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  • The liquid portion of circulating blood is plasma, while the cells and platelets comprise the formed elements; upon clotting, some proteins are removed from plasma and others are released from platelets, forming a new liquid termed serum.

  • Important protein components of plasma include albumin, diverse α- and β-globulins, proteins of the complement system, and fibrinogen, all of which are secreted within the liver, as well as the immunoglobulins.

  • RBCs or erythrocytes, which make up the hematocrit portion (~45%) of a blood sample, are enucleated, biconcave discs 7.5 μm in diameter, filled with hemoglobin for the uptake, transport, and release of O2, and with a normal life span of about 120 days.

  • WBCs or leukocytes are broadly grouped as granulocytes (neutrophils, eosinophils, basophils) or agranulocytes (lymphocytes, monocytes).

  • All leukocytes become active outside the circulation, specifically leaving the microvasculature in a process involving cytokines, selective adhesion, changes in the endothelium, and transendothelial migration or diapedesis.

  • All granulocytes have specialized lysosomes called azurophilic granules and smaller specific granules with proteins for various cell-specific functions.

  • Neutrophils, the most abundant type of leukocyte, have polymorphic, multilobed nuclei, and faint pink cytoplasmic granules that contain many factors for highly efficient phagolysosomal killing and removal of bacteria.

  • Eosinophils have bilobed nuclei and eosinophilic-specific granules containing factors for destruction of helminthic parasites and for modulating inflammation.

  • Basophils, the rarest type of circulating leukocyte, have irregular bilobed nuclei and resemble mast cells with strongly basophilic-specific granules containing factors important in allergies and chronic inflammatory conditions, including histamine, heparin, chemokines, and various hydrolases.

  • Lymphocytes, agranulocytes with many functions as T- and B-cell subtypes in the immune system, range widely in size, depending on their activation state, and have roughly spherical nuclei with little cytoplasm and few organelles.

  • Monocytes are larger agranulocytes with distinctly indented or C-shaped nuclei, which circulate as precursors of macrophages and other cells of the mononuclear phagocyte system.

  • Platelets are small (2-4 μm) cell fragments derived from megakaryocytes in bone marrow, with a marginal bundle of actin filaments, alpha granules and delta granules, and an open canalicular system of membranous vesicles; rapid degranulation on contact with collagen triggers blood clotting.


Blood is a specialized connective tissue consisting of cells and fluid extracellular material called plasma. Propelled mainly by rhythmic contractions of the heart, about 5 L of blood in an average adult moves unidirectionally within the closed circulatory system. The so-called formed elements circulating in the plasma are erythrocytes (red blood cells), leukocytes (white blood cells [WBCs]), and platelets.

When blood leaves the circulatory system, either in a test tube or in the extracellular matrix (ECM) surrounding blood vessels, plasma proteins react with one another to produce a clot, which includes ...

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