|
Bio 220 Lecture 26:
Osmoregulation I
Campbell,
5th, pp. 873-883
I.
Homeostasis
As we discuss various aspects of
animal physiology, we keep coming back to
homeostasis, the need to maintain a constant internal
environment. Today, we're
going to start discussing osmoregulation, the
regulation of water and solute
levels. We'll continue this on Friday with a detailed
examination of the human kidney. We'll conclude with an
examination of the solutions of desert animals to the
special problems in their environment by discussing the
article by McClanahan next Wednesday.
Make a note as we go along of
how elements of the nervous and endocrine systems are
involved both in sensing imbalances and in coordinating
responses that help correct
those imbalances.
II.
Water balance
Water moves into and out of
animal cells by osmosis, just as it did in
plants. One difference is that
most animal cells are not protected by cell walls, so there
is not a pressure difference across the cell membrane. Water
movement is governed by differences inys,
the solute potential, based on
the total osmolarity of all
solutes. An exception is certain cells (like fish eggs) that
are surrounded by a protein "shell" that helps to
counterbalance the pressure of the incoming water, like the
plant cell wall.
There
canbe pressure
differences between two compartments separated by a
barrier. An example is the
difference in pressure between the blood coming into
capillaries and the surrounding interstitial
fluid.
As a
result of this pressure difference, water is forced out of
the capillaries into the surrounding tissue.
A.
Environmental challenges
The earliest animals probably had
a salt and mineral composition identical to the environment.
But the seas have become saltier and animals have radiated
out into practically every possible environment. There are 3
main environments that animals live in and they all create
problems with water balance.
Some
animals continue to be osmoconformers, they maintain
the sameysas
their
|
1 2 3
4 5
6 