Osmoregulation | Osmosis and diffusion | CSIR NET NEET exam

Osmoregulation 

Whenever there is a stress to the homeostatic mechanism as a result of a primary cause such as injury, disease, trauma or infection, problems of fluid imbalance and electrolyte imbalance arise. Otherwise, in a normal living body, the osmoregulatory and excretory systems work in unison to 

control the osmotic pressure, ionic composition, and volume of the body fluids. 

Osmoregulation is the ability of an organism to maintain both the total electrolyte content and 

fluid content of its cells at a constant level. 

Fluid content and electrolyte content of a cell are, however, related as one is functionally inseparable from the other. 

The osmoregulatory capacity of a species determines its survival and it expresses the degree to which an organism can tolerate changes in its external environment. In other words, it is the ability of a species to maintain a 

constant internal environment as against a continuously changing external environment. 

The physicochemical processes of all cells are affected by the osmotic pressure of their environment. 

Perhaps the best illustration of the above statement can be given by the fact that the development of higher mental processes is hampered by an extracellular osmolarity equivalent to more than 1 % NaCI. 

ln the same way, if the volume of body fluids was not carefully controlled, the dilution or concentration of the extracellular fluids would result into a chaotic environment (altered metabolism) within the cells and will also lead to a disruption of the dynamic equilibrium operating between 

extracellular compartments like the blood capillaries and the interstitial fluid. 

Osmoregulation in vertebrates can be achieved mainly by five mechanisms: 

(i) regulation of water entry into the living body by a change in drinking behaviour or by a change in the permeability of external surfaces such as skin (as in amphibians) or gills (as 

in fishes), 

(ii) regulation of water loss from the body by changes in kidney function or by an alteration in the working of the external surfaces mentioned earlier, 

(iii) regulation of the rate of excretion of electrolytes by the renal system, 

(iv) regulation of the rate of excretion or entry of electrolytes through the extra-renal organs like skin or lungs, and 

(v) by an alteration of the osmotic pressure of body fluids brought about by a change in the concentration of physiologically inactive solutes. 

Maintenance of the tonicity of the body fluids is largely accomplished by regulation of water intake and water loss. Water intake is controlled mainly by the 'thirst' mechanism. 

Cellular dehydration is sensed by the receptors localized in neurons in the lateral preoptic areas which in 

turn stimulate a sensation of thirst. 

Another stimulation for thirst is a reduction in extracellular volume. The stretch receptors located in large veins near the heart and in the right atrium respond to reduction in extracellular volume and thereby stimulate a sensation of thrist by activating reflex 

pathways which leads to renin release and production of angiotensin II. 

Juxtaglomerular apparatus 

in the kidney also acts as a volume receptor and liberates renin when the blood pressure in the kidney declines. 

Renin later goes to form angiotensin II. 

The later plays a direct role in stimulating the thirst centers. 

Water loss, on the other hand in most mammals in regulated by the action of a 

neurohypophyseal peptide hormone, arginine vasopressin (antidiuretic hormone ADH). The osmoreceptors located in the supraoptic region of the hypothalamus are sensitive even to an increase of as little as 1 % to 2% in serum osmolality and they stimulate the secretion of ADH. 

ADH causes an increase in the permeability of the collecting tubules, allowing increased water resorption and decrease in urine volume. As a result a concentrated urine is secreted. 

In contrast, if water intake is more, ADH secretion is suppressed as tonicity of body fluids falls. The kidney now elaborates a large volume of dilute urine.

The principal cation in extracellular fluids is sodium and as it has the potential for altering osmotic shifts and blood volume, sodium levels and fluid balance are always closely related. 

Cells in the hypothalamus region called osmoreceptors are sensitive to the electrolyte concentration of the 

fluid surrounding them. When the surrounding fluid is hypertonic to the osmoreceptor cells, osmosis 

of water from the intracellular region to the exterior takes place. The resulting shrinkage of cells excites the hypothalamus which in turn sends impulses to the posterior pituitary. The posterior pituitary releases large quantities of ADH to minimize water loss. The reabsorbed water dilutes 

the hyperosmolarity induced by sodium retention. 

The most important hormone secreted by the adrenal glands for salt balance is aldosterone. In the absence of this hormone, large quantities of sodium are excreted by the kidney and survival 

is only possible if a 0.9% NaCI solution is available in place of drinking water. 

Aldosterone, a steroid hormone acts on the kidney to cause sodium resorption in the distal convoluted tubule, 

partly in exchange for potassium. 

Increased sodium reabsorption raises the solute concentration 

of body fluids. Adrenal hormones in mammals also facilitate the resorption of sodium and chloride 

from sweat and saliva and also cause increased sodium resorption in the gut. 

Low sodium ion concentration triggers the secretion of aldosterone. But other body conditions like increased 

potassium concentration, decreased cardiac output and stress too can induce aldosterone secretion.

The organisms living in the sea, like the elasmobranchs (sharks, skates, rays) and some others have developed another method for their osmoregulation. They maintain isosmosity with their environment by retaining inert solutes like urea and trimethylamine oxide (TMAO).These 

compounds are relatively inert, non-toxic and readily diffusible. 

Also, retention of these can be accomplished even upto a concentration of 0.5 M. Many species can also biosynthesize them and therefore these two compounds and specially urea, find favour with most marine vertebrates. 

In addition to the different osmoregulatory processes mentioned above, there are other special 

ways of osmoregulation adapted by animals and birds living in varied environments. The terrestrial 

inhabitants of the desert have evolved various solutions to the problem of water loss. 

For example, the insects living in deserts develop waxy cuticle to prevent water loss. The thick skin of toads 

also serves the same purpose. Desert dwelling animals have also developed some behavioural patterns along with the physiological adaptations. Burrowing and nocturnal activity allow them to 

avoid the heat and aridity of the desert. Food habits of the desert animals also are indicative of their behavioural adaptation. Most of them live on plant seeds and food material which is high in 

fat content as oxidation of 1 gram of fat yields 1.07 grams of water in contrast to proteins and carbohydrates which yield 0.4 gram and 0.6 gram of water respectively per gram of material oxidised.

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source: Biophysical Chemistry, Upadhyay Upadhyay and Nath, 2001, India