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Human Excretory System

Excretion : Removal of nitrogenous substances formed during metabolism from the body of human is called excretion. Normally excretion means the release of nitrogenous excretory substances like urea, ammonia, uric acid etc.
The main excretory organs of human are as follows –
(i) Kidneys
  • The main excretory organ in human and other mammals is a pair of kidneys. Its weight is 140 grams.
  • There are two parts of it. Outer part is called cortex and the inner part is called medulla.
  • Each kidney is made up of approximately 1,30,00000 kidney ducts which are called nephrons.
  • Nephron is the structural & functional unit of the kidney.
  • There is a cup like structure in the every nephron called Bowman's capsule.
  • Glomerulus of thin blood vessels are  found in the Bowman's capsule which is made up of two types of arterioles.
(a) Afferent arteriole : Which carries the blood to the glomerulus.
(b) Efferent arteriole : By which the blood is taken out of the glomerulus.
  • The process of filtration of liquids into the cavity of Bowman's capsule, is called ultra filtration.
  • The main function of the kidneys is purification of blood plasma i.e. to excrete the unwanted nitrogenous waste substance through urination.
  • The supply of blood to kidneys takes place in large quantity in comparison to other organs.
  • In the kidneys average 125 ml per minute blood is filtrated i.e. 180 litres per day. Out of it 1.45 liters urine is formed daily and the remaining is absorbed back by the cells of nephron and mix into the blood.
  • In the normal urine there is 95% water, 2% salt, 2.7% urea and 0.3% uric acid.
  • The colour of the urine is light yellow due to the presence of urochromes in it. Urochrome is formed by the dissociation of haemoglobin.
  • Urine is acidic. Its pH value is 6.
  • The stone formed in the kidney is made up of calcium oxalate.
(ii) Skin : Oil gland and sweat glands found in the skin respectively secretes sebum and sweat.
(iii) Liver : Liver cells play the main role in excretion by converting more and more amino acids and ammonia of blood into urea.
(iv) Lungs : The lungs excretes two types of gaseous substances carbon dioxide and water vapour. The excretion of some substances like garlic, onion and some spices in which vapour component excreted by the lungs.