NCERT Book Class 11 Biology Chapter 16 Excretory Products and their Elimination

EXCRETORY PRODUCTS AND THEIR ELIMINATION 205

C HAPTER 16
EXCRETORY PRODUCTS AND
THEIR E LIMINATION

16.1 Human Animals accumulate ammonia, urea, uric acid, carbon dioxide, water
Excretory and ions like Na+, K+, Cl–, phosphate, sulphate, etc., either by metabolic
System activities or by other means like excess ingestion. These substances have
to be removed totally or partially. In this chapter, you will learn the
16.2 Urine Formation
mechanisms of elimination of these substances with special emphasis on
16.3 Function of the common nitrogenous wastes. Ammonia, urea and uric acid are the major
Tubules forms of nitrogenous wastes excreted by the animals. Ammonia is the
most toxic form and requires large amount of water for its elimination,
16.4 Mechanism of
whereas uric acid, being the least toxic, can be removed with a minimum
Concentration
loss of water.
of the Filtrate
The process of excreting ammonia is Ammonotelism. Many bony fishes,
16.5 Regulation of aquatic amphibians and aquatic insects are ammonotelic in nature.
Kidney Function Ammonia, as it is readily soluble, is generally excreted by diffusion across
body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys
16.6 Micturition
do not play any significant role in its removal. Terrestrial adaptation
16.7 Role of other necessitated the production of lesser toxic nitrogenous wastes like urea
Organs in and uric acid for conservation of water. Mammals, many terrestrial
Excretion amphibians and marine fishes mainly excrete urea and are called ureotelic
16.8 Disorders of the animals. Ammonia produced by metabolism is converted into urea in the
Excretory
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
System
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 and are
called uricotelic animals.