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Excretion – Section 39.3
Excretion
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Process that rids the
body of substances:
– toxic chemicals
– excess water
– salts
– carbon dioxide
Maintains osmotic
(water) and pH balance
Excretion
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Lungs
– Excrete CO2 and water
vapor in exhaled air
Kidneys
– Excretes nitrogen wastes,
salts, water, and other
substances in urine
Skin
– Excretes water, salts,
small amounts of nitrogen
wastes, and other
substances in sweat
Excretion
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Urea
– Main nitrogenous waste
of body
– Produced in liver from
ammonia (NH3
– Formed during
metabolism of proteins
and nucleic acids
How the Kidney Works
The Kidneys
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Maintains balance of salt,
water, minerals, vitamins,
pH, and volume of blood
Each Kidney composed of
~ 1 million microscopic
units called a nephron that
filter the blood
Three phases occur as
blood flows through a
nephron:
– Filtration
– Reabsorption
– Secretion
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Nephron Structure
Filtration
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Occurs in Bowman’s capsule
where there is a network of
arteries called a glomerulus
Blood pressure inside the
capillaries forces fluid
containing water, salt,
glucose, amino acids, and
urea into the hollow interior of
Bowman’s capsule.
This fluid is called filtrate.
Blood cells, proteins, and
other molecules too large to
cross the membrane remain
in the blood.
Filtration
Filtration
Reabsorption
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Filtrate passes from the
Bowman’s capsule into the
renal tubules.
As the filtrate passes through
the renal tubules useful
molecules; glucose, ions, and
some water, is reabsorbed.
These substances reenter the
bloodstream through
capillaries that wrap around
the tubule
About 99% of water is
returned to blood by osmosis
Reabsorption
Reabsorption
Secretion
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When substances pass from the blood into the filtrate
Eliminate toxic substances from blood
Some Interesting Facts!
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Blood passes through
kidneys 300 times a day.
Nephrons clean all your
blood in 45 minutes
Every day the nephrons
send about six cups of
urine to the bladder
Urine Formation
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Filtrate contained in
nephron is urine
Formed from the water,
urea, and salts that are
left after the absorption
and secretion processes
Urine leaves the kidneys
through the ureters and
travels to the urinary
bladder
Urinary bladder stores
urine and expands as
more urine enters
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Excretion
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Stretch receptors send
nerve impulses to spinal
cord
Spinal cord returns
impulses to bladder and
urethra simultaneously
Impulses cause
contractions of bladder’s
muscular wall and
relaxes muscle of
urethra
Urine is emptied through
the urethra
Animation

Click here for Urinary System Animation
Review Questions
1.
2.
3.
4.
Identify how the carbon dioxide in your body
is produced and excreted.
Relate the following terms to the formation
of urine: filtration, reabsorption, and
secretion.
Summarize how urine is stored and
eliminated from the body.
Concept Generalization: circulatory system,
respiratory system, and excretory system