Chapter: 11
Excretion in humans
Grade: 9
Subject: Biology
BCISW/GR-9/Chpter -13/Bio/01120
Excretion
Excretion is the removal of the following substances:
•toxic materials
•waste products of metabolism
•excess substances from organisms
Excretion is not the same as egestion - which is the passing out of
undigested food through the anus as faeces.
LO: State that carbon dioxide is excreted through the lungs
Excretion is the removal of the waste substances of metabolic reactions (the chemical
reactions that take place inside cells), toxic materials and substances in excess of
requirements
The Need for Excretion
Some of the compounds made in reactions in the body are potentially toxic (poisonous)
if their concentrations build up. Ammonia is made in the liver when excess amino acids
are broken down.
• Carbon dioxide must be excreted as it dissolves in water easily to form an
acidic solution (carbonic acid)which can lower the pH of cells. This can reduce
the activity of enzymes (denatures the enzyme) in the body which are
essential for controlling the rate of metabolic reactions. For this reason, too
much carbon dioxide in the body is toxic
• Ammonia is very alkaline and toxic. It is converted to urea which is much less
poisonous, making it a safe way of excreting excess nitrogen. Urea is also toxic to
the body in higher concentrations and so must be excreted
Excretory organs
Liver
Lungs
Kidney
Skin
The liver breaks
down excess
amino acids and
produces urea.
The yellow/green
bile pigment,
bilirubin, is a
breakdown
product of
haemoglobin
The lungs supply
the body with
oxygen, but they
are also excretory
organs because
they get rid of
carbon dioxide.
The kidneys
remove urea and
other nitrogenous
waste from the
blood. They also
expel excess
water, salts,
Sweat consists of
water, with sodium
chloride and traces of
urea dissolved in it.
When you sweat, you
will expel these
substances from your
body so, in one
sense, they are being
excreted.
hormones
LO: State that the kidneys excrete urea and excess water
and salts
LO: Identify on drawings, diagrams and images kidney, the ureters,
bladder and urethra
Human Urinary System
Contains blood
with lower urea
concentration
Sphincter Muscle
Ring of muscle which controls flow
of urine from bladder to urethra
Contains blood with high
urea concentration
Pathway of Excretion
The kidneys receive blood from the
renal artery, remove urea and a
variable amount of water from it and
returns the modified blood to the
circulation through the renal vein. The
wastes removed from the blood are
eventually expelled from the body
through the urethra after being stored
in the bladder.
LO: Outline the structure of the kidney,
limited to the cortex and medulla.
Structure of Kidney
Each kidney has four parts:
• Cortex – the brown outer layer jammed
pack full of filters called nephrons. Filters
the blood.
• Medulla – the middle reddish layer which
has the tubes carrying filtered wastes to the
centre of the kidney. Contains Loop of
Henle and collecting duct.
• Pelvis – area where all collecting ducts
come together and connect with ureter.
• Ureter – transports urine to the bladder.
LO:
Outline the structure and function of a nephron and its
associated blood vessels, limited to:
(a) the role of the glomerulus in the filtration from the blood
of water, glucose, urea and ions
(b) the role of the nephron in the reabsorption of all of the
glucose, some of the ions and most of the water back into
the blood
(c) the formation of urine containing urea, excess water and
excess ions
Nephron
Part of a nephron (glomerulus, renal capsule and
renal tubule)
• Each arteriole leads to a glomerulus.
• This is a capillary repeatedly divided and
coiled, making a knot of vessels.
• Each glomerulus is almost entirely
surrounded by a cup shaped organ
called a renal capsule, which leads to a
coiled renal tubule.
• This tubule, after a series of coils and
loops, joins a collecting duct, which
passes through the medulla to open into
the pelvis.
• There are thousands of glomeruli in the
kidney cortex and the total surface area
of their capillaries is very great.
There are roughly one million
nephrons in each kidney.
Cortex
Nephron
R. vein
Renal artery
The unit of a kidney is the
nephron – it carries out
filtering and reabsorption.
6/4/2024
Ureter
Medulla
20
PROCESSES OF THE NEPHRON
Blood is filtered
in the bowman's
capsule
Reabsorpti
on of useful
substances
at the
convoluted
tubule
Water levels of
urine adjusted
To bladder
FILTRATION
Renal artery
PROCESSES
OF THE
NEPHRON
Glomerulus
Bowman’s capsule
Distal
convoluted
tubule
6/4/2024
• Blood
arrives
from the renal artery.
Capillaries
off renal
artery
• Blood
enters a ball of capillaries called Glomerulus.
• These
capillaries
sit in a filter called the Bowman’s
Proximal
convoluted tubule
or renal capsule.
• Small molecules leave the capillary and enter the
capsule – glucose, amino acids, salts, urea and
Loop of Henle
Urine collecting duct
water.
23
Renal artery
Glomerulus
Bowman’s capsule
Capillaries off renal
artery
REABSORBTION
PROCESSES OF
THE NEPHRON
Convoluted or
kidney tubule
• The filtered blood
moves down the
nephron.
• In the convoluted
tubule cells reabsorb
the good stuff –
glucose, amino
Distalacids,
most salt andconvoluted
water.
tubule
• The rest – water and
urea – moves on
through the nephron.
Urine collecting duct
• As the urine
moves on more
water is saved
in the loop of
Henle and
Glomerulus
collecting duct
Bowman’s
capsule
• This
adjusts
water loss to
suit the level of
water in the
body.
Loop of Henle
RELEASE OF WASTE
PROCESSES OF THE
NEPHRON
Convoluted
tubule
Urine collecting duct
• Renal artery: brings blood to the kidneys
Contains waste products.
• Renal Vein: takes blood away from the kidneys.
Purified blood
• Convoluted Tubules: reabsorption of useful
substances happens here; including Glucose,
most water and some salts
• Bowman’s Capsule: This is where filtration takes
place. The filtrate contains useable & waste
products
• Collecting duct: collects urine from nephrons
and transports it to the pelvis
Ultrafiltration :
• The blood pressure in a glomerulus causes part of the blood plasma to leak
through the capillary walls.
• The red blood cells and the plasma proteins are too big to pass out of the capillary, so
the fluid that does filter through is plasma without the protein, i.e. similar to tissue
fluid. The fluid thus consists mainly of water with dissolved salts, glucose, urea and
uric acid. The process by which the fluid is filtered out of the blood by the glomerulus
is called ultrafiltration.
Selective reabsorption :
• The filtrate from the glomerulus collects in the renal capsule and trickles down the
renal tubule. As it does so, the capillaries that surround the tubule absorb back into
the blood those substances which the body needs.
• All the glucose is reabsorbed, with much of the water. Then some of the salts are
taken back to keep the correct concentration in the blood.
• The process of absorbing back the substances needed by the body is called selective
reabsorption.
LO: Explain that the volume and concentration of urine produced is
affected by water intake, temperature and exercise
The relative amount of water reabsorbed depends on the state
of hydration of the body (how much water is in the blood), and
is controlled by secretion of the hormone ADH.
 On a hot day: we sweat more to cool down à the body needs
to conserve water à produce a small amount of concentrated
urine.
 On a cold day: little sweat is being produced à we tend to
produce a larger volume of dilute urine.
Filtered blood returns to the vena cava (main vein) via a renal
vein. The urine formed in the kidney passes down a ureter into
the bladder, where it is stored. A sphincter muscle controls the
release of urine through urethra.
LO: Outline the structure and functioning of a kidney tubule,
including:
• the role of the glomerulus in the filtration from the blood of water,
glucose, urea and salts
• the role of the tubule in the reabsorption of all of the glucose,
most of the water and some salts back into the blood, leading to
the concentration of urea in the urine as well as loss of excess
water and salts (details of these processes are not required)
Function of
the kidneys
Ultrafiltration
Selective
reabsorption
The function of the kidney is to
filter blood, removing urea and
excess H2O, reabsorbing glucose,
some H2O and some mineral salts.
Urine is made by filtration and
selective reabsorption
 As blood passes through the
kidneys, it is filtered. This removes
most of the urea from it, and also
excess H2O and salts.
 As this liquid moves through the
kidneys, any glucose in it is
reabsorbed back into the blood.
Most of the H2O is also
reabsorbed along with some of
the salts.
•Let’s revise
•Each kidney contains around a
million tiny structures
called nephrons, also known
as kidney tubules or renal
tubules
•The nephrons start in
the cortex of the kidney, loop down
into the medulla and back up to
the cortex
•The contents of the nephrons
drain into the innermost part of the
kidney and the urine
collects there before it flows into
the ureter to be carried to
the bladder for storage
Describe the role of the liver in the assimilation of amino acids by converting them to
proteins
• Many digested food molecules absorbed into the blood in the small intestine are carried to the liver
for assimilation (when food molecules are converted to other molecules that the body needs)
• These include amino acids, which are used to build proteins such as antibodies, enzymes,
hormones, fibrinogen, a protein found in blood plasma that is important in blood clotting
• Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination
• Enzymes in the liver split up the amino acid molecules
• The part of the molecule which contains carbon is turned into glycogen and stored
• The other part, which contains nitrogen, is turned into ammonia, which is highly toxic, and so is
immediately converted into urea, which is less toxic
• The urea dissolves in the blood and is taken to the kidney to be excreted
• A small amount is also excreted in sweat
Assimilation and role
of the liver
Assimilation is
the movement of digested
food molecules into the
cells of the body where
they are used, becoming
part of the cells.
Assimilation and role
of the liver
Assimilation is
the movement of digested
food molecules into the
cells of the body where
they are used, becoming
part of the cells.
Role of liver in the metabolism of glucose and
amino acids
Excess glucose in the blood arriving at the liver
is converted into glycogen (animal starch) for
storage, or broken down through respiration,
producing energy for other purposes.
LO: State that urea is formed in the liver from excess amino acids
Define deamination as the removal of the nitrogen-containing part
of amino acids to form urea
LO: Explain the need for excretion, limited to toxicity of urea and
carbon dioxide
Composition of Urine
(what its made of)
• Water
• Urea
• NaCl (sodium chloride)
• KCl (potassium chloride)