The body fluids of all animals are remarkably similar in salt content, and this content is very similar to the salt content in sea water. For soft-bodied marine animals like the sea anemone, water balance is simple. Their body fluids are in equilibrium, or are isotonic, with the surrounding water: osmotic pressures are the same inside and outside the animal’s cells. Animals that don’t actively adjust their internal water balance are called osmoconformers.

Fresh water contains about one hundredth of the salt concentration of seawater. But water tends to diffuse from regions of low salt concentration to regions of higher salt concentration. So freshwater animals must have a way to prevent the surrounding water from entering their cells. Animals that actively adjust their internal water balance to maintain osmotic pressure are called osmoregulators.

Freshwater protists like a paramecium are osmoregulators. They expel excess water by means of a contractile vacuole. Marine protists, which are osmoconformers, don’t usually need or possess contractile vacuoles.

Land animals have the opposite problem to freshwater protists. As we’ll see, they need to prevent water loss from their cells.

No matter what the environment, osmoregulation depends upon active transport across cell membranes. The layer of specialized epithelial cells that regulates the movements of salts is called transport epithelium.

Transport epithelium is usually characterized by: a single sheet of cells facing the external environment or lining a tubular channel that leads to the outside, cells that are connected by impermeable tight junctions, forming a continuous barrier, and a variety of membrane transport proteins that can shuttle ions or molecules across the epithelium.

Transport epithelia help maintain water balance and take part in excretion, as we’ll see later in this activity.

Excretion includes the disposal of nitrogen-containing waste products.

In mammals and adult amphibians, this waste is the compound urea; in birds and reptiles, it is uric acid; and in most aquatic animals, including fish, the waste product is ammonia. Why is there so much nitrogenous waste to get rid of? Proteins and nucleic acids are rich in nitrogen, so a by-product of many chemical reactions in the body is ammonia. Ammonia may be great as a component of floor cleaners and hair dyes, but it is very toxic to the body.

Many aquatic animals excrete ammonia directly into the surrounding water where it is quickly diluted. Other animals convert the ammonia to less toxic compounds before excretion takes place.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education