Blood is composed of cells and cell fragments suspended in a liquid known as plasma. In addition, plasma has many molecules, including proteins and nutrients, dissolved in it for transportation between organ systems. As outlined in Vander's Human Physiology, the cellular components, which include erythrocytes, leukocytes, and thrombocytes, make up approximately 45% of blood.
Erythrocyes (Red Blood Cells)
Erythrocytes, also known as red blood cells, are the most common type of cell in the plasma, equaling 99% of the cells in blood, and have a life span of approximately 120 days. These anucleated, biconcave disks develop in the bone marrow and contain nearly 300 million hemoglobin molecules. Hemoglobin binds to oxygen allowing transportation to the body tissues. Once the oxygen-rich cells reach the tissues, the oxygen unloads, diffuses across the walls of the capillaries, and into the tissue cells. Simultaneously, the carbon dioxide from the tissue cells diffuses across the capillaries into the plasma. The red blood cells return to the lungs where oxygen, which enters the alveoli though inhalation, diffuses across the capillaries from the alveoli to the red blood cells where it quickly bonds to the hemoglobin. Concurrently, the carbon dioxide in the plasma diffuses across the capillaries into the alveoli for exhalation. The erythrocytes, reloaded with oxygen, cycle back through the body by way of the circulatory system, to unload the oxygen in the tissues.
Leukocytes (White Blood Cells)
Leukocytes, also known as white blood cells, play a vital role in immunity, as they protect against infection and cancer. These nucleated cells develop in the bone marrow and either mature there or migrate to the thymus for maturation. As explained in Kuby Immunology, the term "leukocyte" is a general name for all white blood cells, as there are several different types, including monocytes, lymphocytes, neutrophils, basophils, and eosinophils. Each of these leukocytes has different roles in immunity as well as different ways of destroying foreign invaders in the body, such as through phagocytosis. The lifespan of a white blood cell ranges from hours to years, depending on its type and level of maturity, as well as its activity level.
Thrombocytes (Platelets)
Thrombocytes, also known as platelets, are cellular fragments responsible for blood clotting. To form platelets, megakaryocytes (the cytoplasmic portions of large bone marrow cells) pinch off and enter circulation where they have a very active role in hemostasis---the stoppage of bleeding. These small, colorless cell fragments contain numerous granules, which have growth factors and clotting proteins. Platelets have a short lifespan of only five to nine days. According to Vander's Human Physiology, after damage occurs to a blood vessel, collagen is exposed resulting in the activation and aggregation of platelets at the wound site. Next, the formation of a platelet plug occurs along with the activation of a pathway that leads to vasoconstriction. The platelet plug seals off breaks in the blood vessel that allows for blood clotting and vessel repair to occur.
References
- "Vander's Human Physiology: The Mechanisms of Body Function"; Eric P. Widmaier, Hershel Raff, & Kevin T. Strang; 2006
- "The Human Body Book: An Illustrated Guide to it's Structure, Function, and Disorders"; Steve Parker; 2007
- "Kuby Immunology"; Thomas J. Kindt, Richard A. Goldsby, & Barbara A. Osbourne; 2007
