ACKNOWLEDGEMENTS

1. Blood plasma, a watery liquid extracellular matrix that contains dissolved substances, and

2. Formed elements, which are cells and cell fragments.

If a sample of blood is centrifuged (spun) in a small glass tube, the cells

layer on top. Blood is about 45% formed elements and 55% blood plasma. Normally, more than 99% of the formed elements are cells named for their red color—red blood cells (RBCs).

3.2.1 Blood Plasma

When the formed elements are removed from blood, a straw-colored liquid called blood plasma (or simply plasma) is left. Blood plasma is about 91.5% water and 8.5% solutes, most of which (7% by weight) are proteins. Some of the proteins in blood plasma are also found elsewhere in the body, but those confined to blood are called plasma proteins.

3.2.2 Formed Elements

The formed elements of the blood include three principal components: red blood cells (RBCs), white blood cells (WBCs), and platelets. RBCs and WBCs are whole cells; platelets are cell fragments. RBCs and platelets have just a few roles, but WBCs have a number of specialized functions.

Several distinct types of WBCs—neutrophils, lymphocytes, monocytes, eosinophils, and basophils—each with a unique microscopic appearance, carry out different functions.

I. Red blood cells II. White blood cells

A. Granular leukocytes (contain conspicuous granules that are visible under a light microscope after staining)

1. Neutrophils 2. Eosinophils 3. Basophils

B. Agranular leukocytes (no granules are visible under a light microscope after staining)

1. T and B lymphocytes and natural killer (NK) cells 2. Monocytes

III. Platelets

Figure 44: Photomicrograph and Scanning electron micrograph of the formed elements of blood.

3.2.3 Red Blood Cells

Red blood cells (RBCs) or erythrocytes (e-RITH-ro¯ -sı¯ts; erythro-_

red; -cyte _ cell) contain the oxygen-carrying protein hemoglobin, which is a pigment that gives whole blood its red color. A healthy adult male has about 5.4 million red blood cells per microliter (μL) of blood,* and a healthy adult female has about 4.8 million. (One drop of blood is about 50 μL.) To maintain normal numbers of RBCs, new mature cells must enter the circulation at the astonishing rate of at least 2 million per second, a pace that balances the equally high rate of RBC destruction. These cells, like almost all the blood cells, live a much shorter time (120 days) than most other cells in the body, some of which last a lifetime. One purpose of the red cells is to carry oxygen from the lungs to the tissues. The oxygen is bound in the red cells to haemoglobin, a protein that contains iron. The erythrocytes are by far the most numerous of the corpuscles, averaging from 4.5 to 5 million per cubic millimeter of blood.

3.2.4 Leukocytes

The leukocytes, or white blood cells, are very different from the erythrocytes in appearance, quantity, and function. Unlike red blood cells, white blood cells or leukocytes have nuclei and do not contain hemoglobin. WBCs are classified as either granular or agranular, depending on whether they contain conspicuous chemical-filled cytoplasmic granules (vesicles) that are made visible by staining when viewed through a light microscope. Granular leukocytes include neutrophils, eosinophils, and basophils; agranular leukocytes include lymphocytes and mono-cytes.

3.2.5 Granular Leukocytes

After staining, each of the three types of granular leukocytes displays conspicuous granules with distinctive coloration that can be recognized under a light microscope. The large, uniform-sized granules within an eosinophil are eosinophilic, they stain red-orange with acidic dyes. The granules usually do not cover or obscure the nucleus, which most often has two lobes connected by a thick strand of chromatin. The round, variable-sized granules of a basophil are basophilic, they stain blue-purple with basic dyes. The granules commonly obscure the nucleus, which has two lobes. The granules of a neutrophil (NOO-tro-fil) are smaller, evenly distributed, and pale lilac in color, the nucleus has two to five lobes, connected by very thin strands of chromatin. As the cells age, the number of nuclear lobes increases. Because older neutrophils have several differently shaped nuclear lobes, they are often called polymorphonuclear leukocytes (PMNs), polymorphs, or “polys.”

Younger neutrophils are often called bands because their nucleus is more rod-shaped.

3.2.6 Agranular Leukocytes

Even though so-called agranular leukocytes possess cytoplasmic granules, the granules are not visible under a light microscope because of their small size and poor staining qualities. The nucleus of a lymphocyte is round or slightly indented and stains darkly. The cytoplasm stains sky blue and forms a rim around the nucleus. The larger the cell, the more cytoplasm is visible. Lymphocytes may be as small as 6 - 9μm in diameter or as large as 10 - 14 μm in diameter. There are three types of lymphocytes: T lymphocytes (T cells), B lymphocytes (B cells), and natural killer (NK) cells.

3.2.7 Functions of WBC

The most important function of the leukocytes is to destroy pathogens.

Whenever pathogens enter the tissues, for example through a wound, certain white blood cells (neutrophils and monocytes) are attracted to that area. They leave the blood vessels and proceed by ameboid or ameba-like motion to the area of infection. There they engulf the invaders by a process called phagocytosis. If the pathogens are extremely strong or numerous, they may destroy the leukocytes. The collection of dead and living leukocytes, forms pus. A collection of pus localized in one area is known as abscess. The Lymphocytes destroy foreign invaders by attacking the cells directly or by producing antibodies that circulate in the blood and help to destroy the cells.

3.2.8 Platelets

Of all the formed elements, the blood platelets (thrombocytes) are the smallest. Platelets help to stop blood loss from damaged blood vessels by forming a platelet plug. When, as a result of injury, blood comes in contact with any tissue other than the lining of the blood vessels, the platelets stick together and form a plug that seals the wound. Their granules also contain chemicals that, once released, promote blood clotting. Platelets have a short life span, normally just 5 to 9 days. Aged and dead platelets are removed by fixed macrophages in the spleen and liver

Plasma: Plasma is the non-cellular or liquid portion of the blood that makes up about 55% of the blood’s volume. Plasma is a mixture of water, proteins, and dissolved substances. Around 90% of plasma is made of water, although the exact percentage varies depending upon the hydration levels of the individual. The proteins within plasma include antibodies and albumins. Antibodies are part of the immune system and bind to antigens on the surface of pathogens that infect the body. Albumins help tomaintain the body’s osmotic balance by providing an isotonic solution for the cells of the body. Many different substances can be found dissolved in the plasma, including glucose, oxygen, carbon dioxide, electrolytes, nutrients, and cellular waste products. The plasma functions as a transportation medium for these substances as they move throughout the body.