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A Study on Various Modes of Excretion

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Excretion is the removal of waste products from the body of an organism. Based on the type of product, there are three types of animals.  The kidney is the primary excretory organ in humans. Lung, skin, liver and intestine are accessory excretory organs.  

Physiology of Excretion  

The process of excretion has two parts 

1. Formation of urea in the liver by urea cycle or ornithine cycle.  

2. Formation of urine.  

Different Modes of Excretion 

Depending upon excretory products, all total there are five modes of excretion in animals. They are: 

  • Ammonotelism: Excretory product is ammonia.

  • Ureotelism: Excretory product is uric acid. 

  • Aminotelism: Excretory product is amino acids.

  • Uricotelism: Excretory product is uric acid. 

  • Guanotelism: Excretory product is guanine. 


The process in which ammonia is eliminated from the body is called ammonotelism. And the organism which exhibits this as an excreted material known as ammonotelic. Examples of organisms are fish, protozoans, echinoderms, poriferans and crustaceans. Such excretion is mainly shown in aquatic animals, as they excrete ammonia directly into the environment, where dilution of compounds occurs very quickly as this compound is very toxic to tissues


In organisms like mammals and amphibians, waste material is in the form of urea and organisms which excrete urea are known as ureotelic. As ammonia is converted into urea in the liver of animals and released back into the blood. Further kidney filters the urea and excretes it outside the body. Some of the urea remain inside the matrix of the kidney so that they can maintain the required osmoregulation in the organism. Humans always excrete urea and because of this, they are ureotelic in nature. However, in comparison to ammonia, urea is less toxic. 


Organisms that are uricotelic in nature remove their nitrogenous wastes as uric acid in the form of pellets or paste. In these organisms loss of water is very less and is least toxic also. As uric acid is insoluble in water because of this reason, the excreta is in the paste form. Examples of uricotelic organisms are birds and insects.


In this type of excretion, mode excreta is in the form of excess amino acids. This process is known as aminotelism. Example: Mollusca


In this mode of excretion, organisms convert ammonia into guanine before excretion. Some which show such type of excretion are reptiles, birds, earthworms. As guanine is not soluble in water so no water is required. 

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FAQs on A Study on Various Modes of Excretion

1. Explain the Process of Filtration?

Ans. 1. Ultrafiltration – It is a passive process that takes place from the glomerulus into Bowman's capsule. The different arteriole is narrower as compared to the afferent arteriole and a high pressure develops in the glomerulus which enables a large amount of filtrate into the  Bowman's capsule.  

Glomerular Filtration at Rate – It is the filtered blood entering into the Bowman's capsule. In a  normal adult, it is 120 ml per minute and about 180 litres per day An adult man normally passes  1 to 1.8 litres of urine in 24 hours.  

2. Reabsorption – It is the process due to glomerular filtrate being returned into the bloodstream. 3. Tubular Secretion – It is the supporting mechanism for filtration and reabsorption. 

2. Write a Note on Urine?

Ans. Urine is a transparent yellowish fluid with a slightly acidic pH of 6.0. A pigment urochrome causes the yellow colour of urine. Urine contains a bad smelling substance urine due to which it has a bad odour. Urine contains 95% water, 2.6% urea, uric acid, salts, etc.  

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