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Homeostasis and
Excretion
Christen Fechtel
Deanna Dang
Stephanie Daniel
Intro to Homeostasis
• Maintenance of physiological systems
by maintaining a stable internal
environment
– water and solute concentrations
– Metabolism and disposal of metabolic
waste
• Two key processes: osmoregulation and
excretion
IB Syllabus!
Osmoregulation
• How animals regulate solute concentrations and
balance the gain and loss of water in blood, tissues,
and cytoplasm• the homeostasis of osmolarity in body fluids
• Regulates chemical composition of body fluids
– Controls movement of solutes between internal fluids and
external environment
• Animal cells will swell and burst if continuous
uptake of water, or shrivel and die is continuous net
loss of water
• Rates of water uptake and loss must
balance...osmosis!
Osmoregulation-Osmosis
• Osmosis: special case of diffusion in which
water moves across a selectively permeable
membrane when two solutions separated by
the membrane differ in osmotic pressure, or
osmolarity (measure of solute concentration:
moles of solute per liter of solution)
– Isoosmotic: two solutions have same osmolarity- no
net movement of water (water molecules
continuously crossing membrane but at equal rates)
– Hypoosmotic: the more dilute solution when two
solutions differ in osmolarity
– Hyperosmotic: solution with greater concentration
of solutes when two solutions differ in osmolarity
Osmoregulation
• Two ways to balance water gain with water
loss:
– Osoconformer (only marine animals): animal
which does not actively adjust its internal
osmolarity; isoosmotic to surroundings-internal
osmolarity same to its environment so no
tendency to gain or lose water
• Live in water with stable composition so have a constant
internal osmolarity
– Osmoregulator: animal that must control its
internal osmolarity because body fluids not
isoosmotic with outside environment
• Discharge excess water if in hypoosmotic environment
• Take in water if in hyperosmotic environment
• Can live in environments unsuitable for osoconformers
(freshwater, terrestrial)
Osmoregulation
• Two type of organisms:
– Stenohaline: can not tolerate substantial
changes in external osmolarity
– Euryhaline: can survive large fluctuations
in external osmolarity (ex: salmon and
tilapia)
Osmoregulation Adaptation in
Marine Animals
• Seawater dehydrates because much saltier than
internal fluids
–
–
–
–
Lose water by osmosis and gain salts by diffusion
Drink large amounts of seawater to balance water loss
Kidneys remove salt from body
Small amounts of water in very concentrated urine
Osmoregulation Adaptation in
Freshwater Animals
• Osmolarity of internal fluids higher than
surrounding environment
– Gain water by osmosis and lose salts by diffusion
• Adaptation to a lower salinity environment-lower
solute concentrations in body fluids-reduced
osmotic difference reduces energy needed for
osmoregulation
• Excrete large amounts of very dilute urine
Osmoregulation Adaptation in Animals
that Live in Temporary Waters
• Anhydrobiosis “life without water”: lose
almost all their body water and survive in
a dormant state when their habitats dry
up
100 µm
100 µm
(a) Hydrated tardigrade
(b) Dehydrated
tardigrade
Osmoregulation Adaptations in
Land Animals
• Threat of dessication
• Lose water through surfaces in gas exchange
organs, urine, feces, and skin
• Adaptations to reduce water loss and maintain
homeostasis
– Body coverings
• Waxy layers of insect exoskeletons
• Layers of dead, keratinized skin cells of humans
• fur
– Nocturnal: lower temp, higher humidity of night
– Drinking and eating moist foods
– Using metabolic water (water produced during
cellular respiration)
Osmoregulation
• Ultimate function: maintain
composition of cellular cytoplasm
– Done by managing composition of
internal body fluid that bathes cells
– In a closed circulatory system, this is
interstitial fluid (fluid composed of water,
amino acids, sugars, fatty acids,
coenzymes, hormones, neurotransmitters,
and salts) controlled through the
composition of blood
– Specialized structures maintain fluid
composition
Osmoregulation
• Transport epithelium: layer or layers of
specialized epithelial cells that regulate solute
movements
– Essential components of osmotic regulation and
metabolic waste disposal
– Move specific solutes in controlled amounts in
specific direction
– Directly face outside environment or line channels
connected to outside by an opening on body surface
– Impermeable tight junctions
– Form barrier at tissue-environment boundary
• Ensures solute moving between animal and environment
passes through a selectively permeable membrane
– Arranged in complex tubular networks with
extensive surface areas
• Ex: salt glands of marine birds
Nasal salt gland
(a) An albatross’s salt glands
empty via a duct into the
nostrils, and the salty solution
either drips off the tip of the
beak or is exhaled in a fine mist.
Nostril
with salt
secretions
Lumen of
secretory tubule
Vein
Capillary
Secretory
tubule
(b) One of several thousand
secretory tubules in a saltexcreting gland. Each tubule
is lined by a transport
epithelium surrounded by
capillaries, and drains into
a central duct.
Artery
NaCl
Transport
epithelium
Direction
of salt
movement
Blood
flow
Central
duct
Secretory cell
of transport
epithelium
(c) The secretory cells actively
transport salt from the
blood into the tubules.
Blood flows counter to the
flow of salt secretion. By
maintaining a concentration
gradient of salt in the tubule
(aqua), this countercurrent
system enhances salt
transfer from the blood to
the lumen of the tubule.