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What is Excretion?
•
Excretion is the process of removing cellular
wastes.
•
The term excretion refers specifically to the
elimination of wastes produced by cellular activity.
•
It is not removal of undigested food material!
 Goal: Maintain homeostasis!!
What is Excretion? (contd)
• The most important waste products are CO2,
nitrogen compounds, and salts.
• CO2 – produced during cellular respiration
(in addition to water)
• Nitrogen compounds – (ammonia, urea, and
uric acid) produced by the breakdown of
amino acids (protein digestion)
• Salts – produced by metabolism
Why do we need to excrete Ammonia?
• All organisms produce ammonia as they
metabolize nutrients (protein digestion)
• Ammonia is a nitrogenous waste that is toxic
and must be removed from the body
• How an organism removes ammonia depends
upon where it lives
Removing Nitrogenous Waste Products
(2) Urea:
- Many land animals and some
bony fish (amphibians,
mammals) dilute the toxic
ammonia with water.
- This substance is called Urea & is
filtered out by the kidneys.
- The problem is they do lose
water in the process!
- Requires Energy
Removing Nitrogenous Waste Products
(3) Uric Acid
• Not very toxic because
insoluble
• Evolved as an adaptation of
land animals
• Many organisms try to
conserve water & excrete their
Nitrogen waste as a solid!
(little loss of water is involved)
• Examples: insects, reptiles,
birds
The Mystery behind Bird Poop
• Unlike mammals, birds do not urinate.
• Their kidneys extract nitrogenous wastes from the
bloodstream, and excrete it in the form of uric acid.
• Uric acid has a very low solubility in water, so it
emerges as a white paste.
Human Excretion
Where does excretion occur in humans?
Like other animals humans have a system
that excretes Nitrogen wastes as Urea, salt, and water
Urinary System: Kidneys, ureter, urethra, and bladder
Kidneys (located in back) play important role in homeostasis –
- remove waste products from blood
- maintain blood pH
- control water content of blood
How The Kidneys Work
1) Blood enters kidneys from
renal artery
2) Blood is filtered by passing
through millions of
nephrons
3) Wastes pass through ureter
to the bladder as urine
4) Clean blood returns to
body through renal vein
5) Bladder stores urine and
passes urine out of the
body through the urethra
Nephrons: How the Body makes Urine
• Your kidneys are
composed of 1 million
cells called Nephrons.
• These long coiled
tubes are where the
blood is actually
filtered and urine is
produced
The Nephron
• Nephron has three
functions:
1. Glomerular filtration: filter
out water, N-wastes (urea),
salt, glucose, amino acids
2. Tubular reabsorption:
reabsorb materials that the
body still needs (food
molecules, water).
3. Tubular secretion: collect
wastes as urine and pass
them to the bladder (urea,
salts, other substances)
Blood filters through the
kidneys about 6 times a day.
99% of the water is
reabsorbed and not excreted
Anatomy of 1 Nephron
Anatomy of 1 Nephron
What kinds of animals have the most
efficient kidneys?
• Desert animals must be
able to conserve moisture
• Most terrestrial animals
must drink fresh water
often; however the
kangaroo rat does not
need to drink water very
often – its kidneys absorb
every little drop of water
Kidney Failure
If renal failure occurs,
people must get their blood filtered through
Dialysis.
Gout
Is a disease where the human body accumulates Uric
acid in the joints. Heredity, alcohol & kidney failure
lead to this problem.
Kidney Diseases
• Kidney Stones