Introduction:

Osmoregulation signifies the physiological methods which an organism employs to maintain the water balance; that is, to compensate for the loss of water, avoid surplus water gain and maintain the proper osmotic concentration (that is, osmolarity) of the body fluids. Most of the humans are around 55 to 60 percent water by weight (that is, 45 percent in elderly and obese people and up to 75 percent in new-born infants). Most of the jellyfish are 95 percent or more water.

The Need for Osmoregulation in Animals:

Water is essential component of the animal's body; it is needed for maintenance of life and other metabolic processes. It forms main medium and most necessary nutrients in all animals. Water accounts for between 60% and 95% of animal's body weight. Water inside animals may be inside cells as intracellular fluid (ICF) or it may be outside cells in form of extracellular fluid (ECF). ECF itself may be distributed between many smaller compartments, like blood plasma and cerebrospinal fluid. The variety of solutes in form of ions and nutrients are dissolved in fluids. Animals require maintaining suitable and right amounts of water and solutes in the different body fluids. Skill to regulate water and solute concentrations in animals is referred as Osmoregulation. Osmoregulation and excretion are closely related together in animals as most animals utilize their excretory organs for osmoregulatory functions. Animals generally need Osmoregulation to make sure:

a) Protection of animal's internal environment from harmful fluctuations

b) Maintenance of balance in water loss and uptake in animal's body

c) Proper control of animal's internal environment.

Principle of Osmosis:

Term osmosis is best described as movement of water across the selectively permeable membrane that divides two solutions, from the region of water high concentration (that is dilute solution) to the region of lower water concentration (that is a concentrated solution). When 2 aqueous solutions of different solute concentrations are divided by membrane permeable to water but impermeable to solute molecules, water diffuses through membrane from dilute solution to more concentrated solution. What occurs here is known as osmosis. This procedure will continue until equilibrium is established, at which point there is no further net movement of water and concentrations of solution on either side of selectively permeable membrane are equal. The selectively permeable membrane is one that permits only water to go through it and no other substances.

Osmoconformers and Osmoregulators:

Not all the organism's osmoregulate. A few marine animals like the sea stars are osmoconformers, that is, their body fluids are alike to seawater in osmolarity and therefore they gain and lose water at equivalent rates and have no requirement to expend energy expelling water or salt from the body. Though when they are positioned in water more or less concentrated than seawater, their tissues swell or shrink, their organelles and cell membranes are damaged and they die. This is why echinoderms are not found in the estuaries or river mouths where fresh and salt-water meet up and the salinity fluctuates very much. Osmoconformers are stenohaline (steno signifies 'narrow range', and hal signifies 'salt'), not capable to tolerate much variation in the environmental salinity.

Osmoregulators, on the other hand, keep a more or less stable internal osmolarity through physiological means. Terrestrial animals should osmoregulate as they unavoidably lose water through excretion and evaporation and substitute water is not always instantly available. Marine Osmoregulators keep an internal salinity lower than that of seawater and freshwater osmoregulatory keep an internal salinity higher than that of fresh water. Euryhaline (eury signifies 'broad') animals, those capable to tolerate a wide range of environmental salinity, should be good Osmoregulators. The blue crab, Callinectes sapidus, for illustration, thrives in estuaries and needs efficient Osmoregulation to survive there.

How does Osmoregulation work in Animals?

A few species like 'Amoeba' utilize ammonia, to collect excretory waste, from the intracellular fluid through both diffusion and active transport. The vacuole shifts to the surface and disposes the contents to the environment, as osmotic action pushes water from the atmosphere into the cytoplasm. Kidneys play a main role in the human Osmoregulation, regulating the quantity of water in urine waste. By the assistance of some hormones like antidiuretic hormone, aldosterone and the human body can raise the permeability of the collecting ducts in the kidney to reabsorb water and prevent it from being excreted. One of the major ways that animals have evolved to osmoregulate is by controlling the quantity of water excreted via the excretory system.

How is Osmoregulation achieved in Vertebrates?

Osmoregulation is accomplished in Vertebrates via the given four processes:

1) Filtration:

Fluid part of blood (plasma) is filtered from the nephron (that is, functional unit of vertebrate kidney) structure termed as the glomerulus into the Bowman's capsule (that is, a cup-like sac at the starting of the tubular component of a nephron in the mammalian kidney which carries the first step in the filtration of blood to form urine) or glomerular capsule (that is, in the kidney's cortex) and flows down the proximal convoluted tubule to a 'u-turn' termed as the Loop of Henle (that is, loop of the nephron) in the medulla part of the kidney. A glomerulus is a capillary tuft which carries the primary step in filtering blood to form the urine.

2) Re-absorption:

The majority of viscous glomerular filtrate is returned to the blood vessels which surround the convoluted tubules.

3) Secretion:

The remaining fluid becomes the urine that travels down collecting ducts to the medullar are of the kidney.

4) Excretion:

The urine (that is, in mammals) is stored in the urinary bladder and exits through the urethra; in other vertebrates, the urine mixes by other wastes in the cloaca (that is, cloaca is the posterior opening which serves up as the only such opening for the intestinal, reproductive and urinary tracts of certain animal species) prior to leaving the body; (frogs as well encompass a urinary bladder).

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