Homeostasis and
Water Balance
BIOL 1407
Homeostasis
• Tendency to
maintain a
relatively
constant internal
environment
despite changes
in external
environment
• Crucial for
survival
Why is Homeostasis Crucial?
• Proper cell function
depends on specific
environmental
conditions within
internal
environment:
– Fluid that surrounds
cells
Why is Homeostasis Crucial?
• Membrane functions
– Fluidity is affected by
temperature
– Membrane potential
(charge) affected by
chemical balance
Why is Homeostasis Crucial?
• Proteins
function within
limited
chemical
range
– Temperature
– pH
Response
Effector
Stimulus
Integration Center
Homeostatic Range
• Variable controlled within an acceptable
range
• Minimum and Maximum
• Example of Temperature Fluctuations in
Adult Male Mice (a) and Adult Male
Humans (b):
– http://www.ehponline.org/members/2002/110p61-65frank/fig2.jpg
Negative
Feedback:
Change in
one
direction 
Change in
opposite
direction
Positive
Feedback:
Change in
one
direction 
More
change in
same
direction
Strategies:
Conformers
vs.
Regulators
Water Balance
• Regulates movement of water molecules between cells
and internal environment
Osmoconformers
• Isoosmotic to
surrounding seawater
• As salinity of
seawater changes 
internal body fluid
composition changes
Osmoregulators
• Body fluids are not
isoosmotic to
surrounding
environment
• ATP must be used by
organism to maintain
the difference
Marine Bony Fish
Freshwater Fish
Terrestrial Animals
Water out
through
evaporation
(most) and
urine (little)
Water in
through
food
(most)
and
drinking
(little)
Photo Credit for Kangaroo Rat: Courtesy of Royal Alberta Museum, 2006, Wikimedia Commons
Salt Glands
Marine Birds and Reptiles
Countercurrent
Exchange
The End
Unless otherwise specified, all images in this presentation came from:
Campbell, et al. 2008. Biology, 8th ed. Pearson Benjamin Cummings.