Blood is a red vascular connective tissue. It forms 6 to 10% of body weight. Generally, 90 ml/kg of blood is present in human body. It is somewhat sticky and slightly heavier than water. It is slightly alkaline, having pH 7.4.
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Blood transports vital requirements and waste products of the body. Blood consists of two types of components—
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A. Blood Cells,
B. Plasma.
Blood Cells:
There are three types of blood cells
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1. RBC: Red blood cells/corpuscles or Erythrocyte.
2. WBC: White blood cells/corpuscles or Leucocyte.
3. Platelets or Thrombocytes.
Erythrocyte (RBC):
Erythrocytes are red blood cells.
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These are bi-concave, non-nucleated, discs like cells. The central area is thin, while edges are thick. The edges contain haemoglobin, which transports respiratory gases, i.e. oxygen and carbon dioxide (02 and C02).
The shape favors flexibility for absorbing and releasing of gases quickly. The size of RBC is 7.2 micron (μ). In a mature male, about 5 to 5.5 millions/ cubic mm of blood, and in females, about 4 to 4.5 million per cubic mm of blood are present. The life period of RBC is about 90 to 120 days (Figs 2.1A Plate 1, and 1B).
Leucocyte (WBC):
These are colorless, nucleated cells. Normally, 4,000 to 10,000 WBC per cu. mm. of blood are present. There are two different types of WBCs, classified on the basis of presence of granules in their cytoplasm.
A. Granulocyte
B. Agranulocyte
Granulocyte:
These are characterized by presence of granules in their cytoplasm. These cells are produced in bone marrow.
Granulocytes are classified on the basis of shape of nucleus and the dye, which they take. These are:
a. Neutrophil
b. Basophil
c. Eosinophil.
Neutrophil:
Neutrophil is also known as polymorph or microphage. There are about 65 to 70% neutrophils present of total WBC. The cytoplasm is full of granules that picks up, both, acidic as well as basic dye.
The nucleus has about 3 to 7 lobes connected by chromatin strand or thread. The life period of neutrophil is about 3 to 5 days. It is a large cell, having diameter 12 to 15 m. Neutrophils are phagocytic in nature.
Basophil:
Cytoplasm of basophil contains small rounded granules, which takes up basic stain. The size is somewhat small with diameter 8 to 10 μ. The nucleus is usually kidney shaped or slightly lobulated. The cytoplasmic granules are central and overlap the nucleus. The life period of basophil is 8 to 12 days. Normally about 0.005 to 1% basophils are present.
Basophil secrets a chemical substance, known as heparine, which prevents the clotting of blood inside the blood vessel.
Eosinophil:
Eosinophils are also known as bilobed cells, as nucleus contains two lobes connected with chromatin strand. It picks up acidic stain. These are circular in shape with diameter 10 to 12 μ. normally, about 15 to 400 cells, i.e. about 1 to 6% of blood is present. Life period of eosinophil is 8 to 10 days.
It protects the body by releasing a chemical substance, called as histamine. Histamine sets off a chain of reactions that ultimately leads to an inflammatory response at the site of infection. Therefore, in case of allergic conditions the number of eosinophil increases.
Agranulocyte:
These are characterized by the absence of granules in the cytoplasm.
These cells are manufactured in lymph gland and spleen. These are of two types:
a. Lymphocytes
b. Monocytes
Lymphocytes:
There are two types of Lymphocytes on the basis of area of cytoplasm.
i. Small Lymphocyte:
In small lymphocyte, the nucleus is relatively large in size and rounded in shape, which occupies a major part of the cell, leaving behind small cytoplasmic area. The diameter of the cell is 8 to 10 μ.
It is about 50% of the total WBCs at the time of birth. The number decreases with the age, at the age of 10 to 15 years, it is 35% and in adults, it is 20 to 30% of the total cells. The life period of small lymphocyte is very short, less than 24 hrs. It produces antibody.
ii. Large Lymphocyte:
The diameter of large lymphocyte is about 15 to 18 μ. The nucleus is centrally placed, small, rounded in shape, leaving behind a large cytoplasmic area. There are about 5 to 10% large lymphocytes presents in adult, however, the number is more in children. The life period of large lymphocyte is small, about few hrs. It produces antibody.
Monocyte:
Monocyte is the largest of all WBC, having diameter 15 to 18 μ. The nucleus is kidney shaped. It can be differentiated from basophil, as the nucleus of monocyte is eccentric. The monocytes are about 2 to 6% of total WBCs, i.e. about 100 to 600 cells per cu mm. The life period of monocyte is about 1 to 2 days. The monocytes are phagocytic in nature.
Platelets or Thrombocytes:
These are the smallest of all blood cells. Platelets are spindle shaped cells, having diameter 2 to 3 μ. These are also known as thrombocytes. Along with another coagulation factors, it helps in the process of clotting of blood.
Thrombocytes are having very short life-span, it is of few hrs. Normally, 1.5 to 4 lakh platelets per cubic mm. of blood are present. These are non-nucleated cells containing very fine granules.
Plasma:
Plasma is a liquid (fluid) part of blood. It contains about 90 to 91% of water. Plasma contains fats, sugar, various proteins and hormones. Inorganic components of plasma includes phosphates, sulfates, sodium chloride, sodium carbonate, magnesium, etc.
These are dissolved in water. Organic components of plasma include albumin, fibrinogen, globulin, etc. Plasma also contains digestive nutrients like glucose, fatty acids, glycerol and calcium, etc. Excretory substances like urea, uric acid are also present in plasma. The other substances present in plasma include antigen, antibodies, vitamins, etc.
Chemical composition of plasma is not constant because continue exchange of various substances from plasma to different organs is going on. In this way, plasma is acting as a transport medium in body.
Functions of Blood:
1. Transport of oxygen from the lungs to the tissues and of carbon dioxide from tissues to the lungs.
2. Transport of metabolic wastes to the lungs, kidneys, skin and intestines for removal.
3. Maintenance of normal acid-base balance.
4. Transport of absorbed fatty acids, monosaccharides and amino acids.
5. Regulation of water balance.
6. Regulation of body temperature.
7. Transport of hormones, vitamins and salts which contain cations such as sodium, potassium, calcium, etc. and anions such as chlorides, phosphates, sulphates and carbonates.
8. Transport of metabolites.
9. Defence against infection by the white cells and by the antibodies.
10. To stop bleeding by clotting.