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| Blood |
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| It is a type of fluid connective tissue, which forms the chief circulating fluid in the body. As a circulating fluid, blood is involved in a wide variety of functions in the body. As a type of connective tissue, blood shows two essential components, namely matrix and cells. |
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| Matrix of blood is known as plasma. Nearly 90% of the plasma is represented by water. The organic substances account for about 9% and the remaining 1% is represented by inorganic substances. The organic substances in the plasma occur in a wide variety of forms. There are carbohydrates mostly in the form of monosaccharides, fats occurring in the form of fatty acids and glycerol. Amino acids and proteins are also present. The plasma proteins are represented by simple proteins called albumins and globulins. The albumins mainly function as buffers preventing a change of pH in blood and other body fluids. The globulins function as antibodies. The plasma also has proteins like fibrinogen and prothrombin, which play an important role in the clotting of blood. The plasma also contains organic compounds in the form of enzymes or vitamins or hormones and metabolic waste products like urea, uric acid and hippuric acid. The inorganic substances in the plasma are represented by salts and ions of some inorganic elements. |
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| Note: |
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| Serum is the liquid part of the blood that remains after the blood clots. |
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| Three types of cells are found suspended in the plasma of blood. They are: erythrocytes (RBC), leukocytes (WBC) and thrombocytes (Platelets). |
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 Erythrocytes (red blood corpuscles) are spherical or oval cells that are responsible for the oxygen carrying capacity of blood. The RBC are characterized by the presence of a pigment called haemoglobin. It is a conjugated protein having hemo as the prosthetic group. Haemoglobin has a very high affinity for oxygen. It readily combines with oxygen to form oxyhaemoglobin. Haemoglobin is also responsible for imparting a red colour to the blood. RBC represent the most common type of cells in the plasma. In the human males the number of RBC is about 50-55 lakhs/mm3 of blood. In females it is about 45-50 lakhs/mm3. In the case of all mammals, RBC appears in form of biconcave discs. Except in the camel, the RBC of mammals are enucleate. |
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| fig. 5.71 |
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| Note: |
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| Erythrocytes make up about 45% of the total blood volume. This volume percentage is known as hematocrit. |
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| RBC take their origin in the bone marrow. The liver also produces RBC, but only during infancy. The average life span of RBC is about 100-120 days. The old, worn out and dead RBC are destroyed mainly in an organ called spleen. Hence, bone marrow is commonly called the cradle of RBC' and spleen is commonly called the 'graveyard of RBC'. |
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 Leukocytes (white blood cells) are amoeboid cells, which play an important role in the defence mechanism of the body by ingesting bacteria and other disease causing germs. WBC represent the least numerous cells in the plasma. The number of WBC in the human blood is about 7000/mm3. The WBC have a very short life span ranging between 1 and 4 days. New WBC are produced in the bone marrow as well as in the lymph glands. The old and dead WBC are destroyed in the spleen. |
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| Based on structural differences, leukocytes are differentiated into |
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| 1. Granulocytes |
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| 2. Agranulocytes |
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 Granulocytes are WBC, in which the cytoplasm is granular and the nucleus is always lobed. The granulocytes, take their origin in the marrow. Based on their staining ability, granulocytes are distinguished into the following three types. |
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| a) Eosinophils are granulocytes, which take up only acidic stains. The nucleus is bilobed. They represent about 1.5% of the total WBC. |
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| b) Basophills are granulocytes, which take up only basic stains. The nucleus is many lobed and usually S shaped. They represent about 0.5% of the total WBC. |
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| c) Neutrophils are granulocytes, which are neutral to stains. The nucleus has three to five lobes. Hence, they are also described as polymorphonuclear leukocytes. They represent about 70% of the total WBC. |
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 Agranulocytes are WBC in which cytoplasm is clear and transparent. Nucleus is always unlobed. Agranulocytes take their origin in the lymph glands like thymus. They can be distinguished into the following two types. |
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| a) Lymphocytes are agranulocytes characterized by the presence of a large usually round nucleus. They represent about 24% of the total WBC. |
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| b) Monocytes are agranulocytes characterized by the presence of a large kidney shaped nucleus. They represent about 4% of the total WBC. |
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| c) Thrombocytes (blood platelets) are tiny irregular cell fragments, which arise from the breakdown of larger cells in the bone marrow called megakaryocytes. The average life span of platelets is about 7 to 10 days. The number of platelets is about 2,50,000/mm3 of blood. The thrombocytes play both mechanical and chemical roles in the clotting mechanism of blood. At the site of the injury, the platelets rupture and release a substance called thromboplastin. It converts the inactive protein, prothrombin into an active form called thrombin. The activated thrombin brings about a conversion of the soluble protein fibrinogen into an insoluble form called fibrin. The fibrin threads entangle the blood cells and result in the formation of a clot. |
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| RBC WBC |
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fig. 5.72 Blood Cells |
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 It transports oxygen to all the parts of the body. |
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 It collects carbon dioxide from the cells and transports it to the respiratory organs. |
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 It distributes the digested food to all the regions of the body. |
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 It collects metabolic waste products from the cells and transports them to the excretory organs. |
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 It carries the raw-materials to the exocrine and endocrine glands. |
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 It distributes the heat generated in the body, due to metabolic reactions (Thermoregulation). |
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 It plays an important role in the defence mechanism of the body, by producing antibodies. |
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 The plasma proteins play an important role in maintaining the acid-base balance of the body. |
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