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Blood is a complex mixture of cells and fluid transported throughout the body by the cardiovascular system. It can be considered a specialized connective tissue, one that provides functional instead of structural connections between all the body's organs. Blood consists of approximately equal volumes of plasma and cells.

  • Plasma is a liquid extracellular matrix that supplies body tissues with material necessary for metabolism, removes wastes, and serves as a dynamic reservoir for maintaining the proper composition of extracellular fluid in the body. Plasma proteins are primarily synthesized in the liver; the kidneys regulate the levels of water and ions.

  • Blood cells, also called the formed elements of blood, consist of anucleate erythrocytes (known as red blood cells, or RBCs), nucleated white blood cells (known as white blood cells, or WBCs), and platelets, which are cell fragments. These formed elements are generated in the bone marrow and thymus continuously throughout life by the division of hematopoietic stem cells and progenitor cells. The mitotic activity of these cells is controlled and integrated by a homeostatic network of growth factors produced throughout the body.

  • Blood cells transport O2 and CO2 throughout the body (RBCs), provide immune defense (WBCs), and maintain the integrity of blood vessels and aid in blood clotting (platelets).

  • Blood flows to every organ and returns traces of pathology that may be present anywhere in the body. Laboratory analyses of plasma components, blood cell numbers, and blood cell production provide significant diagnostic information for all branches of clinical medicine.


Blood is described either as a fluid or as a specialized connective tissue. When considered as a tissue, blood is comprised of cells suspended in an extracellular matrix, blood plasma, which is normally fluid, although it has the potential of becoming semisolid through the process of clotting. Centrifugation of fluid blood provides a simple method for separating these two components in a small tube, with blood cells found at the bottom and plasma on top (Figure 8-1A). Blood is sometimes described as a discontinuous tissue because its cells are produced in the bone marrow and the thymus, but they function elsewhere, either in blood or in other tissues throughout the body. Similarly, blood cells do not produce their matrix—several other organs are responsible for producing and regulating plasma. Blood is a substantial tissue; the combination of blood and the principal blood cell producing tissue, hematopoietic bone marrow, constitute more than 10% of the body's weight.

Figure 8-1:

Composition and formed elements of blood. A. Separation of blood into plasma and cells by centrifugation. The percent values listed provide a typical range of volumes, with the major variation due to individual differences in hematocrit (volume of packed erythrocytes). B. Overview of blood cells. The range of cell numbers considered normal can vary among clinical laboratories. Neutrophils typically account for ...

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