Revision: Biology Zoology >> Excretion Biology (Botany and Zoology) HSC Science Class 11 Tamil Nadu Board of Secondary Education

During different metabolic activities taking place in our body, the body produces many substances, of which some are useful and some are useless.
The process of removing useless and harmful metabolic waste substances is called excretion.

The process of removal of chemical wastes (Mainly Nitrogenous) from the body is known as ''excretion''. It plays an important role in maintaining the homeostatic (steady-state) condition of the body.

Kidneys are the primary excretory organs, eliminating nitrogenous wastes (chiefly urea) from the blood and throwing it out in the form of urine.

During different metabolic activities taking place in our body, the body produces many substances of which some are useful and some are useless.
If retained in the body the unwanted substances may become poisonous and cause much harm and in severe cases, even death. The organs which remove these unwanted and toxic substances from the body are called excretory organs.

The blood flows through the glomerulus under great pressure which causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtration under high force is called Ultrafiltration.

Glomerular filtration rate is the amount of glomerular filtrate formed in all the nephrons of both kidneys per minute. In a healthy individual, it is about 125 ml/minute.

The artificial process which cleans and filters the blood in a person where one or both the kidney may stop working properly is called dialysis.

Inside the kidney, there are millions of microscopic tubes called renal tubules or nephrons. It is the structural and functional unit of the kidney.

Ultrafiltration is the process in which blood is filtered under high pressure in the glomerulus, allowing water and small solutes to pass into the Bowman’s capsule.

Glomerular filtrate is the fluid formed after ultrafiltration that enters the renal tubule and contains water, urea, salts, glucose, and other small molecules.

Selective absorption is the process by which only useful substances like glucose, some salts, and water are absorbed from the renal tubule back into the blood, without disturbing its normal concentration.

Tubular secretion is the active transfer of certain substances, such as ions and drugs, from the blood into the renal tubule during urine formation.

• Excretion removes nitrogenous wastes from the body to maintain internal homeostasis.
• In ammonotelism, animals excrete ammonia, which is highly toxic and needs a large amount of water; seen in aquatic invertebrates, bony fishes and larval amphibians.
• In ureotelism, nitrogenous waste is excreted mainly as urea, which is less toxic and requires a moderate amount of water; typical of mammals, cartilaginous fishes and most adult amphibians.
• In uricotelism, waste is excreted as uric acid, which is almost insoluble and conserves water, so it is common in birds, many reptiles, land snails and insects.
• Ammonotelic animals usually eliminate ammonia via body surface, gills or kidneys, whereas ureotelic and uricotelic animals convert ammonia to urea or uric acid in the liver before excretion.