Blood is considered a connective tissue because it shares key characteristics with other connective tissues. Specifically:

• Origin: Blood, like all connective tissues, originates from the embryonic mesoderm layer, specifically from mesenchyme during development.
• Matrix: Connective tissues are defined by having an extracellular matrix surrounding their cells. In blood, this matrix is the liquid plasma, which consists mostly of water and dissolved proteins and other substances.
• Cells: Blood contains different cell types suspended in this matrix — primarily red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.
• Function: Blood connects various parts of the body by circulating through all organs, transporting oxygen, nutrients, waste, and immune cells. This linking and supporting role is a hallmark of connective tissue.
• Mobility: Blood is often described as a "fluid connective tissue" because its matrix is liquid, allowing it to move freely throughout the body.

Thus, blood meets the structural and functional criteria of connective tissue, though it is specialized as a mobile, fluid tissue with a unique role in transport and defense.