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Excretion
AP Biology
Unit 6
Osmolarity
• Osmolarity = moles of osmotically active
particles per liter of solvent
• 1 M Glucose = 1 Osmolar solution
• 1 M NaCl = 2 Osmolar solution
– 2 osmotically active particles because NaCl
dissociates to become Na+ and Cl- in water
Question…
• What would happen if your body did not
maintain proper osmolarity?
• You would either have an excess of water
(bloated) or too little water (dehydrated)
• Cells wouldn’t have the correct balance of
solutes and H2O  won’t function properly
Osmoconformers
• Animals whose internal osmolarity changes
in relation to their external environment
– Equilibrate with the environment
– There are limits to this– too high or too low
will cause death
• Marine invertebrates
• Ex. Brine Shrimp (Artemia)
Osmoregulators
• Animals who can maintain their internal
osmolarity at a particular level regardless of
the external environment
• Ex. Fish, humans, lots of other animals 
Question…
• How does the environment one lives in
affect how osmoregulation takes place?
– Depending on the environment one lives in,
osmoregulation can be very different
Saltwater Fish
• Challenge: Prevent too much water from
leaving the body (to go into the outside
environment)
– Higher osmolarity outside compared to inside
of body
• Solution…
Freshwater Fish
• Challenge: Prevent too much water from
coming into the body from the outside
– Higher osmolarity inside body compared to
outside
• Solution…
Birds: Salt Glands
• Many birds who live by the
sea may take in sea water
along with the food they eat
• Get rid of the extra salt in
by excreting it through nasal
salt glands  sneeze or
shake off the salt droplets
Nitrogen Waste
• Nitrogenous wastes are a type
of metabolic waste that must be
removed from the body.
• Carbohydrates broken down
into CO2 and H2O
• Fats broken down into CO2 and
H2O
• Proteins and Nucleic Acids
broken down into NH2 groups
(urea, ammonia, uric acid)
Ammonia
• Ammonia is the most
common nitrogen waste
• Toxic at certain
concentrations
• To prevent toxicity to the
animal ammonia must be
– continuously excreted (keep
internal levels low) OR
– Converted to a nontoxic
molecule (urea or uric acid)
before excretion
Excretion in Aquatic Animals
• For most aquatic animals, excreting
ammonia is not an issue - why?
– Ammonia is highly soluble in H2O, diffuses
away rapidly (won’t stay concentrated around
them)
– Aquatic animals continuously lose ammonia
from their bodies through diffusion across their
gill membranes
Ammonotelic
• Animals that excrete
nitrogen waste mostly as
ammonia are
ammonotelic
• Ex. Aquatic
invertebrates, bony fish
Question…
• Why don’t terrestrial animals and some
aquatic animals just excrete dilute ammonia
in liquid?
– Since ammonia is toxic even at fairly low
levels, it would have to use a lot of water to
dilute it
– Too much water loss = dehydration
Ureotelic
• Animals that excrete
nitrogen waste mostly
as urea are ureotelic
• Ex. Mammals (us!),
amphibians, sharks,
rays, some bony fish
Uricotelic
• Animals that excrete
nitrogen waste mostly
as uric acid are
uricotelic
• Helps conserve H2O
because it isn’t very
soluble in water  semi
solid
• Ex. Birds, reptiles,
insects, some
amphibians
Excretory Process
• The main steps in producing urine (fluid
waste) are:
• Filtration
• Selective Reabsorption
• Secretion
Filtration
• nonselective process in
which water and small
solutes are filtered across a
membrane into the excretory
system
– Small solutes include salts,
nitrogen wastes, sugars, amino
acids
– Filtrate = liquid produced
from this step
Selective Reabsorption
• Useful/”good” molecules are
reabsorbed back into the body
from the excretory system by
active transport
– Sugars, amino acids, some salts
Secretion
• More waste (toxins, extra salts,
etc) are transported into the
filtrate
– By active transport
• Selective reabsorption and
secretion also causes water to
move in /out of filtrate
• Urine = whatever is left of the
filtrate after it has completed
all 3 steps
Nephrons
• The filtering units in the mammalian
excretory system are the nephrons.
• Random fact: there are approximately 1
million nephrons in each kidney
Major structures of the nephron
•
•
•
•
•
•
Glomerulus
Bowman’s Capsule
Proximal Tubule
Loop of Henle
Distal Tubule
Collecting Duct
Summary of Urine Creation
• Urine is created as solutes are taken out of
blood due to:
– The differences in osmolarity in the tubules and
the surrounding tissues
– Differences in permeability in the tubule
membranes
Random fact: each day you create
approximately 2-3 liters of urine
Glomerulus & Bowman’s Capsule
• Glomerulus = dense ball of
capillaries
• High pressure blood from
arteries flows into the
glomerulus
• substances are filtered from
the glomerulus into
Bowman’s capsule due to the
high pressure
– Everything small enough gets
across (red blood cells, proteins
too big to fit through)
Selective Reabsorption
• In the proximal tubule, many useful solutes
are reabsorbed back into the body fluid
– Glucose, amino acids, salts, some water
Loop of Henle
• The main function of the is
to reabsorb water from the
urine
– Ascending limb reabsorbs
NaCl into the surrounding
tissue; NOT permeable to
H2O
– Descending limb IS
permeable to H2O H2O
flows out of filtrate due to
osmosis (high salt
concentrations all around)
Descending
limb
Ascending
limb
Loop of Henle
As H2O is
reabsorbed
from the
filtrate, the
osmolarity in
the solution
increases
As NaCl is
actively
transported
out osmolarity
decreases
Question…
• Desert animals (kangaroo rats) usually have
long loops of henle, while freshwater
animals (beavers) have relatively short
loops of henle. How does the environment
one lives in relate to the length of the loop
of henle?
– Loop of henle is for water reabsorption. Living
in a drier area means you need to save more
water from urine  need a longer loop to do
this
Distal tubule
• More secretion and
reabsorption takes
place in the distal
tubule
Collecting Duct
• More water is
reabsorbed in the
collecting duct 
urine is
concentrated
• Pathway of urine:
kidney  ureter
 bladder 
urethra
Control of Excretion Processes
• Antidiuretic hormone
(ADH) increases the
permeability of the
collecting ducts to
increase the reabsorption
of water
– Urine is concentrated
further
– Also called vasopressin
Control of Excretory Processes
• Aldosterone also
increases salt and water
reabsorption in the distal
tubule
– Usually a response to low
blood pressure/volume
– The last step in a series of
reactions involving other
hormones
Interfering with ADH
• Caffeine and alcohol are diuretics
– substances that cause more frequent urination
• Caffeine and alcohol interfere with the
actions of ADH  more urine produced
– Alcohol = prevents ADH from being secreted
– Caffeine = prevents ADH from binding to
receptors on collecting duct