Blood | Definition, Composition, Functions, & Types | Biologist World

Study of Blood – Hematology


  • Blood is a connective tissue (Fluid).
  • It is a vital fluid on which life depends of all the organisms.
  • It is basic (base) in nature and its pH lies in between 7.3 to 7.4.
  • Blood is approximately 7 to 9 percent of the total body weight.
  • The volume of blood in a healthy man is 4 – 6 liters and in women, it’s 4-5 liters.
  • Blood is heavier than the water.


  • Blood is composed of Blood cells, Blood platelets, and also of Plasma.
  • Blood cells and blood platelets collectively called as Formed elements.

A. Blood cells

  • Also known as Hematocyte, Hemocyte, and Haematopoietic cells.
  • Hematopoiesis is the process by which blood cells produced.
  • They together form 45% volume of the total volume of blood and the rest of is by Plasma (55%).
  • Type of blood cells are – RBCs, WBCs, and Platelets.

1. RBCs

  • Also known as Erythrocytes.
  • Normally, their shape is of disk type.
  • The absence of Nucleus.
  • Red blood cells are consist of hemoglobin which is attached with 4 Iron atoms.
  • Hemoglobin is an oxygen-carrying pigment.
  • An average life-span of RBCs is 120 days.
  • Normal adult human, contain 4 – 5 million RBCs.
  • Erythropoietin is a hormone which is released by the kidney and also responsible for the blood cells production.
  • Bone marrow is the production house of RBCs and their maturation period is of 7 days and later on released into the bloodstream.
  • Graveyard of RBCs is spleen because it plays many roles like filtration, removes the excessive amount of RBCs and also filtered out dead RBCs.


2. WBCs

  • Also known as Leucocytes.
  • Normally, they are irregular in shape.
  • White blood cells are of different types, their nucleus shapes are different. They consist of Cytoplasm and also of a nucleus.
  • They provide immunity to our body and also known as soldiers, guard or nurse cells of the body.
  • WBCs are of two types – Granulocytes and Agranulocytes.
Granulocytes –  Those have granulated cytoplasm. They are of three types –
  • 1. Neutrophils – 
  •  They form the 60-65% of the total WBCs.
  • They are consist of a multilobed nucleus.
  • Bone marrow is the site of production.
  • Their life-span in tissue is about 4-5 days and in blood is 4-8 hours.
  • Neutrophils are phagocytic in nature. They protect our body against foreign particles like microbes.


  • 2. Basophils – 
  • They form only 0.5 – 1% of the total WBCs.
  • Their nucleus is of S-shaped and 3-lobed.
  • Bone marrow is the site of production.
  • Their life-span is same as Neutrophils.
  • They mainly secrets Heparin, Histamine, and Serotonin.
  • Heparin – Anti-coagulent.
  • Histamine – Vaso-dilator, Neuro-transmitter.
  • Serotonin – Increases vascular permeability.


  • 3. Eosinophils – 
  • Also known as Acidophils.
  • They form 2-3% of the WBCs.
  • The nucleus is Bi-lobed.
  • Bone marrow is the site.
  • Life-span is the same as the Neutrophils and Basophils.
  • Their important role in immunity, antiallergic and parasitic reactions.


Agranulocytes – Without granulated cytoplasm. They are of two types-
  • 1. Monocyte – 
  • They form 6-8% of the WBCs.
  • Bean-shaped or kidney-shaped nucleus is found.
  • Bone marrow is the site of formation.
  • Their life-span in tissue is about months or for years, and in blood is 10-20 hours.
  • They are phagocytic in nature, as a result,  they contribute in immunity of the body.


  • 2. Lymphocyte – 
  • They form 20-25% of the WBCs.
  • They have a round-shape nucleus.
  • The site of formation is Thymus, Lymph node spleen, Peyer’s patches and also Bone marrow.
  • Their life-span can be of a few days and also of years.
  • Lymphocytes are the main providers of the immunity. They secrets some antibodies in the body which kills microbes and their toxins also.


3. Platelets

  • Also known as Thrombocytes.
  • They don’t consist of a nucleus.
  • Their body shape is of Biconvex disk-like, round and also oval.
  • Life-span of the platelets is about a week.
  • When a blood vessel is injured, platelets get clumped at the site of injury and release certain chemicals called Platelet factor. They are responsible for Blood coagulation.


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B. Plasma

  • It’s of pale yellow color.
  • It makes up 55% of the total volume of the blood.
  • Plasma = Blood – ( RBCs + WBCs + Platelets). The portion of blood which is lack of Formed elements.
  • Plasma is an extracellular fluid of the blood.
  • It consists of many plasma proteins like serum albumin, fibrinogens, Prothrombins, and also serum globulins.
  • Plasma carries salt, water and also enzymes
  • The blood pH is maintained by Plasma also by neutralizing strong acid and base.
  • Blood components are traveled throughout the body from one organ to another due to plasma because it carries them.
  • It’s also conducting heat to the skin for dissipation and body immunity.
Component of Plasma
  • Water – 90-92% of plasma is made up of water.
  • Plasma proteins –  They form only 7-8% of plasma.
  • Various organic and inorganic components like mineral salts, nutrients, dissolved gases forms the remaining part of the plasma.

Blood Group

  • Blood group depends on the classification of the Blood type.
  • In blood, there is two types of Antigen A and B, and also two types of Antibody ‘a’ and ‘b’.
  • Antigen ‘A’ and Antibody ‘b’ is compatible with each other and whereas Antigen ‘B’ is compatible with Antibody ‘a’.
  • Antigen ‘A’ and Antibody ‘a’ are incompatible with each other and cause self-clumping and also same for antigen and antibody ‘B’ and ‘b’.
  • The antigen present on the surface of RBCs and antibody present in the plasma.
  • Universal donor -> O-
  • Universal recipient -> AB+

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