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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 formed elements and a pale yellow liquid called serum. Serum contains growth factors and other proteins released from platelets during clot formation, which confer biological properties very different from those of plasma.

Collected blood in which clotting is prevented by the addition of anticoagulants (eg, heparin or citrate) can be separated by centrifugation into layers that reflect its heterogeneity (Figure 12–1). Erythrocytes comprise the sedimented material, and their volume, normally about 44% of the total blood volume in healthy adults, is called the hematocrit.


Composition of whole blood.

A tube of blood after centrifugation (center) has nearly half of its volume represented by erythrocytes in the bottom half of the tube, a volume called the hematocrit. Between the sedimented erythrocytes and the supernatant light-colored plasma is a thin layer of leukocytes and platelets called the buffy coat. The concentration ranges of erythrocytes, platelets, and leukocytes in normal blood are included here, along with the differential count or percent range for each type of leukocyte represented in the buffy coat. A cubic millimeter of blood is equivalent to a microliter (μL). (Complete blood count [CBC] values in this chapter are those used by the US National Board of Medical Examiners.) (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008).

The straw-colored, translucent, slightly viscous supernatant comprising 55% at the top half of the centrifugation tube is the plasma. A thin gray-white layer called the buffy coat between the plasma and the hematocrit, about 1% of the volume, consists of leukocytes and platelets, both less dense than erythrocytes.

Blood is a distributing vehicle, transporting O2, CO2, metabolites, hormones, and other substances to cells throughout the body. Most O2 is bound to hemoglobin in erythrocytes and is much more abundant in arterial than venous blood (Figure 12–2), while CO2 is carried in solution as CO2 or HCO3, in addition to being hemoglobin-bound. Nutrients are distributed from their sites of synthesis or absorption in the gut, while metabolic residues are collected from cells throughout the body and removed from the blood by the excretory organs. Hormone distribution in blood ...

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