Blood Components

Blood Components

The blood is made up of cells, fragments of cells, and dissolved biochemicals containing nutrients, oxygen, hormones, and wastes (FIGURE 17-1). It helps to distribute body heat and maintain stable interstitial fluid. Blood is the only fluid connective tissue, with its cells suspended in a liquid, the extracellular matrix. It is heavier and thicker than water, is the only fluid tissue in the body, and is a homogenous liquid. This means it has a similar composition throughout. Blood contains formed elements such as erythrocytes (red blood cells) or RBCs, leukocytes (white blood cells) or WBCs, and platelets. Of these, only the WBCs are complete cells, containing nuclei and organelles. Most of the formed elements exist in the bloodstream for only a few hours or days before they are replaced by new cells. Also, most of them do not divide and are replaced by stem cells that continuously divide in the red bone marrow. The liquid portion of blood is called plasma. The combination of plasma and formed elements is called whole blood. When a blood sample is spun in a centrifuge, the heavier formed elements sink, whereas the plasma rises. A thin, whitish­-colored layer called the buffy coat is present­at the point where the RBCs join the plasma. This layer is made up of leukocytes and platelets.