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A brief overiew of hematology, the study of blood.

Hematology (the study of blood) is a field of medicine devoted to understanding the mechanisms functions, diseases, and treatments related to the blood system. There are many components of the blood system: three general cell types (White Blood Cell, Red Blood Cell, and platelets), proteins that control bleeding and clotting, and many other proteins that serve essential organ specific functions. It is difficult to understand hematology without first appreciating that there is a strong interplay with practically every system in the body.

Blood formation

In the beginning, the human fetus begins making blood cells at about 4 weeks after conception (gestation). The blood cells are made in a structure known as the yolk sac and later the production is transitioned into the liver before finally making it to the bone marrow in the last trimester of pregnancy. Newborn babies are born with an abundance of large red blood cells because of the low oxygen environment of the uterus.

After birth, the red blood cell (RBC) count gradually diminishes to adult levels as the baby's body adjusts to breathing air, with its high oxygen content. White blood cell (WBC) and platelet levels are also elevated and slowly fall as the body matures. After birth, production of blood originates in the bone marrow within the hips and long bones of the body. The marrow provides a hospitable environment for all three cell types to mature and to learn their role in the peripheral circulation. Any abnormality in the formation and maturation of these cells can lead to hematologic problems.

Function of Blood Cells

Each cell type in the blood serves an important function.

White blood cells are the "soldiers" of the immune system that fight infection and protect the body against viruses, bacteria, and fungi. Abnormalities and function of the white blood cells are best discussed in the immunology primer contained in this web site.

Red blood cells deliver oxygen to the body's organs and remove tissue waste products. If there is increased consumption or destruction of circulating blood cells either as a result of infection, or other systemic diseases, the marrow accordingly increases production to replace the loss. This cycle or feedback is critical to maintaining the body's ability to preserve vital organ functions. When the marrow is suppressed (for example, by viruses) or impaired by infiltration of abnormal blood cells (as in the case of leukemia), the body is susceptible to infection, severe anemia, and bleeding. The function of red blood cells greatly depends on their shape and a substance called hemoglobin. Abnormal hemoglobin production can lead to shortened red blood cell lifespan in the circulation and ultimately anemia.

platelets play a primary role in forming blood clots and controlling bleeding whenever the skin is damaged. Platelet defects usually stem from abnormal surface proteins that prevent proper "stickiness."

Bleeding and Clotting

The blood also contains clotting and anti-clotting proteins (factors) that form a delicate balance. The body's clotting system is comprised of two parts: the intrinsic and extrinsic pathways. The extrinsic pathway is the critical mechanism triggered in response to trauma while the intrinsic pathway plays a role in the ongoing formation of fibrin clots. There is overlap in between these two pathways. Any disturbance in one or more clotting proteins can lead to bleeding disorders. The process of controlling bleeding involves interplay between blood vessel, platelets, and clotting systems (extrinsic and intrinsic pathways). Briefly, the sequence is as follows: When the blood vessel is damaged it shrinks. Subsequently the platelets arrive to the scene and start forming a clump primed by the carefully regulated clotting factors. Anti- clotting factors regulate this process to prevent excessive clot formation and platelet consumption. Variation in quantity or effectiveness of any of the clotting or anti-clotting factors can lead to disease

Just as the body carefully regulates its response to blood vessel injury and clot formation, there is an equally important balance between production and consumption of blood cells (white blood cells, red blood cells, and platelets). On a similar note, as a whole, the blood system is a complex network of cells and proteins that regulate one another to provide the perfect response to an injury or threatening source of infection. The blood system also provides vital nutrients to organs and removes harmful waste products. Understanding the dynamics of hematology can be important for physicians attempting to make a diagnosis or trying to distinguish similar disease processes.