Algonquin College
11.6 - Blood Cell Types
The blood is composed mostly of plasma, a watery mixture of electrolytes, soluble and insoluble plasma proteins. The remainder of the blood is composed of several types of blood cells: red blood cells, white blood cells and platelets.

Red blood cells (Erythrocytes)

Erythrocytes are by far the most common blood cells. They are very small biconcave disc-shaped cells without a nucleus. The high concentration of red blood cells (millions per cubic millimetre) means that red blood cells are one of the major contributing factors in blood viscosity.

The lack of a nucleus and other organelles leaves more room inside the cell for hemoglobin; the iron-containing protein complex that transports oxygen from the respiratory system to body cells. The hemoglobin content is slightly higher in men than in women.

An abnormally high concentration of red blood cells is a condition called polycythemia. An abnormally low concentration of red blood cells is a type of anemia; a lack of oxygen-carrying capacity in the blood. Anemia can also be caused by a lack of iron or hemoglobin.

White blood cells (Leukocytes)

Leukocytes are relatively uncommon in the blood relative to erythrocytes. They are the only blood cells with a nucleus and a full range of organelles and can contain granules or be agranulate (without granules).

Leukocytes have diverse functions in body defense including fighting infections, parasites and cancerous cells. They also clean up cellular debris from dead cells or pathogens. Leukocytes are attracted to where they are needed by positive chemotaxis; a process by which attractant chemicals (cytokines) are released by injured or infected cells or other white blood cells. To be able to reach all tissues in the body leukocytes cross between the blood and the interstitial fluid by diapedesis. They then "crawl" through body tissues by ameboid motion.

An abnormally high concentration of white blood cells is leukocytosis and an abmormally low concentration of white blood cells is leukopenia.

Platelets (Thrombocytes)

Platelets are not complete cells and have no organelles or nucleus. They are cell fragments that bud off of megakaryocytes, very large cells from the bone marrow.

Platelets participate in the blood clotting response by producing clotting factors and temporarily sealing leaks in blood vessels.