excretory products and its elimination NCERT
Channel: ISHTA’s BIO ACADEMY
Date: 28/12/2022

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water
and ions like Na +
, K+
, Cl–
, phosphate, sulphate, etc., either by metabolic
activities or by other means like excess ingestion. These substances have
to be removed totally or partially. In this chapter, you will learn the
mechanisms of elimination of these substances with special emphasis on
common nitrogenous wastes. Ammonia, urea and uric acid are the major
forms of nitrogenous wastes excreted by the animals. Ammonia is the
most toxic form and requires large amount of water for its elimination,
whereas uric acid, being the least toxic, can be removed with a minimum
loss of water.
The process of excreting ammonia is Ammonotelism. Many bony fishes,
aquatic amphibians and aquatic insects are ammonotelic in nature.
Ammonia, as it is readily soluble, is generally excreted by diffusion across
body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys
do not play any significant role in its removal. Terrestrial adaptation
necessitated the production of lesser toxic nitrogenous wastes like urea
and uric acid for conservation of water. Mammals, many terrestrial
amphibians and marine fishes mainly excrete urea and are called ureotelic
animals. Ammonia produced by metabolism is converted into urea in the
liver of these animals and released into the blood which is filtered and
excreted out by the kidneys. Some amount of urea may be retained in the
kidney matrix of some of these animals to maintain a desired osmolarity.
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 a