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Blood is a complex and unique fluid of variable composition circulating through the heart, arteries, capillaries, and veins, known as the vascular system of the body. It is a tissue in which cellular constituents are suspended in a liquid medium performing specialized functions. The prime function of blood is to carry oxygen from the lungs to the body tissues and carbon dioxide from the tissues to the lungs. Blood carries fluid to and from the tissues, thus maintaining proper fluid balance throughout the body (explained later in this paragraph). The average pH value of blood is 7.40.

Blood also carries nutrients or food supplies from the digestive system to the body cells or tissues and transports waste products for the tissues to the kidneys and bowel for excretion to prevent accumulation. In response to trauma or infection, blood cells and antibodies are carried in the blood to a point protection against the causative agents of disease, or to transport blood-clotting substances to a break in a blood vessel to promote the clotting process when injury is caused by bleeding or hemorrhage.

The blood also carries hormones from the endocrine glands to the target organs, and it assists in the regulation of the body temperature by carrying excess heat from the interior of the body to the surface layers of the skin, where the heat is dissipated to the surrounding air.

To perform these complex functions, blood is complex and is composed of two main parts. One part is composed of blood cells, the particles suspended in the plasma, making up approximately 45 percent of total blood volume and including erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). The red and white blood cells are known as corpuscles. The other part is plasma, the fluid portion of the blood, which consists primarily of water in which are dissolved proteins and many inorganic and organic substances carried by the blood to and from the tissues. Plasma makes up 55 percent of the blood.


A typical cell is composed of a single nucleus embedded in cytoplasm. The living substance of the cell is grayish, viscous liquid called protoplasm. Protoplasm is enclosed in the cell interfaces by a cell membrane that selectively regulates the interchange of materials between the cell and its environment. A typical cell is shown in figure 1-1; however, some cells have different forms.

Figure 1-1. Typical cell structure.

The nucleus is a spherical oval body surrounded by a thin membrane (nuclear membrane). Contained in the nucleus is a sphere called the nucleolus. The nucleus is thought to be an organizing center for the cell and can have the capacity for cell production. The absence of nuclei signifies the end of cell development. Also found within the nucleus is a network of nuclear fibrils made up of DNA (deoxyribonucleic acid) and protein called chromatin. It is thought that the decreasing growth activity of a cell during maturation is regulated by chromatin.

Surrounding the nucleus is a mass of protoplasm called cytoplasm. Contained within the cytoplasm are numerous granules, filaments, and globules. These structures are divided into two groups known as organoids (organelles) and inclusions. The organoids are thought to perform most of the metabolic functions of the cell. Mitochondria, Golgi apparatus, fibrils, centrioles, and the chromatin substance are classified as orgnanelles. Cytoplasm inclusions are usually seen as granulation. The granulation is an accumulation of proteins, lipids, carbohydrates, pigments, and secretory granules.


Erythrocytes. An erythrocyte (red blood cell) is an elastic, non-nucleated, biconcave disc having a diameter of approximately 7.2 microns. The mature red cell contains about 34 percent hemoglobin (a complex iron-bearing pigment that transports oxygen). Hemoglobin is contained in the interior of the cell, and the outer surface of the cell is surrounded by a cell membrane. When unstained, the cell has a pale, greenish- yellowish appearance. It is buff pink with an accented central zone of pallor when stained with Wright’s stain.

The production of erythrocytes or erythropoiesis, occur primarily in the red marrow of the spongy bones. Erythrocytes make up the great majority of cells found in the peripheral blood. Their vast surface area is important in the transport of oxygen from the lungs to the tissues because of quick exchange of oxygen in both sites that occurs across the red cell surface. Erythrocytes are subject to many alterations in shape, size, staining properties, and structure in different disease processes. An adult female has approximately 4.8 million/cu mm red cells, and an adult male has approximately 5.4 million/cu mm red cells. The erythrocytes have an average life span of 80 to 120 days. See table 1-1 for purposes of the blood cell tree as described in this subcourse.

Table 1-1. Blood cell tree.

Leukocytes. Leukocytes are commonly known as white blood cells because of their lack of color in unstained preparations. They are nucleated cells that have an average diameter of 8 to 12 microns. There are 5,000-10,000 leukocytes per cubic millimeter of blood. Leukocytes are divided into three groups: (1) granulocytes (neutrophils, eosinophils, and basohils) that can phagocytize or ingest bacteria or other particles; (2) lymphocytes which participate in humoral (B lymphocyte) and cell mediated (T lymphocyte) immunity and (3) monocytes that phagocytose bacteria cellular debris and interact with lymphocytes in the processing of antigens in the immune reaction. In humans, there are 3 types of phagocytes: macrophages, granulocytes, and monocytes. All three types are attracted to bacteria and certain other particles by substances released by the particles (the process of chemotaxis). They then pin the particle against a surface and engulf it. They are differentiated by the specific nuclear and cytoplasmic staining properties.

Thrombocytes. Thrombocytes are found commonly known as platelets. They are the morphologically recognizable precursors of platelets and are detached fragments of the cytoplasm-megakaryocytes that are found in the bone marrow (others coming from red bone marrow include: promegakarycocytes and megakaryoblasts). Unstained platelets appear as small hyaline structures with a diameter of approximately two microns. When stained with Wright’s stain, they have a pale blue cytoplasm with a dark granular center. Platelets are very fragile and live for a period of about 3 to 5 days.


Plasma. Blood plasma is a clear, yellowish fluid that accounts for about 55 percent of the total volume of the blood. The chemical nature of plasma is very complex. It consists of 92 percent water, 7 percent proteins (albumin, globulin, and fibrinogen), carbohydrates (glucose), lipids (fats, lecithin, and cholesterol), dissolved gases (oxygen, carbon dioxide, and nitrogen), non-protein nitrogenous substances, and less than 1 percent of inorganic salts. Blood plasma is the fluid portion of the blood before clotting occurs. Put another way, plasma from which fibrinogen has been removed is called serum.

Serum. Serum is the fluid portion of blood after the clotting process is complete. Fibrinogen, the precursor of fibrin, is removed from plasma to form the framework of the blood clot.


David L. Heiserman, Editor

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Revised: June 06, 2015