Formed Elements

Once plasma is removed, what is left of blood is its cellular fraction, collectively called the formed elements: red blood cells (erythrocytes), white blood cells (leucocytes) and platelets. Each of the three has a completely different job and a completely different structure suited to that job - red cells carry gases, white cells defend the body, and platelets plug wounds - and the next three sub-sections describe each in turn. All three formed elements ultimately trace back to stem cells in the bone marrow, though white cells continue their development in lymphoid tissue as well. Counting and examining these cells (a full blood count) is one of the most routine and informative tests in medicine, because a shift in any one type's numbers - too many or too few red cells, an unusual white-c …