Excretory System of Human Body

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Excretory System of Human Body

Waste materials excreted in animals are of many kinds : Nitrogenous Substances, Carbon – Dioxide, Pigments, Excess Water, etc.

The Nitrogenous Waste can be excreted in the following forms :

  • Ammonia : Highly Toxic, Soluble in Water and has to be immediately excreted. In aquatic animals (called Ammoniotelic).
  • Urea : Far less toxic than ammonia. Less water required than ammonia. In Mammals, Alligators, Turtles (called Ureotelic).
  • Uric Acid : In animals where conservation of water is needed. In Birds, Insects, Land Reptiles (called Uricotelic).

Excretory System of Mammals

1. Kidneys

Left kidney is higher than the right kidney.

A kidney is composed of 1.2 million microscopic structural and functional units called nephrons or uriniferous tubules.

2. Ureters

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Open into Urinary Bladder

3. Urinary Bladder

Size depends upon the amount of mine in it (up to 700 – 800 ml).

The lower part is guarded by 2 Sphinctors : inner involuntary and outer voluntary.

4. Urethra

Leads to the exterior.

In females, it carries only urine. Opens by urinary aperture in the vulva in front of vaginal aperture.

In males it carries urine as well as spermatic fluid.

Act of passing urine is called Micturition.


Functional unit of kidney. It is a long tube differentiated into 4 regions.

1. Bowman’s Capsule

Contains a globular bunch of capillaries, the glomerulus.

The Bowman’s capsule and the glomerulus together form a globular body called Malpighian body.

In glomerular filtrate : water (99%), inorganic salts, amino acids, fatty acids, glucose, urea, uric acid, hippuric acid, hormones, vitamins are present.

2. Proximal Convoluted Tubule (PCT)

Starts from the neck of Bowman’s capsule and is highly twisted.

Almost whole of the glucose and vitamins, about 75% of amino acids, about 70% of Na and K, a large amount of Ca and Mg and 75% water is absorbed.

3. Loop of Henle

U – shaped. Consist of 2 straight limbs : descending and ascending. No water is absorbed in ascending limb.

4. Distal Convoluted Tubule (DCT)

  • Greatly twisted.
  • Reabsorbs some Na and Cl.
  • Leads to Collecting Tubules which lead to ureters.

Note :

The yellow colour of urine is caused by the pigment urochrome, which is a breakdown product of haemoglobin from worn out RBCs.

The urine on standing gives a pungent smell. It is due to conversion of urea in to ammonia by bacteria.

pH of urine is between 5 – 8. Average pH is 6.0 (slightly acidic).

Kidney Stones : Calcium Oxalate

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