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Describe human blood and give its functions.

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Blood Composition:

1. Blood is a red coloured fluid connective tissue derived from embryonic mesoderm. 

2. It has two components – the fluid plasma (55%) and the formed elements i.e. blood cells (44%). 

3. Plasma is a straw coloured, slightly alkaline and viscous fluid having 90% water and 10% solutes such as proteins, nutrients, nitrogenous wastes, salts, hormones, etc. 

4. Blood corpuscles are of three types, viz. erythrocytes (RBCs), white blood corpuscles (WBCs) and thrombocytes (platelets).

(5) Red blood corpuscles or Erythrocytes:

1. Erythrocytes or red blood corpuscles. They are circular, biconcave, enucleated cells. 

2. The RBC size : 7 pm in diameter and 2.5 pm in thickness. 

3. The RBC count : 5.1 to 5.8 million RBCs/ cu mm of blood in an adult male and 4.3 to 5.2 million/cu mm in an adult female. 

4. The average life span of RBC : 120 days. 

5. RBCs are formed by the process of erythropoiesis. In foetus, RBC formation takes place in liver and spleen whereas in adults it occurs in red bone marrow. 

6. The old and worn out RBCs are destroyed in liver and spleen. 

7. Polycythemia is an increase in number of RBCs while erythrocytopenia is decrease in their (RBCs) number.

Functions of RBCs:

1. Transport of oxygen from lungs to tissues and carbon dioxide from tissues to lungs with the help of haemoglobin. 

2. Maintenance of blood pH as haemoglobin acts as a buffer. 

3. Maintenance of the viscosity of blood.

(6) White blood corpuscles / Leucocytes:

1. Leucocytes or White Blood Corpuscles (WBCs) are colourless, nucleated, amoeboid and phagocytic cells. 

2. Their size ranges between 8 to 15 pm. Total WBC count is 5000 to 9000 WBCs/cu mm of blood. The average life span of a WBC is about 3 to 4 days. 

3. They are formed by leucopoiesis in red bone marrow, spleen, lymph nodes, tonsils, thymus and Payer’s patches, whereas the dead WBCs are destroyed by phagocytosis in blood, liver and lymph nodes. 

4. Leucocytes are mainly divided into two types, viz., granulocytes and agranulocytes. 

5. Granulocytes : Granulocytes are cells with granular cytoplasm and lobed nucleus. Based on their staining properties and shape of nucleus, they are of three types, viz. neutrophils, eosinophils and basophils.

(I) Neutrophils:

1. In neutrophils, the cytoplasmic granules take up neutral stains. 

2. Their nucleus is three to five lobed. 

3. It may undergo changes in structure hence they are called polymorphonuclear leucocytes or polymorphs.

4. Neutrophils are about 70% of total WBCs. 

5. They are phagocytic in function and engulf microorganisms.

(II) Eosinophils or acidophils:

1. Cytoplasmic granules of eosinophils take up acidic dyes such as eosin. They have bilobed nucleus.

2. Eosinophils are about 3% of total WBCs. 

3. They are non-phagocytic in nature. 

4. Their number increases (i.e. eosinophilia) during allergic conditions. 

5. They have antihistamine property.

(III) Basophils:

1. The cytoplasmic granules of basophils take up basic stains such as methylene blue.

2. They have twisted nucleus.

3. In size, they are smallest and constitute about 0.5% of total WBCs. 

4. They too are non-phagocytic. 

5. Their function is to release heparin which acts as an anticoagulant and histamine that is involved in inflammatory and allergic reaction.

6. Agranulocytes : There are two types of agranulocytes, viz. monocytes and lymphocytes. Agranulocytes do not show cytoplasmic granules and their nucleus is not lobed. They are of two types, viz. lymphocytes and monocytes. 

(I) Lymphocytes: 

1. Agranulocytes with a large round nucleus are called lymphocyte. 

2. They are about 30% of total WBCs. 

3. Agranulocytes are responsible for immune response of the body by producing antibodies.

(II) Monocytes:

1. Largest of all WBCs having large kidney shaped nucleus are monocytes. They are about 5% of total WBCs. 

2. They are phagocytic in function. 

3. They can differentiate into macrophages for engulfing microorganisms and removing cell debris. Hence they are also called scavengers. 

4. At the site of infections they are seen in more enlarged form.

(7) Thrombocytes/Platelets:

1. Thrombocytes or platelets are nonnucleated, round and biconvex blood corpuscles. 

2. They are smallest corpuscles measuring about 2.5 to 5 mm in diameter with a count of about 2.5 lakhs/cu mm of blood.

3. Their life span is about 5 to 10 days. 

4. Thrombocytes are formed from megakaryocytes of bone marrow. They break from these cells as fragments during the process of thrombopoiesis. 

5. Thrombocytosis is the increase in platelet count while thrombocytopenia is decrease in platelet count. 

6. Thrombocytes possess thromboplastin which helps in clotting of blood. 

7. Therefore, at the site of injury platelets aggregate and form a platelet plug. Here they release thromboplastin due to which further blood clotting reactions take place.

(8) Functions of blood:

1. Transport of oxygen and carbon dioxide 

2. Transport of food

3. Transport of waste product 

4. Transport of hormones 

5. Maintenance of pH 

6. Water balance 

7. Transport of heat 

8. Defence against infection 

9. Temperature regulation 

10. Blood clotting/coagulation 

11. Helps in healing

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