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After studying this chapter you should know:

  • The components of the blood.

  • The methods used to assess these components.

  • The identity and function of the major hematopoietic and lymphoid tissues: the bone marrow, spleen, thymus, and lymph nodes.

Regardless of what kind of doctor a student decides to become, a well-grounded understanding of disorders of the blood and the hematopoietic tissues is essential. Primary hematologic disorders are commonly encountered in community and hospital-based clinical practices, and a wide variety of other diseases come to attention by producing secondary abnormalities of the blood. Beyond their everyday clinical importance, studies of hematologic diseases have yielded seminal insights into the molecular pathogenesis of cancer and basic aspects of stem cell biology. These lessons have had far-reaching influences on biomedical research and are beginning to shape the practice of molecular medicine.

The goal of this book is to provide students with a foundation in the area of blood disorders that they can build on as they train to become physicians and/or research scientists. In this chapter, we will commence with a description of the cast of characters—the blood, the primary blood-forming (hematopoietic) tissues, and the secondary hematopoietic tissues that support the maturation and function of some of the elements of the blood—accompanied by a brief description of commonly used laboratory tests. In Chapter 2, the origins of blood cells and the regulation of blood cell production will be discussed in detail. The remainder of the text will then be devoted to specific hematologic disorders.


Normal adult men have about five liters of blood, whereas women average closer to four. The blood has two main components—the blood cells (or formed elements)—and the plasma, the fluid phase of the blood. These two components can be separated from one another by centrifugation at low speed. Red cells are dense and collect in the bottom of the tube, whereas white cells and platelets are of intermediate density and tend to collect at the interface between the red cells and the plasma in a thin gray-white layer referred to as the buffy coat.


Most formed elements can be recognized by their morphologic appearance in smears of whole blood, which can be prepared, stained, and visualized under the light microscope in just a few minutes' time. In days past, clinicians skilled in the art of morphology could sometimes confirm a suspected diagnosis based on a patient's history and physical findings through the inspection of blood smears prepared at the bedside. In the modern era, the evaluation of blood is usually left to laboratory technicians and pathologists, but to fully understand blood disorders, students must have a working knowledge of the morphology of the formed elements and ...

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