One of the fundamental biological processes in living beings and, above all, for those that inhabit aquatic ecosystems is the osmoregulation, Also known as osmotic balance.
All metabolic reactions necessary for life take place in an aqueous or liquid medium. For the correct operation of these reactions, it is necessary that the concentrations of water and solutes (all those low molecular weight organic compounds that help maintain the osmotic balance) oscillate within relatively narrow margins, in a process called osmoregulation.
We can define the osmoregulation as the method that maintains the homeostasis of the body, which is nothing other than the ability of living organisms to maintain their internal condition stable depending on the changes that may occur outside through the exchange of matter and energy with it.
All of this depends in a crucial way on the controlled movement of solutes existing in internal fluids and those found in the environment. This leads us to the regulation in the water movement play a fundamental role.
This regulation of the movement of water is carried out by osmosis, which is a physical phenomenon based on the movement of a solvent liquid through a semipermeable membrane. This phenomenon arises thanks to a diffusion which does not require energy expenditure and is crucial for the correct cellular metabolism of living beings.
Ultimately, the osmoregulation helps to ensure that concentrations of solutes existing inside organisms (for example, in cells) and the environment that surrounds them tend to balance each other through flow through membranes semipermeable. This circumstance allows regulating the osmotic pressure (pressure exerted in order to stop the flow of solvent penetrating a membrane).
Osmotic balance in animals
In most animals, the fluids that supply cells are isosmotic compared to the fluids that coexist inside cells. This means that the fluids inside and outside cells have a similar osmotic pressureThis prevents the cell from swelling excessively, as would occur in a hypotonic solution, or wrinkling, something that happens in the hypertonic solutions.
To be able to keep those fluids isosmotic On both sides of the plasma membrane, many cells use active ion transport (e.g. pumping Na+ outwards) which requires energy expenditure, complementing passive processes.
Animal cells see in a isosmotic solution a medium suitable for its correct functioning and development. In plants, this is not the case: plant cells that are found in a isosmotic solution may suffer from hair loss turgor, since its cell wall retains solutes and relies on high internal pressure.
Passive and active transit of water and ions
El passive transit does not involve energy consumption: ions They diffuse from the medium from higher to lower concentration and, by osmosis, the water moves in the opposite direction. The rate of ionic diffusion can be affected by the temperature, while osmosis depends on the solute gradient.
El active transit requires metabolic energy. It is used for eliminate excess ions (metabolic waste) or for absorb necessary substances that go against the gradient. In fish, this transport occurs mainly in gill epithelial cells, in the intestine and in the kidney.
Hormones and endocrine control of osmoregulation
Osmoregulation is modulated by hormonesIn marine fish, the cortisol promotes the excretion of salts in the gills; in freshwater fish, prolactin promotes ion absorption and water retention. calcitonin influences the management of calcium and permeability of membranes. In addition, the axis GH/IGF-1 (growth hormone/insulin factor) facilitates acclimatization to saline environments, and teleosts use the mineralocorticoid receptor with cortisol as a functional ligand to regulate ionic transport.
Osmoregulation in aquatic animals
Aquatic animals have adapted to a wide range of habitats, from freshwater (with very few solutes) to hypersaline waters (with abundant solutes). This confronts them with problems of osmotic balance very different. In addition, each species functions within a ambient osmolarity range determined.
- Pinholes: organisms that tolerate a narrow range of salinity of the environment, both in fresh and salt water.
- Euryhalines: organisms that tolerate a wide range of salinity, being able to live and move between fresh, brackish and marine water, for example some that migrate between rivers and sea.
There are mainly two ways to achieve this: osmoregulation:
El osmoconformism refers to animals that are in osmotic balance with the environment in which they live, that is, their body fluids are almost isosmotic with respect to the environment. They are usually marine organisms, especially many invertebrates and some cartilaginous vertebrates that accumulate u and other osmolytes to equalize ambient osmotic pressure.
Animals osmoregulators maintain their internal osmolarity distinct from that of the medium, actively adjusting the hydric balance and ions. The energy cost varies according to the permeability of the body surface. If the osmolarity of body fluids is greater than that of the environment, the animal is hyperosmotic; if it is less, it is hypoosmotic.
Acclimatization and salinity change
The species euryhaline (for example, some that migrate between rivers and sea) face additional challenges. Their acclimatization involves gradual changes in the expression of ionic transporters in gills and intestine, adjustments in the renal function and a fine one hormonal regulation (cortisol, prolactin, GH/IGF-1). These changes require tiempo and energy; therefore, sudden variations in salinity can generate osmotic stress.
Osmoregulation in freshwater fish
In freshwater fish, the concentration of ions body is greater than that present in water. This causes a diffusion of water into the interior of the fish through the epithelium of the gills and skin. Unregulated, this flow could swell the tissues and impair vital functions.
To compensate, the kidney of these fish generates large volumes of urine very diluted (high glomerular filtration), which allows the expulsion of excess of water. As their salt concentration exceeds that of the environment, the fish lose electrolytes by diffusion, so they must reabsorb salts through specialized cells in the gills and obtain them through the eating.
In the branchial epithelium, ion exchange is linked to the ion exchange itself. metabolism. Carbon dioxide is converted into bicarbonate and is exchanged with ions chloride, Whereas the ammonium (from protein catabolism) can be expelled by exchanging it with sodium. Thus, the excretion of waste is coupled with the maintenance of the ionic homeostasis.
El pH of water conditions these exchanges: in more environments acids, Na+ uptake is difficult, and sodium may accumulate in the blood and cause edemas or ascites in sensitive species. Maintain a stable pH and within the range of the species it is essential to avoid osmotic disturbances.
In aquariophilia, it is common to add small amounts of non-chlorinated salt in freshwater facilities that have recently been cycled when biological stability is not yet present. The presence of certain ions in the water it facilitates the exchange in gills and helps to control ammonia during the maturation phase of the system. It should be done with criterion and according to species, since some are sensitive to increases in conductivity.
Osmoregulation in saltwater fish
In marine fish, the external environment is hyperosmotic with respect to its internal fluids. Therefore, water tends to leave the body by osmosis and the ions from the sea enter by diffusion through the gillsThe main risk is the dehydration if not actively corrected.
To avoid dehydration, marine fish they drink sea water and absorb water in the intestine after precipitating and segregating part of the salts. The excess of NaCl It is eliminated in the gills by chloride cells (rich in mitochondria) that secrete chlorine through specific channels and expel sodium by paracellular routes. Some of the remainder is excreted by heces y urine.
Unlike freshwater fish, many marine fish produce little urine and with high signal concentration. This is related to a lower presence of glomeruli in the kidney; some species, such as seahorses, develop kidneys aglomerular. To recover water and limit losses, they have long renal tubules and effective reabsorption mechanisms.
In marine cartilaginous fish (not common in domestic aquariums), the strategy is different: they are osmoconformers that accumulate u and other osmolytes to equalize its osmotic pressure with the sea, expelling excess salts through specialized glands. This mention illustrates the diversity of evolutionary solutions for the same osmotic problem.
El Stress alters osmoregulation: sudden changes in salinity, poor water quality or inadequate management destabilize the hormones and ionic transporters. Although the cortisol facilitates acclimatization to salt water, chronic stress compromises the epithelial barrier and water balance, increasing susceptibility to pathogens.
Implications in aquaculture
In aquaculture production, water salinity is a factor critical for growth. Osmoregulation involves a energy expenditure which, if it is high, takes away resources from the for Growth already the feed conversion. Adjust the salinity range optimal by species and stage, along with temperature y photoperiod, maximizes productivity and well-being. In marine teleosts, exposure to a hyperosmotic environment forces them to intensify the excretion of salts and raises the metabolic cost; therefore, aquaculturists modulate salinity to improve performance y supervivencia.
Osmotic balance may seem complex, but it is essential for life. Understanding it helps to interpret the comportamiento and the needs of fish, both in the wild and in the aquarium. The key is to respect the environmental ranges of each species, avoid changes abrupt and ensure water quality that sustains its protection mechanisms osmoregulation without unnecessary energy overcosts.
