CHAPTER XI
EXCRETORY SYSTEM
Objective
• Define the term excretion and describe how this
process helps maintain homeostasis.
• Name the major metabolic wastes and the
processes by which they are formed.
• Describe the types of vertebrate kidney.
• Explain the parts of urinary system and the
processes of urine formation.
• As an organism carries out its life processes,
• Waste products build up in the body fluids.
• If these metabolic wastes are not removed from
the body, the organism would die.
• Therefore, the organism must be able to remove
metabolic waste and other excess substances
that build up over time.
Excretion
• Excretion is the process by which these waste
and excess substances are removed from the
organism.
• The process of excretion also removes excess
heat from the body, thus helping to keep the
temperature of the body constant.
• In human and other
complex animals,
• The organs of excretion
are the lungs, kidneys,
liver, and skin.
• These organs work with
the circulatory, nervous,
and endocrine system to
keep the body’s internal
environment constant.
• On other words, these
organ systems maintain
homeostasis.
Major Metabolic Wastes And
Their Excretory Organ
• The most important of the metabolic wastes
1. Carbon dioxide
2. Water
are
formed
during cellular respiration
3.Nitrogen compounds ,such as ammonia,
urea, and uric acid ,are produced by the
breakdown of amino acid.
4.Mineral salts, such as sodium chloride and
potassium sulfate, build up during metabolism.
-All of these wastes are poisonous in high
concentrations.
• Many people confuse excretion with elimination.
• Elimination, or defecation, is the removal from
the digestive tract of unabsorbed and
undigested food in the form of feces.
• Since these materials have never entered the
body cells, they are not metabolic wastes.
• The liver removes harmful substances, such as
bacteria, certain drugs , and hormones from
the blood.
• Within the liver, these substances are changed
into inactive or less poisonous forms.
• Thus, the liver purifies or detoxifies, the blood.
• The inactive substances formed in the liver are
returned to the blood stream and
• Are finally excreted from the body by the
kidneys.
Type of vertebrate kidney
- Vertebrate kidneys, or nephroi, are built in
accordance with a basic structural pattern consisting
of
1. Glomeruli , usually incorporated in renal corpuscles;
2. Tubules, surrounded by peritubular capillaries and
3. A pair of longitudinal ducts.
- Variations in the details from fish to man are
primarily in the number and arrangement of
glomeruli and in the relative length of the
tubules.
- In present –day vertebrates
- The uriniferous tubules
develop antero-posteriorly in
two or three stages
- In succession these stages
are
-pronephros,
-mesonephros, and
- metanephros.
- These stages have evolved
from the original
archinephros.
-
-
Pronephros
Pronephros develops in the anterior most part
of the nephrotome
There are only 1 to 13 uriniferous tubules in
each, one pair to each segment.
Near each tubule is a glomerulus but
Bowman’s capsule and peritoneal funnel are
lacking.
The glomeruli are called external glomeruli
- The uriniferous tubules of each
pronephros open into a
common pronephric duct
- Which runs backward to enter
the embryonic cloaca.
- A pair of pronephroi become
functional only in some
cyclostomes and embryonic of
all anamniote.
- In other vertebrates, they
degenerate during
development .
-
-
-
Mesonephros
Mesonephros develops from that part of the
nephrotome which lies behind the
pronephros.
At first it consists of paired segmental
uriniferous tubules,
Each with a peritoneal funnel opening into the
coelom,
and a glomerulus enclosed in a Bowman’s
capsule.
- These mesonephric
uriniferous tubules join the
existing pronephric duct on
each side,
- Which is called mesonephric
duct or Wolffian duct.
- Later, the mesonephric
tubules undergo budding to
form hundred of tubules,
- So that their segmental
arrangement is lost.
- The later tubules have no
peritoneal funnels.
- Mesonephroi form the
adult functional kidneys in
some fishes and
amphibians
- They form the kidneys of
embryo of amniotes,
which they degenerate in
the adult.
Metanephros
• The first embryonic hint of a metanephros is
the formation the metanephric duct that
appear as a ureteric diverticulum arising at
the base of the preexisting mesonephric duct.
-The ureteric diverticulum grows dorsally into
the posterior region of the nephric ridge.
-Here it enlarges and stimulates the growth of
metanephric tubules that come to make up the
metanephric kidney .
• The metanephros becomes the adult kidney of
amniotes ,and the metanephric duct is called
the ureter.
The Urinary System
-The urinary system is made up of the
-kidneys
-ureters
-bladder
-urethra
-The two kidneys are the organs that produce
urine.
-Urine passes from each kidney through a tube
called a ureter to the urinary bladder, where it
is stored.
-During urination ,the stored urine travels from
the bladder to the outside of the body through
the uretra.
Urine Formation
Urine is made in the nephrons in two stages:
1.
filtration
(𝑓𝑖𝑟𝑠𝑡 𝑠𝑡𝑎𝑔𝑒)
→ both useful substances
and wastes are removed
from the blood .
2.
Reabsorption
𝑠𝑒𝑐𝑜𝑛𝑑 𝑠𝑡𝑎𝑔𝑒
→ some of useful
substances reenter the
blood to be used by the
body .
Filtration
Filtration takes place in the glomeruli and
Bowman’s capsules.
The blood that enters a glomerulus is under
pressures.
The pressure forces the filtrate, which include
1.Water
2.Urea
3.Glucose
4.Amino acid
5.Various salts
through the thin walls of the glomerulus
into Bowman’s capsule.
- Blood cells and blood proteins, however, are
too large to pass through the walls of the
glomerulus.
- These substances remain in the blood.
- The filtrate that enters Bowman’s capsule is
like blood plasma, but it does not contain
proteins.
- If all of the filtrate that is formed were
excreted.
- The body would lose too much water along
with important nutrients and salts dissolved in
that water.
plasma protein
These substance
Remain in capillary cells
platelets
glucose
water
urea
These substance
amino acid
Enters into
Bowman’s capsule various salts
Reabsorption
- After the filtrate has left Bowman’s capsule,
reabsorption occurs in the renal tubule.
- It is the process of reabsorption that reduce
the volume of filtrate and
- return various important substances to the
blood.
- Normally , as the filtrate passes through the renal
tubules of the nephrons,
- about 99 percent of the water,
- all of the glucose and amino acid and
- many of the salts are reabsorbed.
- These substances are reabsorbed in to the blood
by the capillaries that surround the tubules.
- The reabsorption of water from the renal
tubules is an important means of water
conservation in mammals.
- Since most of the water is reabsorbed, the
substances left in the filtrate are highly
concentrated.
- While water is reabsorbed by osmosis,
- Glucose
Amino acid and
need active transport to be
salts
reabsorbed
- ATP, the energy source for active transport, is
supplied by the many mitochondria found in the
cell of the renal tubule.
- The tubules are lined with microvilli that
greatly increase the surface are through which
reabsorption can occur.
- The large area allows the reabsorption of huge
amounts of water and other substances.
s
After reabsorption
- The fluid remaining in the
tubule is urine
- The urine is made up of
1. Water
2. Urea and
3. Various salts.
- Urine flows from the
tubule into the collecting
ducts
- It passes out of the
kidneys through the
ureters to the bladder,
which is emptied from
time to time through the
urethra.
Formation of Urea
- Amino acids are the
breakdown products of
proteins.
- Because excess amino acids
cannot be stored in the
body, they are broken down
in the liver.
- The parts of the amino acids
are charged into other
substances.
Form each amino acid
The amino group is changed into ammonia
(N𝐻3 )
- The remainder of amino acid molecule either
is changed into pyruvic acid and used as an
energy source in cellular respiration or is
changed into glycogen or fat storage.
- Because the ammonia produced from the
amino group is very poisonous
- It is changed into the less harmful substance
urea by a series of enzyme-catalyzed
reactions.
-The urea diffuses from the liver into the blood
stream.
-The bloodstream, then carries the urea to the
kidneys.
-The kidneys filter urea from the blood, and
-It is finally excrete from the body in the urine