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Osmoregulation is the active regulation of the osmotic pressure of body fluids to maintain the homeostasis of the body's water content, that is it keeps the body from becoming too dilute or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution the more water wants to go into the solution. The pressure that must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water. Animals in all environments (aquatic and terrestrial) must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion: getting rid of metabolic wastes and other substances such as hormones which would be toxic if allowed to accumulate in the blood via organs such as the skin and the kidneys; keeping the water and dissolved solutes in balanced is referred to as osmoregulation.


Examples of osmotic pressure

  • Hypertonic is a solution with higher solute concentration (higher osmotic pressure) than another thus water wants to move in.
  • Hypotonic is a solution with lower solute concentration (lower osmotic pressure) than another thus water wants to move out of it.
  • Isotonic is solution with the same solute concentration (same osmotic pressure) as another; no net movement of water.

Forms of osmoregulation

Two major types of osmoregulation are osmoconformers and osmoregulators. Osmoconformers match their body osmolarity to their environment . It can either be active or passive. An example are marine fish. By drinking in sea water, and actively excreting salt out from the gills, the fish will gain salt as it produces an isotonic urine. Osmoregulators tightly regulate their body osmolarity which always stays constant and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example are freshwater fish. The gills actively uptake salt from the environment. Water will diffuse into the fish where the fish will excrete a very hypotonic urine to expel all the excess water.

Osmoregulation in plants

There are no specific osmoregulation organs in higher plants. Control of water intake and loss is by means of those internal and external factors which affect the rate of transpiration.

Plants share with animals the problems of obtaining water and in disposing of the surplus. Certain plants develop methods of water conservation. Xerophytes are plants in dry habitats such as deserts which are able to withstand prolonged periods of water shortage. Succulent plants such as the cactus have water stored in large parenchyma tissues. Other plants have leaf modifications to reduce water loss, such as needle-shaped leaves, sunken stomata and thick, waxy cuticles as in the pine. The sand-dune marran grass has rolled leaves with stomata on the inner surface.

Osmoregulation in protoctists and animals

Amoeba makes us of a contractile vacuole to collect excretory waste, such as ammonia, from the intracellular fluid by both diffusion and active transport. As osmotic action pushes water from the environment into the cytoplasm, the vacuole moves to the surface and disposes the contents into the environment.

Kidneys play a very large role in human osmoregulation. Kidneys regulate the amount of water in urine waste. With the help of naturally producing hormones such as antidiuretic hormone, aldosterone, and angiotensin II, the human body can increase permeability of the collecting ducts in the kidney to reabsorb water and prevent it from being excreted.

A major way animals have evolved to osmoregulate is by controlling the amount of water excreted through the excretory system.

Vertebrate excretory systems

Waste products of nitrogen metabolism

Ammonia is a toxic by-product of protein metabolism and is generally converted to less toxic substances after it is produced then excreted; mammals convert ammonia to urea while birds and reptiles form uric acid to be excreted with other wastes via their cloacas.

How osmoregulation is achieved in vertebrates

Four processes occur:

  • filtration - fluid portion of blood (plasma) is filtered from nephron (functional unit of vertebrate kidney) structure known as the glomerulus into Bowman's capsule or glomerular capsule (in the kidney's cortex) and flows down the proximal convoluted tubule to a "u-turn" called the Loop of Henle (loop of the nephron) in the medulla portion of the kidney.
  • reabsorption - most of the viscous glomerular filtrate is returned to blood vessels which surround the convoluted tubules.
  • secretion - the remaining fluid becomes urine which travels down collecting ducts to the medullary region of the kidney.
  • excretion - the urine (in mammals) is stored in the urinary bladder and exits via the urethra; in other vertebrates the urine mixes with other wastes in the cloaca before leaving the body; ( frogs also have a large penis).


  • E. Solomon, L. Berg, D. Martin, Biology 6th edition. Brooks/Cole Publishing. 2002


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