23.8:
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.
23.8:
What Are Osmoregulation and Excretion?
Organisms must keep bodily fluids at a constant temperature and pH while maintaining specific solute concentrations in order to support life functions. Osmoregulation is the process that balances solute and water levels.
Osmosis is the tendency of water to move from solutions with lower ion concentrations, or osmolarities, to those with higher ion concentrations. Osmosis occurs in response to differences in the molecular concentrations of solutions separated by a semipermeable membrane.
Bodily fluids, which are separated by such membranes, contain water, non-electrolytes, and electrolytes—solutes that dissolve into ions in water. Both electrolytes and non-electrolytes influence osmotic balance. However, since the more important factor to osmosis is solute number, rather than size, the contribution of electrolytes is more significant.
Unlike water, electrolytes cannot diffuse passively through membranes but rely on facilitated diffusion and active transport. In facilitated diffusion, protein-based channels move solutes across membranes. Conversely, energy is used to move ions against concentration gradients in active transport.
When animals ingest food, material that cannot be used is excreted from the body. Excretory systems in nature involve tradeoffs between conserving energy and water.
Nitrogen is among the most significant kinds of waste in the body. Excess nitrogen forms ammonia, which is toxic and must be discarded. Some animals directly excrete ammonia; others first convert it into urea or uric acid, which are less toxic. Ammonia conversion requires more energy than direct excretion, however, it conserves more water.
Transport epithelia often mediate osmoregulation and excretion. These specialized cells move solutes and are found in excretory organs throughout the animal kingdom: from insect Malpighian tubules to fish gills to vertebrate kidneys.
Typically organized in tube-shaped networks with large surface areas, transport epithelia often assist with both water balance and waste removal. For example, some seabirds have nasal glands that remove salt from the blood and excrete it from the nostrils, enabling them to consume seawater.
Suggested Reading
Pedersen, Stine Falsig, András Kapus, and Else K. Hoffmann. "Osmosensory mechanisms in cellular and systemic volume regulation." Journal of the American Society of Nephrology 22, no. 9 (2011): 1587-1597. [Source]
Weiner, I. David, William E. Mitch, and Jeff M. Sands. "Urea and ammonia metabolism and the control of renal nitrogen excretion." Clinical Journal of the American Society of Nephrology 10, no. 8 (2015): 1444-1458. [Source]