Connective tissues have cells scattered throughout an extracellular matrix. They connect different body systems. Blood is considered as a type of connective tissue because of two reasons.

$\Rightarrow$ Like the other connective tissues, blood is mesodermal in origin.
$\Rightarrow$ It connects the body systems, transports oxygen and nutrients to all the parts of the body, and removes the waste products. Blood has an extra-cellular matrix called plasma, with red blood cells, white blood cells, and platelets floating in it.