Like bone, blood will later get its own (Unit 15). In the meantime, it‘s hard to bend one‘s mind around the idea of blood as a connective tissue, but here we are. Blood consists of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). The first two of these are cells. Platelets are cell fragments. For this reason, these components are collectively called formed elements. A liquid connective tissue matrix (plasma) surrounds these formed elements. Plasma is primarily salt water plus proteins.

Lymph is a filtrate of blood with most of the cells and some of the protein removed. Defensive white blood cells move around in the lymph to guard against invaders.