Blood represents a fluid connective tissue consisting of cells and liquid extracellular material called plasma. Propelled mainly by rhythmic contractions of the heart, 5 or 6 L of blood in an average adult move 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 into the extracellular matrix (ECM) surrounding blood vessels, plasma proteins react with one another to yield a localized clot, which includes formed elements and releases a viscous, yellowish liquid called serum. Serum contains cellular growth factors and other proteins secreted by platelets during clot formation, which give serum biological properties very different from those of plasma.
Figure 12–1 shows that when a tube of collected blood, blocked from clotting by the addition of anticoagulants (eg, heparin or citrate), undergoes centrifugation it produces layers reflecting its composition. Erythrocytes comprise the sedimented material, called hematocrit, about 44% of the total blood volume in healthy adults.
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 percentage 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.)
The straw-colored supernatant comprising 55% at the top half of the centrifugation tube represents plasma, three or four times more viscous than water. A thin gray-white layer called the buffy coat between the plasma and hematocrit, only 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. Blood O2 binds the erythrocyte protein hemoglobin and as shown in Figure 12–2 is much more abundant in arterial than in venous blood. Blood CO2, in addition to being hemoglobin-bound, moves dissolved in plasma, some as HCO3–. Blood distributes nutrients from their sites of synthesis or absorption in the gut and collects metabolic residues in tissues throughout the body for removal from the blood by the excretory organs. Hormone distribution in blood permits the exchange of chemical messages between distant organs regulating normal organ function. Blood also ...