INTRODUCTION
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. Every organism, from the smallest protest to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination.Example of toxic waste materials are ammonia, urea, uric acid , CO2 , H2O ,and ions like Na+, K+ ,Cl- ,phosphate ,sulphate, etc.Through excretion organisms control osmotic pressure - the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism’s internal environment
In single-celled organisms, waste products are discharged directly through the surface of the cell. Multicellular organisms utilize more complex excretory methods. Animals have special excretory organsand different excretory organs constitute the excretory system. . In man this is primarily carried out by the lungs, kidneys and skin.[1]
The main function of excretory system is to eliminate the nitrogen waste.In this chapter you will learn the mechanism of elimination of nitrogenous wastes. Ammonia, Urea,and Uric acid are the major forms of nitrogenous wastes. Ammonia is highly toxic and water soluble.The process of excreting ammonia is known asammonotelism.Such animals are called ammonotelic. It is found in aquatic animal groups like sponges, coelentrates, crustaceans, echinoderms, bony fish, tadpole larvae, and salamander.
Mammals, many terrestrial amphibians and marine fishes mainly excrete urea.They are called ureotelic.It is commonly found in man , whales, seals, desert mammals like kangaroo rats , camels, toads, frogs, cartilaginous fishes, aquatic and semi aquatic reptiles like alligator, terrapinsand turtles.Urea is far less toxic than ammonia. Ammonia produced by metabolism is converted into urea to the liver of these animals and released into the blood which is filtered and excreted out by the kidneys. To maintain a desired osmolarity,some urea may be retained in the kidney matrix of these animals.
Reptiles, birds, land snails, and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water. They are called uricotelic animals.Uric acid is formed from ammonia mostly in the liver and to some extent in kidneys. But itis much less toxic than both ammonia and urea and it is almost insoluble in water and can beeliminated from the body in nearly a solid state, saving a lot of water.
Other Nitrogenous wastes –Allantosm, creatine, creatinine and hippouric acid are some nitrogenous wastes products excreted by mammals.
In most of the invertebrates, the excretory organs are simple tubular forms whereas in vertebrates , the excretory organs are complex.For example flate worms like Planeria have the simplest tubular excretory system called the Protonephridia or flame cells.
Rotifers, some annelids and cephalochordaes (Amphioxus) also possess protonephrida.
Protonepridia are generally concerned with osmoregulation i.e., the regulation of ionic and fluid volume. Earthworms possess a similar simple tubular excretory system called metanephridia .
\In cockroaches and other insects , the malpighian tubules are the excretory organs.
Crustaceans like Cray fish have a pair of specialised excretory structures called ‘antennal gland’ or‘green gland’ that is situated at the base of the second antennae.
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