Describe the role of kidney as a part of Excretory System.

As a result of the complex chemical reactions taking place within the body, certain metabolic waste products are formed.

For example : large amounts of CO2 and water are produced from metabolism of carbohydrates, fats and proteins. We are going to discuss about Excretory System, products from Excretory System & structure of kidney.

Excretory System

Ammonia, urea and uric acid are formed from other nitrogenous compounds. These products become toxic or harmful if retained inside.

They tend to alter the normal internal environment of the body. Unless these are kept within the normal range, cell functioning will deteriorate.

It may cause eventual death of the cell and possibly of the individual. Therefore, the metabolic wastes must be removed from the body for the continuation of life.

The elimination (removal) of all harmful and unwanted metabolic wastes (especially nitrogenous wastes) from the body is called Excretion.

The organs concerned with this function are termed excretory organs or organs of Excretory System. The major excretory organs in man and other vertebrates are kidneys.

Some other organs such as lungs, liver, large intestine and skin also eliminate certain metabolic wastes besides normal functions. They are known as accessory (additional) excretory organs.

The excretory system serves to eliminate from our body-

  1. nitrogenous waste products, such as, ammonia, urea and uric acid,
  2. excess of water and mineral salts and
  3. unwanted substances, such as, pigments. All these substances are removed in the form o1 an aqueous solution called urine. Removal of excess water, acid, mineral salts, etc. helps in keeping the volume, composition, pH and osmotic pressure of the blood constant.


I. Respiratory waste products 
The result of oxidation of glucose in the cells is the production of CO, and water with the release of energy. CO, being toxic is eliminated from the body during expiration from the lungs.

Excess of water is removed either by the formation of sweat or urine via Excretory System.

II. Nitrogenous waste products 
Nitrogen containing waste products are produced by the breakdown of excess amino acids or proteins or nucleic acids.
The nitrogenous waste products are of the following three types :

  1.  Ammonia : Ammonia is produced by the oxidative deammination of amino acids. It is highly toxic. Excretion of ammonia is called ammonotelism. The animals which excrete ammonia are called ammonotelic. e.g., Hydra, bony fishes, tailed amphibians, etc.
  2. Urea : Urea is the most common excretory product of vertebrates and i some invertebrates. It is produced in the liver by the combination of ammonia and CO2. It is poisonous and if allowed to accumulate in the blood to a certain level, causes death. It is excreted out through kidneys. It is highly soluble in water. It is removed from the body in the form of urine. The removal of urea is called ureotelism. The animals which excrete urea are termed ureotelic.
  3. Uric acid : Uric acid is least toxic amongst all the three nitrogenous waste products. It is the chief excretory product in insects, reptiles, birds and desert animals. Removal of uric acid is called uricotelism. The animals which excrete uric acid are termed uricotelic.

III. Mineral salts 
Excess of mineral salts taken along with food are also toxic. These may lead to serious disorders in the body. The excess of mineral salts are eliminated from the body along with sweat, urine or faeces via Excretory System.

IV. Water 
65% weight of our body is made of water. But, if it exceeds a normal limit it may lead to serious disorders, such as, oedema, increased blood pressure, etc.

Water is taken in with food and some drink it too. It is also produced due to the oxidation of food. Water is eliminated along with urine, sweat, faeces and expired air.

V. Bile pigments 
Bile pigments chiefly yellow coloured bilirubin are the breakdown products of haemoglobin of dead RBCS.

These are excreted out by liver cells from blood and secreted into the bile juice poured into the duodenum by common bile duct. These give faeces their yellowish brown colour. Some of these pigments a excreted in urine.


All the body organs which help the anima) in excretion collectively constitute the excretory system. The excretory system consists of –

  1. A pair of kidneys
  2. A pair of ureters,
  3. A urinary bladder and
  4. Urethra.

Kidneys in Excretory System

The human kidneys are reddish brown, bean shaped, slightly flattened organs. They are present just below the stomach, one on either side of the vertebral column, protected by the last two ribs. They are asymmetrical in position i.e. present at different levels.

The right kidney is about 1-1.5 cm lower than the left kidney, because the right kidney is pushed downward by the large right liver lobe.

Each kidney is about 10 cm long, 6 cm wide and 3.5 cm thick. It is a bean-shaped structure. Outer margin of kidney is convex and inner margin is concave.

In the middle of inner concavity is present a notch called hilus or hilum. Hilus leads to a space called renal sinus. The renal artery, renal vein, nerve fibre, lymph vessels and ureters enter or leave the kidney through hilus.

Internal structure of kidney in Excretory System

The kidney is bounded externally by a thin sheet of white fibrous tissue called capsule. In each kidney about 1.25 million, highly coiled tubules called uriniferous tubules or nephrons are present. These are the structural and functional unit of kidney.

Each uriniferous tubule is blind at one énd and opened at the other end, where it opens into a collecting tubule. All the nephrons are held together by connective tissue. There are also present a large number of blood vessels, lymph vessels and nerve fibres.

In a longitudinal section, the kidney is differentiated into two regions. The outer, darker region is called cortex. Whereas, the inner light region is called medulla.

The cortex is dotted in appearance. The nephrons are highly coiled in this region. Whereas, the medulla is striated in” appearance. The nephrons run almost straight in this region.

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