Class 11 Biology


Blood is a connective tissue which is composed of a fluid matrix (plasma) and formed elements.


Plasma is a straw-coloured and viscous fluid. Plasma constitutes about 55% of the blood. About 90% of plasma is water and about 6-8% is composed of proteins.

The major plasma proteins are: fibrinogen, globulins and albumins.

Fibrinogen plays important role in blood coagulation. Globulins are mainly involved in defense mechanism and albumins help in osmotic balance.

Small amounts of minerals, like Na+, Ca++, Mg+ +, HCO3 and Cl; are also present in plasma.

Plasma also contains glucose, amino acids, lipids, etc. because these substances are always in transit in the body. Factors for clotting of blood are present in the plasma in an inactive form. Plasma without the clotting factors is called serum.

Formed Elements

The formed elements constitute about 45% of the blood. Erythrocytes, leucocytes and platelets are collectively called formed elements.

Erythrocytes or Red Blood Cells (RBCs)

The RBCs are the most abundant cells in blood. In a healthy adult, about 5 million to 5.5 million RBCs are present per cubic mm of blood. RBCs are formed in the bone marrow in adults.

In most of the mammals, nucleus is absent in the RBCs. RBCs are biconcave in shape.

The red colour is because of an iron containing protein complex, called haemoglobin. In a healthy adult, each 100 ml of blood contains 12-16 gm haemoglobin.

The average lifespan of RBCs is 120 days. RBCs are finally destroyed in the spleen and hence, spleen is also called the graveyard of RBCs. RBCs play a significant role in transport of respiratory gases.

Leucocytes or White Blood Cells (WBCs)

The WBCs are nucleated and are relatively lesser in number than RBCs. In a healthy adult about 6000-8000 WBCs are present per cubic mm of blood. Leucocytes are generally short lived.

There are two main categories of WBCs:

  1. Granulocytes , e.g., neutrophils, eosinophils and basophils
  2. Agranulocytes. e.g., Lymphocytes and monocytes.

Neutrophils: Neutrophils are the most abundant cells among WBCs and comprise about 60-65%.

Monocytes: Monocytes comprise about 6-8% of WBCs. Neutrophils and monocytes are phagocytic cells.

Basophils: Basophils secrete histamine, serotonin, heparin, etc. They are involved in inflammatory reactions.

Eosinophils: Eosinophils comprise 2-3% of WBCs. These resist infections and are also associated with allergic reactions.

Lymphocytes: Lymphocytes comprise about 20-25% of WBCs. There are two major types of lymphocytes, viz. B and T types. Both the types are responsible for immune responses of the body.


Platelets are also known as thrombocytes. They are cell fragments produced from megakaryotcytes. Megakaryocytes are special cells in the bone marrow. Usually, one cubic mm of blood contains 150,000-350,000 platelets. Platelets can release a variety of substances. Most of these substances are involved in blood coagulation.

When a person is suffering from dengue, he/she needs to keep watching his platelet count. Platelet count falls drastically in dengue patients. If it falls below a certain level, the patient needs to get blood transfusion to restore the platelet count.

Blood Groups

Two such groupings – the ABO and Rh – are widely used all over the world.

ABO Grouping

ABO grouping is based on the presence of absence of two surface antigens on the RBCs, viz. A and B. Antigens are chemicals which can induce immune response. The plasma also contains two natural antibodies. Antibodies are proteins produced in response to antigens.

Importance of Blood Group

During blood transfusion, the donor blood needs to be carefully matched with the blood of a recipient. Transfusion of unmatched blood can lead to severe problems of clumping, i.e. destruction of RBC.

Blood Groups and Donor Compatibility
Blood GroupsAntigen on RBCAntibodies in PlasmaDonor’s Group
AAAnti-BA, O
BBAnti-AB, O
OOAnti-A, BO

The blood group O can be donated to persons with any other blood group and hence, an individual with O group is called universal donor. A person with AB blood group can accept blood from all blood groups and hence, such an individual is called universal recipient.

Rh Grouping

The Rh antigen is similar to one present in Rhesus monkeys. It is also observed on the surface of RBCs of majority (nearly 80%) humans. Such individuals are called Rh positive (Rh +ve). A person without Rh antigen is called Rh negative (Rh –ve).

Significance of Rh Group

An Rh -ve person, if exposed to Rh +ve blood, will form specific antibodies against the Rh antigens. Hence, Rh group should also be matched before transfusions.

Rh Incompatibility of Foetus and Mother

A special case of Rh incompatibility is observed between the Rh –ve blood of a pregnant mother with Rh +ve blood of the foetus. Rh antigens of the foetus do not get exposed to the Rh –ve blood of the mother in the first pregnancy because the two bloods are well separated by the placenta.

But during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of Rh +ve blood from the foetus. In such an eventuality, the mother starts preparing antibodies against Rh in her blood.

In case of her becoming pregnant again, the Rh antibodies from the mother (Rh –ve) can leak into the blood of the foetus (Rh +ve) and destroy the foetal RBCs. This can prove fatal to the foetus or can cause severe anemia and jaundice to the baby. This condition is called erythroblastosis foetalis. This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Coagulation of Blood

Blood coagulates in response to an injury or trauma. Coagulation is a mechanism to prevent excessive loss of blood in case of injury. Clot is a dark reddish brown scum which is formed at the site of an injury over a period of time. This is formed by a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.

Process of Blood Clotting

Inactive fibrinogen is present in plasma. It is converted by the enzyme thrombin into active fibrin. Thrombin is formed from the inactive prothrombin. An enzyme complex, thrombokinase, is responsible for this conversion. This complex is formed by a series of linked enzymatic reactions (cascade process). The process involves a number of factors present in the plasma in an inactive state. An injury stimulates the platelets to release certain factors which activate the mechanism of coagulation. Calcium ions play a very important role in clotting.

Haemophilia: Haemophilia is a clotting disorder which prevents blood clotting. A person suffering from haemophilia is always at a risk of excessive blood loss in case of injury.

Lymph (Tissue Fluid)

When the blood passes through the capillaries in tissues, some water; along with many small water-soluble substances move out into the spaces between the cells. Larger proteins and most of the formed elements are left in the blood vessels. This fluid is called the interstitial fluid or tissue fluid. Exchange of nutrients, gases, etc. between the blood and the cells always occur through this fluid.

There is an elaborate network of vessels called the lymphatic system. The lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called lymph.

Lymph is a colourless fluid. It contains specialized lymphocytes. Lymph also carries nutrients, hormones, etc. Fats are absorbed through lymph in the lacteals present in the intestinal villi.