We will not be discussing the entire chapter in detail, but will focus a bit on water balance in vertebrates and
excretion and the function of the vertebrate kidney in particular (including the hormones ADH & aldosterone, which
we’ll use as our bridge between this lecture and the one on the endocrine system).
We already discussed several examples of
, the ability of an animal to maintain a constant internal
environment, in previous chapters.
is an animal that uses homeostasis to maintain a specific condition,
like a ‘thermoregulator’ maintains a constant temperature.
is the idea of maintaining the osmotic
balance of the cells of the body.
In other words, you don’t want water rushing into your cells and popping them, and
you don’t want water rushing out of your cells and leaving a shriveled cell that has too high a concentration of salts,
etc. to function correctly, so cells want to stay isotonic relative to their surrounding media, if possible.
This is tough
to do on an individual cell-by-cell basis, but it is easy to do if we just regulate the entire interstitial fluid (which
means controlling ionic balance in the blood, since interstitial fluid is derived from blood).
Most cellular waste
products (except CO
) end up dissolved in water, so getting rid of the waste in a timely fashion is crucial to
maintaining osmotic balance.
The most important waste products that build up in our bodies are
– products of the breakdown of proteins and nucleic acids (once they have been broken down to be fed into
the Krebs cycle and other biosynthetic pathways).
See Fig. 44.9.
The primary waste product produced by cells is ammonia
, which is an extremely toxic compound to cells.
Ammonia is very water-soluble, so it is easy to very quickly build up toxic concentrations of dissolved ammonia in
the body fluids.
Ammonia can act as a base to pick up hydrogen ions from solution and form ammonium ions
), which means that ammonia can be dangerous to organisms because it can change their pH.
In addition, lots
of ammonia seems to interfere with the function of ion channels in the plasma membranes of cells, and these are
very important to maintain the proper ionic balance (and charge) between the inside of the cell and the outside of the
Some species secrete ammonia directly into the environment, but they need lots of water to do it, because they can’t
let the ammonia build up to too high a concentration or it is toxic.
Some organisms, like freshwater fish, can
actively secrete ammonia in the form of NH
directly from the cells of the gills.
Organisms that don’t live in a freshwater aquatic environment have to process ammonia into other compounds for