Formed Elements

∞ generated and posted on 2015.12.29 ∞

Cells along with cell-like structures found in blood.

We use the term Formed Elements to distinguish those materials that are dissolved or instead suspended in blood, including as multimolecular complexes, from those that possess plasma membranes and are at least cell like.

These included the white blood cells, also known as leukocytes, and the red blood cells (erythrocytes) as well as the platelets. Contrast plasma, which is the fluid portion of blood.

Note that platelets are not cells but instead are membrane-bound fragments of cells that are involved in blood clotting. Erythrocytes, in mammals, also are technically not cells as they lack a cell nucleus and also lack mitochondria and other organelles. This is not the case for all animals, however, where, for example, bird's erythrocytes in fact are true cells as are also the erythrocytes of most other non-mammal vertebrates.

Not all animals possess blood, however. Even among animals that have circulatory systems, not all such circulatory systems are open circulatory systems, meaning the fluid flows from and then back to the heart within continuous tubes (i.e., blood vessels). Such animals instead possess hemolymph, which not only is lacking in erythrocytes but is employed to carry primarily nutrients rather than oxygen to animal tissues.