Osmoregulation, its mechanism and control

  • Osmoregulation is a homeostatic process by which the water content and the concentration of salts in the body of an animal is maintained.
  • The term osmoregulation was coined by Hober in 1902.
  • Different organisms have different limits of tolerance, both for water and salt gain and for water and salt loss due to different environmental conditions.
  • Animals having body fluids of the same concentration as that of surrounding medium (isotonic) never face the problem of osmoregulation.
  • However, animals which live in a medium of a lower salt concentration (hypotonic) have to face the difficulty of dilution of body fluids due to endosmosis. Hence, to get rid of excess of water that has entered the body, such animals have developed special mechanisms.
  • Animals which live in a medium of high salt concentration (hypertonic) have to face the difficulty of excessive exosmosis which may cause the shrinkage of the body fluids. To avoid this, they have to evolve certain regulatory mechanisms.

Mechanism of osmoregulation:

  • Animals tend to maintain an optimum osmotic concentration for a given environment. To maintain their internal osmotic concentration at a constant level, they have developed various types of adaptations as follows:
    1. By developing an impermeable covering (such as exoskeleton or cuticle) over the body surface to reduce the area of permeability.
    2. Reduction in the permeability of the exposed areas of the body.
    3. By increasing the absorption of water and salts through excretory organs, gut wall or the body wall to compensate their loss.
    4. By direct intake of food and drinks.
    5. By removing wastes in semisolid form to conserve water.
  • Osmoregulatory mechanisms in vertebrates are more efficient than in invertebrates because of the reduced permeability of body surface and the development of kidneys which play a great role in this phenomenon.
  • All fresh water animals are hypertonic to their surrounding water because the body fluids are more concentrated than the surrounding water. Therefore, there is a tendency for the inflow of water inside their body and the outward diffusion of salt through their semipermeable body wall. If this process goes on without some regulatory mechanism, the useful salts of the body will be lost and it will result in the dilution of blood. In fresh water vertebrates, the kidneys excrete hypotonic urine, i.e. dilute urine, the urine with plenty of water and far less solutes. In such animals, the conservation of water is not a problem.
  • On the contrary, the terrestrial habitat lacks both water and salts in the surrounding air. Therefore, terrestrial animals often face the problem of water and salt losses. Water loss may be due to sweat and urine. Water loss may be due to evaporation of water from the general body surface or through sweat or urine. Salt loss may be due to sweat or urine. This problem can only be solved by maintaining a balance between water and salt loss and water and salt gain. That is why such animals usually drink large amount of water and feed on food rich in salts, to compensate the deficiency of water and salts. Their kidneys excrete hypertonic urine, i.e. the concentration of solutes is more in comparison to blood plasma.
  • During summer months, when excessive sweating occurs, the urine is hypertonic while during winter months, when no sweating occurs, the urine is hypotonic. The volume and composition of urine varies according to the physiological state of the body. In spite of wide fluctuations in water and salt intake, the kidneys maintain the body fluid composition at a speedy state by controlling the amount of nitrogen wastes, salts, fluid and ionic concentration in blood.
  1. Maintenance of osmotic pressure:
    • The kidneys control osmotic pressure of extracellular body fluid by regulating the amount of water lost from the body. If a large amount of water is taken in, more water is eliminated by the kidneys. On the contrary, if a large amount of water is lost, the kidneys excrete relatively less amount of urine. This is done by reabsorption of water from the body of Henleā€™s Loop. In terrestrial mammals which produce concentrated urine, the loops of Henle are very long.
  2. Maintenance of ionic concentration:
    • The concentration of different ions of sodium, potassium, chloride, bicarbonate etc. in the blood is regulated by selective tubular reabsorption, mainly in the proximal convoluted tubule. At normal rates of excretion, all the sodium ions which are first filtered out during ultrafiltration are later reabsorbed in the proximal tubule and returned to the blood. The excess of Na+ ions are secreted by the tubule from the blood into urine, thus, maintaining a proper ionic concentration of the blood.
  3. Maintenance of acid-base balance:
    • The kidneys also regulate the acid base balance in the body fluids by removing non-volatile acids like lactic acid, sulphuric acid, phosphoric acid and ketone.

Control of osmoregulation:

  • The regulation of water balance and salt concentration in the body is achieved by the effects of certain hormones such as anti-diuretic hormone (ADH), aldosterone and other hormones which act on the distal convoluted tubule and collecting ducts.
    1. Regulation of water content:
      • This regulation is brought about by changing the permeability of distal convoluted tubule and collecting ducts. The change in permeability is affected by anti-diuretic hormone (ADH) or vasopressin released from the posterior lobe of the pituitary. When the amount of water decreases in the blood plasma, it stimulates the secretion of ADH and more water is reabsorbed from the filtrate in the collecting duct. This results in the concentrated urine passing out of the body. Contrarily, when the amount of water increases in blood, it inhibits the secretion of ADH and so, less water is absorbed from the filtrate. In this case, dilute urine is excreted out of the body. Alcohol and certain drugs suppress ADH secretion and more urine is discharged.

    1. Regulation of salts:
      • Regulation of different salts in the blood is under the influence of aldosterone. A decrease in sodium in blood stimulates a group of secretory cells, the juxtaglomerular complex (situated between the distal convoluted tubule and the afferent arteriole) to release an enzyme renin. Renin activates a plasma globulin produced in the liver to form the active hormone angiotensin and this releases aldosterone from the adrenal cortex. It makes the wall of the renal tubule and collecting duct highly permeable to sodium, potassium and chloride ions. As a result, all Na+, K+ and Cl- ions are reabsorbed from the filtrate into the blood. Then, as this diluted (hypotonic) fluid passes through the distal tubule and collecting duct, some more of the remaining ions are reabsorbed and a more hypotonic urine passes out of the body.

Osmoregulation, its mechanism and control