The hematologic system is blood and the blood forming structures. Blood is comprised of fluid and cells. The term for the fluid component is plasma . The cellular blood components are red blood cells, white blood cells and platelets. All of each type of cell originates from identical ancestor cells –hematopoietic stem cells.
The blood forming structures are the bone marrow and lymphoid tissue. The bone marrow is the spongy tissue in the center of many bones that produces blood stem cells. Those precursors in turn develop into red blood cells, white blood cells and platelets. Because of their role in blood production they constitute a subdivision of the system in general. It is thus termed the hematopoietic system. The World Health Organization recognizes and classifies diseases of the hematologic system in its current ICD publications as those of the blood and blood-forming organs.
The other functions of the hematologic system are the coordination of immune system function and the transport of oxygen and nutrients to tissues and cells and waste products from tissues and cells. The latter serves to integrate processes of the respiratory system, cardiovascular system and other systems of the body which are essential for life.
Pronunciation of Terms
Clotting proteins – Is the group of 13 different proteins in blood involved in the two pathways of coagulation – blood clot formation. They are vital for preventing or stopping bleeding into deep tissues and cavities of the body. The site of production is the liver.
Erythrocyte – Is another name for red blood cell.
Erythropoiesis – Is the production of red blood cells. It normally occurs in the bone marrow but in certain disease states can occur in other lymphoid tissues – namely the liver, spleen and lymph nodes. The name for this atypical production of red blood cells is extramedullary erythropoiesis.
Hematocrit – The volume of packed red cells in the bottom of a test tube of blood after spinning of the blood in a centrifuge expressed as a percentage. For example, if half of the cells settle to the bottom, the hematocrit is 50 percent.
Hemoglobin (HB) (HGB) – Is the molecule that transports most of the oxygen in the blood of humans and other warm-blooded creatures. As the name denotes, it has heme and globin portions. The globin part is a sphere-shaped protein, of which there are 4 subunits. All 4 differ to varying degrees in their chemical structure but do bear some resemblance. Embedded within a cleft in the side of each globin subunit is a heme group. The heme group is a ring of carbon atoms with an iron ion (Fe++) embedded in its center. Because of its high affinity it takes up oxygen from the lungs where it is in high concentration. It then releases it to other tissues where the amount of oxygen is much lower. A portion of oxygen dissolves in blood plasma but typically 98.5% of it is bound to hemoglobin to form oxyhemoglobin.
Hemoglobin saturation is the extent to which the protein molecule is fully loaded with oxygen. It is a percentage of the maximum oxygen carrying capacity for a given pressure in blood which dissolved oxygen exerts. Hemoglobin gives blood its red color. The brightness of the red correlates with the degree of saturation though. That’s why blood in arteries is bright red and that in veins is dark red.
Oxygen carrying capacity is the maximum amount of O2 that a person’s blood can carry. The numbers can vary to some degree depending upon the partial pressure of O2 and if there is CO in the blood. But In most instances, 98.5% of O2 binds to HB; the remaining 1.5% dissolves in plasma. Therefore, it is for the most part, a function of the amount of Hb present in the blood.
Leukocyte – Is another name for white blood cell.
Lymphoid tissue – It is the tissue in the body that produces blood cells and/or antibodies. As such, it functions as a part of both the hematologic system and the immune system. The two main lymphoid organs are the bone marrow and thymus. Secondary lymphoid tissue includes the lymph nodes, lymph vessels, the spleen, tonsils, adenoids, Peyer’s patches and appendix. The bone marrow is where ancestors of all the blood cells originate. But these other sites are where all-important cell proliferation, maturation and differentiation occur.
Platelet – Is the smallest of the cellular blood components. It is one of the many irregularly shaped particles which form in the bone marrow from the fragmentation of its precursor – the megakaryocyte. It is essentially a portion of the cytoplasm of the cell and does not contain a nucleus. Its primary role is hemostasis – the body’s response to stop blood loss. In contrast to clotting proteins, platelets prevent or stop bleeding from small blood vessels and into superficial sites such as the skin and other epithelial structures such as the lining of the gastrointestinal tract or respiratory tract.
Red blood cells (RBCs) – Are the reddish cells in the blood which contain hemoglobin. They transport oxygen bound to the hemoglobin from the lungs to other tissues. In contrast to white blood cells they don’t have a nucleus.
Thrombocyte – Is an alternate term for platelet.
White blood cell (WBC) – Is any of the various types of colorless cells in the blood which have a nucleus and are involved in immune system function.