• The kidneys are two reddish-brown bean-
shaped organs found in vertebrates.
• They are located on the left and right side
in the retroperitoneal space, and in adult
humans they are about 12 centimeters in
• The kidneys excrete a variety of waste
products produced by metabolism into the
Introduction External view Internal view
Pronephric kidney is a transient embryonic organ that
serves as a kidney. It gives rise to mesonephric kidney.
A mesonephric kidney is an embryonic organ that
disappear is all mammals when the permanent
kidney is functional.
A metanephric kidney is a highly complex organ that filters
waste products from circulation. It forms a permanent
Archinephros: the Ancestral Kidney
• Excretory organ of ancestral vertebrates.
• Also called holonephros or complete kidney as it extends the
entire length of the coelom.
• Composed of a pair of archinephric ducts.
• Each duct has a pair of tubules to a segment.
• Each tubule is opened by a nephrostome into the coelom.
• All tubules are drained into a common archinephric duct, which
opens into the cloaca.
• The first kidney tubules to appear in the embryos of all
vertebrates called Pronephros.
• Tubules are segmented, one pair at each segment.
• Pronephric tubules: simple, become long, and
• Glomerulus: Malpighian tuft (capillary blood vessels)
removes urine and toxins from the blood.
• No. of pronephric tubules : 3 in the larval frog, 7 in
human embryos, and 12 in chicks.
• Functional only in the embryonic or larval stage of
amphibians and fishes.
Pronephros : The Larval Kidney
Pronephros Mesonephros Metanephros
Mesonephros: Kidney of Anamniotes
• Mesonephros – “ middle kidney ”.
• Main excretory organ of aquatic vertebrates (
• Temporary kidney in higher vertebrates
• When the pronephros degenerates the
persistent pronephric duct is called
mesonephric duct or the Wolffian duct.
• In fishes and amphibians, mesonephros is
functional both in embryos as well as the adult.
• In amniotes ( reptiles, birds, and mammals )
mesonephros is functional only in the embryos
replaced by metanephros in the adults.
• In Sharks and Caecilians, mesonephric tubules
extend posteriorly throughout the length of
coelom - Opisthonephros
Metanephros: Kidney of Amniotes
• Kidney of amniotes.
• Most posterior and last to develop in both ontogeny and
• Permanent kidney (reptiles, birds, and mammals)
develop in embryos from the lower part of the Wolffian
duct and replacing the embryonic mesonephros.
• When metanephric tubules develop, all the
mesonephric tubules disappear except those
associated with the testis in male and forming vasa
• Consists of compact paired organ containing many
nephrons; a ureter separate from the Wolffian duct
leads from the metanephros to the bladder
1. Excretory organs A pair of kidneys, urinary ducts,
and a urinogenital sinus. No
A pair of kidneys, ureters a
urinary bladder and cloaca.
A pair of kidneys, a urinary
bladder and cloaca. No
A pair of kidneys, a pair
of ureters and cloaca.
Include a pair of kidneys, paired
ureters, a bladder and urethra
2. Kidneys Adult kidneys greatly elongated,
ribbon-like flat. Each kidney has 2
distinct parts Anterior narrow part.
Posterior broader part is functional
kidney and called
Adult kidneys are elongated,
oval, flat. Not differentiated
into parts and are
Adult kidneys are small,
irregular. Each kidney is
bilobed. Kidneys are
Adult kidneys are small,
flat, dorsally. Each kidney
is trilobed. Kidneys are
Adult kidneys are small, bean
shaped. Kidneys are
metanephric and not divided
3. Histology of
Covered ventrally by peritoneum,
not differentiated into cortex and
medulla and made of a compact
mass of coiled uriniferous tubules.
Same as in fishes Same as in fishes and
Kidney covered ventrally
differentiated into cortex
and medulla and contains
a very large number of
Each kidney covered ventrally
by peritoneum, differentiated
into an outer cortex and inner
medulla, and made of much
convoluted uriniferous tubules
4. Uriniferous tubules Have a special urea absorbing
segment. Loop of Henle absent.
Lack a urea-absorbing
segment and loop of Henle.
segment and loop of Henle.
Lack urea absorbing
segment, but water
absorbing loop of Henle
Absorb urea through
glomerular filteration and
tubular reabosrption and also
have water absorbing loop of
Comparative Account Of The Excretory
System Of Vertebrates
5. Peritoneal funnel Nephrostomes present. Nephrostomes present. Nephrostomes absent. Nephrostomes absent. Nephrostomes absent.
6. Ureters Mesonephric ureters of
both sides run over ventral
surface of kidney and open
into a urinogenital sinus,
which leads into cloaca.
Ureters open separately in
male but by a common
aperture on a urinary
papilla in female.
Mesonephric ureters arise
and run along outer side of
kidneys and open behind by
separate apertures directly
into cloaca. A urinogenital
sinus is absent.
Metanephric kidney ducts
or ureters run ventrally over
kidneys and open dorsally
and separately into middle
chamber of cloaca, called
As in reptiles, ureters are
metanephric. They run
ventrally over kidneys and
open behind separately into
urodaeum through its roof.
Ureters begin from a wide
funnel-like cavity in kidney,
called pelvis. Cloaca absent.
7.Urinary bladder Absent A large thin-walled
membranous elastic bilobed
urinary bladder opens
ventrally into cloaca by a
Small, thin-walled, inelastic,
undivided sac opening
ventrally into coprodaeum
Absent Large, median, pearshaped,
muscular sac. Its neck,
called urethra, opens at the
tip of penis in male and into
vestibule of female which
opens to outside through
8. Nature of excretion Predominantly
excrete more ammonia than
predominantly urea along
semisolid uric acid and
urates which are not much
soluble in water
Like reptiles, brids are also
urecotelic excreting mainly
uric acid and urates in a
Urecotelic since chief
excretory product in urine is
urea dissolved in water.