FORMED ELEMENTS

A. Erythrocytes: Erythrocytes are also called red blood cells, or RBCs. They are the most abundant formed elements in the blood (4-6 x millions/uL). Because of their presence in most tissues and organs, erythrocytes are useful to histologists and pathologists in estimating the size of other tissue and organ components (through estimates of multiples or fractions of RBC diameter).
1. Normal structure and function. RBCs are structurally and functionally specialized to transport oxygen from the lungs to other tissues. Their cytoplasm contains the oxygen binding protein hemoglobin. Their small diameter (7-8 um) and biconcave shape (in humans) help to maximize their surface-to-volume ratio, facilitating oxygen exchange. Mature RBCs lack nuclei and cytoplasmic organelles, which they lose during differentiation. Because they lack mitochondria, the energy needed to maintain the hemoglobin in a functional state must be derived from anaerobic glycolysis. Because they lack ribosomes, the glycolytic enzymes and other important proteins cannot be renewed. Mature erythrocytes therefore have a limited lifespan (120 days) in the circulation before they are removed by macrophages in the spleen and bone marrow.2. Abnormalities a. Anisocytosis refers to the presence of a high percentage of RBCs with unusually great variations in size. Those larger than 9 ┬Ám in diameter are termed macrocytes, and those smaller than 6 um are termed microcytes. b. Nuclear fragments. In some disease states, nuclear fragments, or Howell-Joliy bodies, remain in otherwise mature RBCs. When these form circular filaments they are termed Cabot rings. c. Reticulocytes. Some RBCs recently released from the bone marrow contain a small amount of residual RER and ribosomes that can be precipitated into blue, netlike structures with the vital dye brilliant cresyl blue. When these reticulocytes constitute more than about 1% of the circulating RBCs, they indicate an increased demand for oxygen carrying capacity leg, from loss of RBCs due to hemorrhage or anemia, or to recent ascent to a higher altitude).

COMPOSITION OF PLASMA

A. Water: Plasma contains 90% water by volume.
B. Solutes: Plasma contains 10% solutes by volume. These solutes include plasma proteins and other organic compounds as well as inorganic salts.
1. Plasma proteins. Plasma contains a rich variety of soluble proteins, 7% by volume. Important examples include: a. Albumin. This is the most abundant plasma protein (3.5-5 g/dL of blood) and is mainly responsible for maintaining the osmotic pressure of blood. B. Globulins. Alpha, beta, and gamma globulins are globular proteins dissolved in the plasma. The gamma globulins include the antibodies, or immunoglobulins, synthesized by plasma cells. c. Fibrinogen. This protein is converted by blood-borne enzymes into fibrin during clot formation. Fibrinogen is synthesized and secreted by the liver. 2. Other organic compounds. Other organic molecules in plasma, include nutrients such as amino acids and glucose, vitamins, and a variety of regulatory peptides, steroid hormones, and lipids. 3. Inorganic salts. Inorganic salts in plasma, 0.9% by volume, include blood electrolytes such as sodium, potassium, and calcium salts.

GENERAL FEATURES OF THE BLOOD

A. Two Divisions: Humans have a total blood volume of about 5 L (depending on body size). Blood is divisible into 2 parts, the formed elements, which include the blood cells and platelets, and the plasma, or liquid phase, in which the formed elements are suspended and in which a variety of important proteins, hormones, and other substances are dissolved.
B. Basic Cell Types: There are 2 basic types of blood cells, the erythrocytes, or red blood cells, and the leukocytes, or white blood cells.
C. Clotting: Outside the blood vessels, blood undergoes a complex reaction called clot formation or coagulation, which plays an important role in repairing damaged vessels and preventing blood loss.
D, Hematocrit: When anticoagulants (heparin, citrate, etc) are added, blood samples can be separated in a centrifuge into 3 major fractions. The erythrocytes constitute the densest fraction and end up at the bottom of the tube. The hematocrit is the percentage of packed erythrocytes per unit volume of blood. In adults, normal hematocrit values vary from 35 to 50% and are sex dependent. Leukocytes are less dense and less numerous (about 1 % Of blood volume) and form a thin white or grayish layer over the erythrocytes. On top is a thin layer of platelets. The least dense is the clear layer of plasma, which constitutes 42 47% of the blood.
E. Differential Cell Count: Blood is also studied by spreading a drop on a slide to produce a single layer of cells (blood smear). The cells are stained, differentiated by type, and counted to reveal disease-related changes in their relative numbers. The smears are usually stained with Romanovsky-type dye mixtures containing eosin and methylene blue.
F. Staining Properties: All of the descriptions of the staining properties of blood cells refer to their appearance after staining with Romanovsky-type mixtures leg, Wright's or Giemsa). Blood cells and their components exhibit 3 major staining properties that allow the cell types to be distinguished:
1. Basophilia is an affinity for methylene blue. Basophilic structures stain purple to black. 3. Eosinophilia, or acidophilia, is an affinity for eosin. Eosinophilic structures stain salmon pink to orange.
4. Neutrophilia is an affinity for a complex of dyes (originally thought to be neutral) in the mixture. Neutrophilic structures stain salmon pink to lilac.