Human Blood

Blood is a never-stationary fluid and it is always in motion from the heart to the arteries and back through the veins.

  • Colour: Bright red when taken from an artery and deep red when taken from a vein.
  • Volume: An average adult human has 5 to 6 litres blood.
  • Taste: Salty, slightly alkaline with a pH of 7.3 to 7.45.

Functions of Blood

  • It forms a clot which serves to prevent the loss of blood and the entry of disease-causing germs.
  • WBC protect the body from diseases by engulfing bacteria which may have entered the body.
  • Antibodies produced by the it neutralise poisonous substances or kill germs which enter the body.
  • It transports digested food from the alimentary canal to the tissues.
  • It transports excretory materials from the tissues to the liver, kidneys or skin for elimination.
  • It helps in keeping the temperature of the body uniform by distributing heat.
  • Haemoglobin of RBCs combines with oxygen to form oxyhaemoglobin which reaches tissues to deliver the oxygen.
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Composition of Blood

It is made up of plasma and the blood corpuscles.


  • It is a light yellow-coloured alkaline liquid.
  • It mainly consists of :
    • Water- 90–92%
    • Proteins – 7–8%
    • Inorganic Salts – 1%
    • Other Substances – Trace amounts

Cellular Elements:

There are three kinds of cellular elements found in it:

  • RBC
  • WBC
  • Platelets

Red Blood Cells (RBCs/Erythrocytes)

  • RBCs are minute, disc-like structures.
  • RBCs are 7 µ in diameter.
  • In adults, RBCs are produced in the marrow of long bones such as ribs, breast bone and ileum of the hip girdle.
  • In children, RBCs are produced in the bone marrow of all the cells until 5 years of age.
  • Mature RBCs do not have nuclei.
  • The average life of RBCs is 120 days.

White Blood Cells (WBCs/Leucocytes)

  • WBCs are amoeboid.
  • WBCs are produced in the bone marrow, lymph nodes and sometimes even in the liver and spleen.
  • The average life of WBCs is about 2 weeks.
  • WBCs are classified into two categories based on their shape and characteristics.
    • Granular
    • Agranular
  • Leukaemia is a cancer in which the number of WBCs increases manifold at the cost of RBCs.
  • Leucopenia is the abnormal decrease in the number of WBCs.

Blood Platelets (Thrombocytes)

  • These are minute, oval or round, non-nucleated structures floating in the blood.
  • Platelets are derived from megakaryocytes in the red bone marrow.
  • Their life span is 3 to 5 days.
  • Platelets play an important role in blood clotting.

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