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.
Reprint 2026-27
, 206 BIOLOGY
A survey of animal kingdom presents a variety of excretory structures.
In most of the invertebrates, these structures are simple tubular forms
whereas vertebrates have complex tubular organs called kidneys. Some
of these structures are mentioned here. Protonephridia or flame cells are
the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria),
rotifers, some annelids and the cephalochordate – Amphioxus.
Protonephridia are primarily concerned with ionic and fluid volume
regulation, i.e., osmoregulation. Nephridia are the tubular excretory
structures of earthworms and other annelids. Nephridia help to remove
nitrogenous wastes and maintain a fluid and ionic balance. Malpighian
tubules are the excretory structures of most of the insects including
cockroaches. Malpighian tubules help in the removal of nitrogenous
wastes and osmoregulation. Antennal glands or green glands perform
the excretory function in crustaceans like prawns.
16.1 HUMAN EXCRETORY SYSTEM
In humans, the excretory system consists
of a pair of kidneys, one pair of ureters, a
urinary bladder and a urethra (Figure
16.1). Kidneys are reddish brown, bean
shaped structures situated between the
levels of last thoracic and third lumbar
vertebra close to the dorsal inner wall of
the abdominal cavity. Each kidney of an
adult human measures 10-12 cm in
length, 5-7 cm in width, 2-3 cm in
thickness with an average weight of 120-
170 g. Towards the centre of the inner
concave surface of the kidney is a notch
called hilum through which ureter, blood
vessels and nerves enter. Inner to the hilum
is a broad funnel shaped space called the
renal pelvis with projections called calyces.
The outer layer of kidney is a tough
capsule. Inside the kidney, there are two
zones, an outer cortex and an inner
Figure 16.1 Human Urinary system medulla. The medulla is divided into a few
conical masses (medullary pyramids)
projecting into the calyces (sing.: calyx).
The cortex extends in between the
Reprint 2026-27
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.
Reprint 2026-27
, 206 BIOLOGY
A survey of animal kingdom presents a variety of excretory structures.
In most of the invertebrates, these structures are simple tubular forms
whereas vertebrates have complex tubular organs called kidneys. Some
of these structures are mentioned here. Protonephridia or flame cells are
the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria),
rotifers, some annelids and the cephalochordate – Amphioxus.
Protonephridia are primarily concerned with ionic and fluid volume
regulation, i.e., osmoregulation. Nephridia are the tubular excretory
structures of earthworms and other annelids. Nephridia help to remove
nitrogenous wastes and maintain a fluid and ionic balance. Malpighian
tubules are the excretory structures of most of the insects including
cockroaches. Malpighian tubules help in the removal of nitrogenous
wastes and osmoregulation. Antennal glands or green glands perform
the excretory function in crustaceans like prawns.
16.1 HUMAN EXCRETORY SYSTEM
In humans, the excretory system consists
of a pair of kidneys, one pair of ureters, a
urinary bladder and a urethra (Figure
16.1). Kidneys are reddish brown, bean
shaped structures situated between the
levels of last thoracic and third lumbar
vertebra close to the dorsal inner wall of
the abdominal cavity. Each kidney of an
adult human measures 10-12 cm in
length, 5-7 cm in width, 2-3 cm in
thickness with an average weight of 120-
170 g. Towards the centre of the inner
concave surface of the kidney is a notch
called hilum through which ureter, blood
vessels and nerves enter. Inner to the hilum
is a broad funnel shaped space called the
renal pelvis with projections called calyces.
The outer layer of kidney is a tough
capsule. Inside the kidney, there are two
zones, an outer cortex and an inner
Figure 16.1 Human Urinary system medulla. The medulla is divided into a few
conical masses (medullary pyramids)
projecting into the calyces (sing.: calyx).
The cortex extends in between the
Reprint 2026-27
