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Hematology is the study of blood and its basic biological components, including red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (erythrocytes). Hematologists study and help treat a variety of hematological malfunctions and diseases, one of the primary being the various anemias. Anemias, like sickle cell anemia, result in a loss of erythrocytes, which reduces the blood's ability to transport oxygen to tissues.

The vital importance of blood to life has been known since at least 400 B.C. However, there were many early misconceptions concerning blood's functions and activities. It was once thought that disturbances in blood "humors" caused diseases. As a result, bloodletting was thought to eliminate contaminated fluids from the body and became a primary, though misguided, therapy for almost every disease. It was not until the seventeenth century that the microscope was invented and the science of hematology moved into the modern era. With this technology, the cellular components of blood were first discovered. In 1852, Karl Vierordt quantitatively analyzed blood cells, which led to correlations between blood cells counts and various diseases.

Blood helps the body to function in many ways. It is the main connection between different body tissues, transferring substances produced by one organ for use by other tissues. As a result, blood is a primary component in proper organ functioning. For example, it transports oxygen from the lungs and releases it in various tissues, providing the essential nourishment for tissue survival and growth.

One of the primary areas of study within hematology is hematopoiesis, or the formation and development of blood cells. Blood is formed by various hematopoietic tissues or organs depending on the stage of life. In humans, the embryo yolk sack begins producing blood usually within 20 days after fertilization. In the third month of embryonic life, the liver takes over with help from the spleen, kidney, thymus, and lymph nodes. Although lymph nodes continue to play a role in blood formation throughout life, bone marrow becomes the primary source of blood production in the embryo at about six months and continues this role after birth.

In contrast to the early theory of blood's relationship to disease, hematologists came to understand that changes in specific blood components are the result, not the cause, of disease. Anemias, for example, are the result of other body organ or tissue malfunctions. In addition to studying and treating various types of anemias, hematologists are concerned with a variety of other blood disorders, like leukemias (cancerous malignancies that occur in the blood and lymph nodes) and blood clotting, which can lead to an embolism (obstruction of a blood vessel).

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