Unit – I B

Blood and its Composition


  • Study of blood is called haematology. Blood is a fluid connective tissue. It is bright red, slightly alkaline (pH 7.4), salty viscous fluid heavier than water.
  • An adult has a blood volume of approximately 5 litres.
  • Blood is the only tissue that exists in both the liquid and solid state simultaneously.

Dissolved Molecules in Blood:


These help to regulate water potential, by maintaining normal blood volume and pressure. They are the most common plasma protein.

Immunoglobins (antibodies):

These are very large proteins that target infection and so cause infected or foreign cells to be attacked by white blood cells (WBC’s). Together with the WBC’s they form the immune system.


These are tightly coiled proteins that unwind to form a blood clot.

Functions of Blood:

1. Delivers digested nutrients to cells
2. Delivers oxygen to cells
3. Delivers enzymes, hormones and other chemical messengers to cells
4. Delivers water, vitamins and minerals to cells
5. Picks up carbon dioxide from cells and carries it to the lungs to be expelled
6. Pick up urea and other chemical wastes and carries them to the liver and kidneys for disposal
7. Picks up excess body heat and brings it to the skin to be excreted
8. Fights disease
9. Helps begin the repair process after a cut or other injury

Composition of Human Blood:

a) Plasma:

  • It is the straw coloured non-living, liquid part of blood.
  • Plasma is composed of about 90 to 92% water and 8% to 10% proteins, salts, hormones, enzymes and other various chemicals. Most of the solute part about 7% is proteins.
  • Plasma makes up about 45 – 50% of blood’s volume
  • Blood corpuscles and platelets are suspended in plasma.

The figure shows separation of blood constituents on centrifuging.

B) Red Blood Corpuscles (Erythrocytes):

  • They are the most common type of blood cell (5.1 to 5.8 million/per cubic mm)
  • They are living cells, having average life span of 120 days.
  • They are non nucleated, small in size, round and biconvex in shape (thin in the middle and thick around the edges).
  • They give red colour to the blood.
  • The formation of RBCs is called erythropoiesis.
  • In foetus they are formed in liver and spleen whereas in adult they are formed in red bone marrow.
  • They contain haemoglobin, a protein that helps in transportation of oxygen and carbon dioxide. Haemoglobin maintains pH of blood and act as buffer. Less amount of haemoglobin results in anaemia.

c) White Blood Corpuscles (Leucocytes):

  •  They are colourless, nucleated, amoeboid and phagocytic cells larger than RBC’s.
  • Their life span is about 3 to 4 days.
  •  Their approximate count in a blood of healthy person is about 5000 to 9000 WBC’s per cubic mm .
  • They are produced in the bone marrow, spleen and thymus. Formation of WBCs is called leucopoiesis.
  • The dead WBCs are destroyed by phagocytosis in blood, liver and lymph nodes.
  • They can move through the walls (squeeze through) of blood vessels into the lymphatic system. This is known as diapedesis.
  • Their numbers increase rapidly in the presence of a pathogen (can go from 8000/drop to over 30 000/drop in a matter of hours). Their number returns to normal when the infection is cleared from the body.
  • WBC’s rid the body of pathogens in the process of phagocytosis:  the WBC surrounds, engulfs and “eats” the invading pathogen.
  • Increase in number of WBCs is called leucocytosis.
  • Decrease in number of WBCs is called leucopenia.

Infection:  An attack on the body’s cells by a pathogen is called infection.

Pus: A whitish substance made of millions of  dead WBC’s and living and dead bacteria. pus usually forms when the pathogen is winning the battle

Leukemia: Pathological increase in number of WBCs is called blood cancer or Leukemia. blood cancer characterized by the bone marrow producing an abnormally high number of  non-functional WBC’s. WBC count does not return to normal levels; instead remains high.[/box]

Types of White Blood Corpuscles (Leucocytes):

Depending upon the presence of granules present in them white blood corpuscles are classified as a) Granulocytes and b) Agranulocytes
Granulocytes on basis of staining agent are classified as i) Neutrophils ii) Eosinophils (Acidophils) and iii) Basophils. While agranulocytes are classified as i) Lymphocyes and ii) Monocytes

Classification of Granulocytes:

i) Neutrophils:

a) They are Stained with neutral dyes
b) Their nucleus is 3 to 5 lobed.
c) They are phagocytic in function
d) They constitute 54 to 62 % of total WBCs
e) They are also called polymorphs.



ii) Eosinophils (Acidophils):

a) They are stained with acidic dyes like eosin.
b) Their nucleus is bilobed.
c) They are non phagocytic in function
d) Their number increases in allergy
e) They constitute 3 % of total WBCs
f) Pathological increase in number is called eosinophilia.

iii) Basophils:

a) They are stained with basic dyes like methylene blue
b) Their nucleus is twisted.
c) They are non phagocytic in function
d) They constitute 0.5 % of total WBCs
e) They release heparin (anticoagulant) and histamine involved in inflammatory and allergic reaction).


Classification of Agranulocytes:
i) Lymphocytes:

a) They have large nucleus
b) They constitute 25 to 33 % of total WBCs
c) They produce antibodies
d) They are responsible for immune response of the body.



ii) Monocytes:

a) They are largest of all WBCs
b) They have large kidney shaped nucleus
c) They constitute 3 to 9 % of total WBCs
d) They are phagocytic in function
e) They remove cell debris
f) They are scavengers of cell.

d) Platelets or Thrombocytes:

1. They are non nucleated
2. They are round and biconvex in shape.
3. They are the smallest element in the blood.
4. They have life span 5 to 10 days
5. Formation of platelets is called thrombopoiesis
6. Increase in number platelets is called thrombocytosis.
7. Decrease in number is called thrombocytopenia
8. Release thromboplastin useful in clotting of the blood.

Clotting of Blood:

  1. Inside blood vessel blood does not clot due to presence of anticoagulants like heparin and antithrombins. As soon as blood vessel ruptures, bleeding starts. The conversion of liquid blood into semisolid jelly is called blood coagulation or blood clotting.
  2. Platelets play an important role in blood clotting, by adhering to the site of the wound and releasing clotting factors known as prothrombin. Prothrombin is inactive.
  3. At the site of rupture the platelets and injured tissues release thromboplastin which initiates formation of enzyme prothrombinase.
  4. In presence of Ca ions prothrombinase convert inactive prothrombin to active thrombin. Thrombin converts soluble fibrinogen into fibrin.
  5. The fibrin forms net to enmesh platelets blood cells and plasma to form clot.

Genetic Disorders:


A genetic disorder of Factor VII is called haemophilia, suffers (all male – why?) may bleed extensively from even a small cut or scrape.


Unwanted clotting of blood within blood vessels can block the flow of blood. If this happens in the brain, brain cells may die, causing a stroke.

Coronary Thrombosis:

In the coronary artery, it may cause the death of heart cells.[/box]

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