Body Fluids and Circulation - Notes | Class 11 | Part 2: Blood Vascular System - Blood & Blood Vessels



It includes Heart, Blood & Blood vessels.


Formed of plasma (55%) & formed elements (45%).


Straw-coloured, slightly alkaline (pH 7.4) viscous fluid.

Constituents of plasma 
  • Water (90-92%): It is a good solvent.
  • Plasma proteins (6-8 %): Include
    • Fibrinogen: For blood coagulation.
    • Globulins: Act as antibodies (for defense of the body).
    • Albumins: For osmotic balance. Regulate blood pressure.
  • Glucose, amino acids, lipids & cholesterol.
  • Inorganic constituents: Na+, Ca2+, Mg2+, Cl-, HCO3- etc.
  • Gases like CO2, O2, N2 etc.
Plasma without clotting factors is known as Serum.


Red Blood Cells (RBC) or Erythrocytes:
  • Biconcave non-nucleated cells. No mitochondria, Golgi complex etc. 
  • Red colour is due to Haemoglobin (iron containing protein). 
  • Normal Hb level is 12-16 g/ 100 ml.
  • Count: 5 - 5.5 millions/ mm3.
  • Formed in: Red Bone marrow.
  • Average lifespan: 120 days. Worn-out RBCs are destroyed in spleen (graveyard of RBCs).
  • Function: CO2 and O2 transports.
White Blood Cells (WBC) or Leucocytes:
  • Colourless nucleated cells.
  • Count: 6000-8000 /mm3.
  • Formed in: Bone marrow, lymph glands, spleen.
  • Average lifespan: Generally short lived (1- 15 days).
  • Function: Part of immune system.
Types of WBC: Granulocytes & Agranulocytes 

1. Granulocytes

They are 3 types:
  • Neutrophils (Heterophils): 60-65%. Soldier of the body.
Function: Phagocytosis.
  • Eosinophils (Acidophils): 2-3%. Resist infections. Cause allergic reactions.
  • Basophils (Cyanophils): 0.5-1%. Secrete histamine, serotonin, heparin etc. Cause inflammatory reactions.
2. Agranulocytes

They are 2 types:
  • Lymphocytes (20-25%): Smallest WBC with largest nucleus. 
Includes B- lymphocytes & T- lymphocytes. 
Function: Cause immune responses. Secrete antibodies.
  • Monocytes (6-8%): Largest WBC.
Function: Phagocytosis.

Platelets (Thrombocytes):
  • Colourless non-nucleated cell fragments.
  • Count: 1.5 - 3.5 lakhs /mm3.
  • Formed in: Megakaryocytes in Bone marrow.
  • Average lifespan: 7 days.
  • Function: Blood clotting.


It is a mechanism for haemostasis (prevention of blood loss through injuries). 

At the site of injury, following events occur: 
Clumped platelets & tissues release thromboplastin 
It forms thrombokinase (Prothrombinase) enzyme 
Thrombokinase hydrolyses prothrombin to thrombin enzyme in presence of Ca2+ 
Thrombin converts soluble fibrinogen to insoluble fibrin 
Fibrin threads trap dead & damaged blood cells to form clot (coagulum).


Blood groups were discovered by Carl Land Steiner.

1. ABO grouping 

It is based on presence or absence of 2 surface antigens (chemicals that induce immune response) on RBCs namely A & B. 

Similarly, plasma contains 2 antibodies (proteins produced in response to antigens) namely anti-A & anti-B.

Blood group



Can donate blood to

Can receive

blood from (Donor’s group)




A & AB

A, O




B & AB

B, O


A, B


AB only

A, B, AB & O



Anti-A & Anti-B

A, B, AB & O

O only

Antigen A reacts with anti-A. Antigen B reacts with anti-B.

If bloods with interactive antigens & antibodies are mixed together, it causes clumping (agglutination) of RBCs.

Persons with O Group are called Universal donors because they can donate blood to persons with any other blood group. 

Persons with AB group are called Universal recipients because they can accept blood from all groups.

2. Rh grouping 

Rhesus (Rh) factor is another antigen found on RBC.

Rh+ve means the presence of Rh factor and Rh-ve means absence of Rh factor. 

Nearly 80% of humans are Rh+ve.

Anti-Rh antibodies are not naturally found. So Rh-ve person can receive Rh+ve blood only once but it causes the development of anti-Rh antibodies in his blood. So, a second transfusion of Rh+ve blood causes agglutination. Therefore, Rh-group should be matched before transfusion.

Erythroblastosis foetalis

It is a Rh incompatibility between the Rh-ve blood of a pregnant mother and Rh+ve blood of the foetus.

Rh antigens do not get mixed with maternal blood in first pregnancy because placenta separates the two bloods.

But during first delivery, the maternal blood may be exposed to small amount of foetal blood (Rh+ve). This induces the formation of Rh antibodies in maternal blood.

In case of her subsequent pregnancies, the Rh antibodies from the mother leak into the foetal blood (Rh+ve) and destroy the foetal RBCs. This is fatal to the foetus or cause severe anaemia and jaundice to the baby. This condition is called Erythroblastosis foetalis.

It can be avoided by administering anti-Rh antibodies to the mother immediately after the first delivery.


Blood vessels are 3 types: Arteries, Veins & Capillaries.
  • Arteries: They carry blood from heart to other tissues. 
They contain oxygenated blood (except pulmonary artery). 
Their smaller branches are called arterioles. 
 Arteries are 3-layered- inner tunica intima (squamous endothelium), middle tunica media (smooth muscles & elastic fibres) and outer tunica externa (fibrous connective tissue).
  • Veins: They carry blood towards heart. 
They contain deoxygenated blood (except pulmonary vein). 
 Their smaller branches are called venules. 
Veins are also 3-layered but tunica media is comparatively thin.
  • Capillaries: In tissues, arterioles divide into thin walled and single layered vessels. They are called capillaries. They unite into venules.
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