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
There are three kinds of cellular elements found in it:
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.
- 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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