Composition of blood
Blood is a fluid connective tissue which consists of 55% plasma and 45% blood cell.
In a healthy adult, the average blood volume is 4.5 litres to 6 litres.
Plasma is a transparent, pale yellowish fluid which is non-living. It contains several substances like hormones, antibodies, etc.
It consists of 90% water and 10% solid in the form of protein, fats, minerals and carbohydrates.
It forms 55% of the total volume of the blood in the human body.
Functions of plasma
1. It transports digested food to different organs and tissues of the body.
2. It removes and carries the waste substances from the tissues to the related excretory organs for their elimination.
3. It removes the major portion of carbon dioxide from the tissues.
4. It transports the hormones secreted by various endocrine and heterocrine glands.
5. It regulates body temperature.
6. It contains protein like fibrinogen and prothrombin which help in the clotting of blood at cuts.
2. Blood cells/Blood corpuscles
Blood cells are scattered in plasma. They consist of Red Blood Corpuscles (RBC), White Blood Corpuscles (WBC) and Blood Platelets.
They cover about 45% of the total volume of blood.
a. Red blood corpuscles (RBC) or erythrocytes
In human, RBCs are circular, binocave and disc-shaped. They have no nuclei.
Each cubic mm of circulating blood contains 45 to 50 lakhs of RBCs.
In adults, RBCs are formed in the bone marrow such as ribs, vertebrate, etc. from where they escape into the general bloodstream.
The lifespan of RBCs is about 4 months (127 days). They are destroyed in the liver and spleen.
RBCs are red in colour due to presence of the most important pigment known as haemoglobin (Hb).
Haemoglobin contains iron due to which RBCs carry oxygen.
The haemoglobin with oxygen is called oxyhaemoglobin (HBO). It is scarlet in (bright red) colour.
When the number of RBCs or their content in haemoglobin decreases, it leads to a disease called anaemia.
Sometimes, due to same reason, there is less amount of oxygen inside the body.
RBCs or erythrocytes are called oxygen carriers as they carry oxygen from the lungs to the body tissues for respiration.
b. White blood corpuscles (WBCs) or leukocytes
WBCs are colourless and irregular in shape. They have nuclei bur no haemoglobin.
They may be granular or non-granular type.
Granular WBCs have multi-lobed nuclei while non-granular WBCs have round nuclei.
Neutrophils, eosinophils and basophils are granular WBCs while lymphocytes and monocytes are non-granular.
Each cubic mm of blood contains about 6000 to 10000 white blood cells. They are formed in the bone marrow and the lymph glands.
Their lifespan is of about 15 days and is destroyed like the RBCs.
If the number of WBCs remains above normal in a person’s body, it is called leukaemia, which is a blood cancer.
WBCs, help in defence of the body by phagocytosis with the production of anti-bodies and by neutralizing the toxic substances, produced by the germs.
Neutrophils ingest and destroy bacteria, eosinophils increase and become active in the case of certain infections and allergies and basophils secrete an anticoagulant.
Similarly, lymphocytes and monocytes are associated with the immune system.
Lymphocytes produce antibodies and monocytes ingest non-bacterial foreign substances during infection.
Immune power pf the body decreases if the number of WBCs reduces below normal. This disease is called leukopenia.
c. Blood platelets or thrombocytes
Blood platelets are small, colourless and are oval in shape. They appear spindle-shaped or rod-shaped in profile.
They are produced in the bone marrow and after a few days of life, they die in the spleen. There is no nucleis in them and the lifespan of blood platelets is of about 2 to 3 days.
Each cubic mm of blood contains 2 to 4 lakhs platelets. They play an important role in case of bleeding and healing of wounds.
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