Scientists categorize blood as a connective tissue for two primary reasons. According to Rutgers University, blood originates in embryo in the mesoderm, one of the three primary layers of cells; blood shares this origin with other kinds of connective tissue. And, like other kinds of connective tissue, blood plays a connective role with respect to the systems within the human body.
MedlinePlus states that blood is living tissue comprising both liquid and solids. The liquid, plasma, consists of protein, salts and water, while the solids the blood contains are platelets and red and white blood cells. The majority of the blood in the human body is plasma.
The different components of the blood play diverse roles in the human body, according to MedlinePlus: red blood cells take oxygen from the lungs to organs and tissue, white blood cells defend the body against infection, and platelets assist with the blood clotting process when a person is wounded. The blood solids have differing life spans as well. White blood cells live for less than a day, while platelets live for around six days and red blood cells live for about four months. The marrow inside bones produces new blood cells to replace the dead cells.