What Are Osmoregulation and Excretion?

To support bodily functions, animals regularly consume food and water. Material that cannot be used is excreted as waste. One of the most significant waste forms is nitrogen, a by-product of the breakdown of proteins and nucleic acids. Nitrogen tends to form ammonia in the body which is toxic, and must be properly disposed of.

Some animals directly excrete ammonia. Others first convert it into a less toxic form, like urea or uric acid. The variety of excretory systems allow animals to balance the conservation of energy and water.

What is maintained in the body is as important as what is removed from it. Regardless of diet, weather, and other external conditions, bodily systems must balance the level of water and dissolved substances called solutes. This is achieved through osmoregulation.

Osmosis is the tendency of water to move across a semi-permeable membrane from solutions with lower ion concentrations, or osmolarities, to those with higher ion concentrations. Bodily fluids, like plasma, and fluids inside and outside of cells are separated by such membranes. Animals control internal solute and water levels by osmoconforming or osmoregulating.

Osmoconformers have an internal osmolarity similar to that of their environment, so they do not generally lose much water. However, they actively keep concentrations of specific solutes distinct from those in their environment. Osmoregulators maintain osmolarities different from their surroundings, which requires energy to regulate water intake and loss.

Animals use specialized structures for osmoregulation. The primary vertebrate osmoregulatory organs are the kidneys, the kidneys continuously filter blood and control blood pressure, playing a key role in the osmoregulatory processes that support vertebrate life in a wide range of environments.