The problem of excretion, and salt and water balance in animals;
We have seen that animals need mechanisms for ridding their bodies
of metabolic wastes --- particularly nitrogenous ones, but many others
as well. The process of releasing such useless, even poisonous , substances
is called excretion. It should not be confused with elimination (defecation).
Whereas excretion is release of wastes that have been inside the cells,
tissue fluids, or blood of the organism, elimination is release of unabsorbed
wastes from the digestive tract.
In general, excretory mechanisms also serve a second very
important function: They help regulate the water and salt balance of the
organism. Our examination of excretion will focus on both these aspects,
which are in most instances inextricably intertwined.
Let us look first at the problem in aquatic animals. As we know,
the first nitrogenous waste formed by deamination of amino acids is ammonia.
Now, ammonia is an exceedingly poisonous compound, and no organism can
survive if its concentration in the body fluids gets very high. But the
small, highly soluble molecules of ammonia readily diffuse across cell
membranes, and there is no great difficulty in getting rid of them if an
adequate supply of water is available. The water keeps the solution dilute
while the ammonia is in the body, acts as a vehicle for its expulsion from
the body, and flushes it rapidly away from the vicinity of the animal.
In an environment with a limited supply of water, as a fish tank, the ammonia
will build up and kill the fish. Bubblers in fish tanks help to move the
ammonia from the water into the air and hence from the tank. In view of
the plentiful supply of water available to aquatic animals, in the seas,
it is not surprising that for many of these animals the characteristic
nitrogenous excretory product is ammonia.
Many marine invertebrates ( no backbone, live in hard skins)
lack special excretory systems, relying instead on release of wastes across
the general surface membranes. Such organisms seldom have any problem with
water balance, because they are essentially isotonic (same osmotic pressure)
with the surrounding sea water, and hence neither take in much excess water
nor lose too much water. A variety of these organisms supplement the excretory
process by phagocytic excretion; i.e. certain cells pick up solid particles
of waste material by phagocytosis (to eat, active engulfing of particles
by a cell) and then move to the outer body surface or to the surface
of the digestive cavity, where the materials are released.
Maintenance of the proper nonfluctuating internal fluid environment
is relatively simple for marine invertebrates as long as they remain in
the sea; it is quite a different matter when they move into hypotonic (lower
osmotic pressure) media such as the brackish water of estuaries or
the fresh water of rivers and lakes. Many marine animals are incapable
of moving into such habitats, because their body fluids always lose salts
until they have about the same salinity and osmotic concentration as the
external fluids. Since their cells generally can not tolerate much change
in the makeup of the fluids bathing them, these animals soon die when they
are put into brackish or fresh water.
Some marine animals, however, have evolved adaptations that enable
them to move into hypotonic media. The adaptation may be of an evasive
character, as in oysters and clams, which simply close their shells and
thereby exclude the external water during those parts of the tidal cycle
when the water in the estuaries is very dilute. But by far the most important
adaptations for survival in dilute media, ---and the ones that have played
the principal role in the evolutionary movement of animals into fresh water---,
are those that enable animals to regulate the osmotic concentrations of
their body fluids and keep them constant despite fluctuations in the external
medium. Such organisms are said to have the power of "osmoregulation".
Without the power of osmoregulation most fish, amphibians, reptiles, birds,
and mammals could not exist.
The shore crab is an example of a marine invertebrate that has
evolved a degree of osmoregulation enabling it to live in both seawater
and brackish water. In seawater the crab's body fluids are in osmotic equilibrium,
but in brackish water they are hypotonic relative to the surrounding medium.
To maintain the internal fluids near their normal concentration in brackish
water, cells on the crab's gills remove salt from the surrounding water
and actively secrete it into the blood, while the excretory organs eliminate
the excess water that constantly pours in.
--
oozing on the muggy shore of the gulf coast
[email protected]
--
The silver-list is a moderated forum for discussion of colloidal silver.
To join or quit silver-list or silver-digest send an e-mail message to:
[email protected] -or- [email protected]
with the word subscribe or unsubscribe in the SUBJECT line.
To post, address your message to: [email protected]
Silver-list archive: http://escribe.com/health/thesilverlist/index.html
List maintainer: Mike Devour
