Respiration and Elimation of Nitrogenous Wastes: Process of Excretion: Removal of Harmful and Unwanted Products

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Removal of all harmful and unwanted products (specially the nitrogenous products) from the body is called excretion. The excretory system consists of organs and tissues participating in the removal of waste products. Some of these excretory organs constitute the urinary system which forms and eliminates urine and helps mainly in the excretion of nitrogenous waste – products, water and some mineral salts. Besides the urinary system, there are some accessory excretory organs and tissues such as the skin, lungs and liver. Urea is the main nitrogenous waste in our body. It is formed by the breakdown of surplus amino acids and nucleic acids in the liver. Blood transports urea to the kidneys for filtration and removal in the form of urine.

Modes of Removal of Nitrogenous Wastes

Depending upon the nitrogenous wastes excreted, animals can be classified as ammoniotelic, ureotelic and uricotelic.

Table Showing Categories of Animals on the Basis of Nitrogenous Waste Produced
Table showing Categories of animals on the basis of nitrogenous waste produced.


Product formed

Solubility in water




(highly toxic)

Highly soluble, therefore needs plenty of water for its excretion

Fresh water aquatic animals e.g. bony fish, Amoeba.



(less toxic)

Less soluble, thus needs less water for excretion

Mammals like man, dog etc., marine fishes and amphibians like frog and toad.


Uric acid

(least toxic)

Insoluble solids or semi solid. Needs very little water just to flush out the uric acid.

Birds, reptiles and insects.

Importance of Excretion

  • It removes the waste products and toxic materials from the body.

  • It eliminates the excess of material from our body like, soluble vitamins, NaCl, bile pigments and drugs.

  • Maintain the pH of body fluids. Removal of excess of water or its retention in case of shortage of water.

Excretory Organs in Cockroach

The main excretory organs of cockroach are Malpighian tubules and few accessory excretory organs are fat body, cuticle and nephrocytes. The Malpighian tubules are fine unbranched, yellow tubules that open into the alimentary canal at the junction of the mid and hind gut. The cells of tubules remove nitrogenous waste and certain salts from the haemolymph and then pump them into the lumen of the tubule. Fluid passes to the hindgut and in the process gets concentrated. This concentrated fluid then moves into the rectum and is excreted as concentrated urine along with feces.

Image showing Excretory organ of cockroach.

Image Showing Excretory Organ of Cockroach

Excretory Organs in Humans

  • The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and the urethra. Kidneys are bean shaped organs located on either side of the vertebral column in the lower abdominal cavity.

  • The outer surface of the kidney is convex, and the inner surface is concave, where it has a deep notch called hilum which leads into funnel shaped space called renal pelvis. The renal cortex contains blood-filtering mechanisms is protected by a fibrous capsule and by layers of fat.

  • The medulla contains 8 to 12 renal pyramids, striated wedges composed mostly of tubular structures. Kidneys filter metabolic wastes from the blood and excrete them as liquid called urine.

  • As kidney form the urine, they also maintain the normal composition of blood, fluid and salt balance throughout the body tissues.

  • Urine formed in the kidney is brought to the urinary bladder by two hollow muscular tubes called ureters.

  • Urine thus formed will be passed to urinary bladder via ureter. Through the urinary opening at urethra, we excrete urine.

Image showing Excretary organs of humans.

Image Showing Excretary Organs of Humans

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