Blood

Hematology is the study of blood. The body contains approximately 4 to 5 L of blood, making up about 8% of the body’s weight. The functions of blood include the following:

Blood divides into solid and liquid portions when spun in a centrifuge (Fig. 12-1). The solid parts, called formed elements, are red blood cells (RBCs), white blood cells (WBCs), and platelets (thrombocytes). Table 12-1 shows the formed elements of the blood. The buffy coat is a mixture of the WBCs and the platelets. The remaining liquid portion of the blood is the plasma.

TABLE 12-1

Formed Elements of Blood

	Cell Type	Description	Function
	Erythrocyte	Biconcave disk; no nucleus; 7-8 µm in diameter	Transports oxygen and carbon dioxide
	Leukocyte
	Neutrophil	Spherical cell; nucleus with connected filaments; cytoplasmic granules stain light pink to reddish-purple; 12-15 µm in diameter	Phagocytizes microorganisms
	Basophil	Spherical cell; nucleus with two indistinct lobes; cytoplasmic granules stain blue-purple; 10-12 µm in diameter	Releases histamine, which promotes inflammation, and heparin, which prevents clot formation
	Eosinophil	Spherical cell; nucleus often with two lobes; cytoplasmic granules stain orange-red or bright red; 10-12 µm in diameter	Releases chemicals that reduce inflammation; attacks certain worm parasites
	Lymphocyte	Spherical cell with round nucleus; cytoplasm forms a thin ring around the nucleus; 6-8 µm in diameter	Produces antibodies and other chemicals responsible for destroying microorganisms; responsible for allergic reactions, graft rejection, tumor control, and regulation of the immune system
	Monocyte	Spherical cell; nucleus round, kidney, or horseshoe shaped; contains more cytoplasm than lymphocytes; 10-15 µm in diameter	Phagocytic cell in the blood leaves the blood and becomes a macrophage, which phagocytizes bacteria, dead cells, fragments, and debris within tissues
	Platelet	Cell fragments surrounded by a cell membrane and containing granules; 2-5 µm in diameter	Forms platelet plugs; releases chemicals necessary to blood clotting

Red Blood Cells

More than 25 trillion RBCs, also called erythrocytes, circulate in the body’s 4 to 5 L of blood (Fig. 12-2). Erythrocytes contain a protein called hemoglobin that carries oxygen to all cells and removes carbon dioxide. Each RBC lives only 90 to 120 days. New cells are manufactured by the red marrow or myeloid tissue in bones (see Chapter 14). A few million new RBCs are made each second in a process called hemopoiesis. The liver and spleen remove dead RBCs and reuse the material.

White Blood Cells

WBCs, also called leukocytes, fight disease and infection. There are fewer WBCs than RBCs, and they are larger. Leukocytes live about 9 days and can move out of the blood vessels as part of the immune process. WBCs remove foreign particles, fight infection, and help prevent disease. Pus consists of WBCs mixed with bacteria.

The five types of WBCs are listed as follows (Table 12-1):

Platelets

Platelets, also called thrombocytes, are the smallest blood cells. They promote clotting to prevent blood loss. As platelets pass over a rough spot in a vessel, they become sticky. They may form a plug to seal small vessels by themselves or start the clotting process. To form a clot, platelets combine with a protein, called prothrombin, and calcium to form thrombin. This mass combines with another protein called fibrinogen to form a gel-like substance called fibrin, which forms the clot. Thrombocytes are usually produced in red bone marrow and live for about 5 to 9 days.

Plasma

Plasma is a pale yellow liquid that is left when the formed elements are removed from blood. Whole blood is 55% plasma. Plasma is 90% water and approximately 10% proteins. It contains nutrients, electrolytes, oxygen, enzymes, hormones, and wastes. The proteins help fight infection and assist in the clotting (coagulation) of blood. Serum is plasma without the clotting proteins. Serum may be used for identification and research of antibodies.

Blood Typing

A person’s blood type is an inherited characteristic of the blood. Before a transfusion is given, many factors are checked to prevent an adverse reaction in the person receiving the blood. Blood type is determined by antigens located on the surface of the RBC. Clumping of the blood cells may occur when the antigens of donated blood react with antibodies in the plasma of the person receiving it. This clumping of incompatible cells blocks the blood vessels and may cause death.

The four major blood types are A, B, AB, and O (Table 12-2). Type AB blood is called the universal recipient because it has no antibodies in the plasma to react with other blood cells and can receive any type of blood safely. Type O blood is the universal donor because the blood cells have no antigens to react with the antibodies in the plasma of the other blood types. Therefore type O blood can be given safely to a person of any blood type.

Answers to Case Studies

are available on the Evolve website:

http://evolve.elsevier.com/Gerdin

Another important aspect of blood typing is the identification of the antigen known as the Rh factor. The Rh factor is found in the RBCs. About 85% of North Americans have this factor and are said to be Rh-positive (Table 12-3). If Rh-positive blood is given to someone with Rh-negative blood, that person’s blood considers the Rh-positive blood a foreign particle and tries to combat it by forming antibodies. A second transfusion of Rh-positive blood can be fatal to an Rh-negative person. The Rh factor also becomes important in the Rh-negative mother having a second Rh-positive baby (Fig. 12-3).

TABLE 12-3

Approximate Distribution of Blood Types in the U.S. Population*

Blood Type	Prevalence (%)
O Rh-positive	38
O Rh-negative	7
A Rh-positive	34
A Rh-negative	6
B Rh-positive	9
B Rh-negative	2
AB Rh-positive	3
AB Rh-negative	1

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