid plasma matrix, and like other connective tissues it develops from the embryonic mesoderm. Functionally it also connects different parts of the body by transporting oxygen, nutrients, hormones and wastes, and it helps in defence and clotting. Because of this cells-in-matrix structure, mesodermal origin and connecting role, blood is classified as a fluid connective tissue.