Osmoregulation – Definition, Mechanism, Types and Examples

What is Osmoregulation?

Osmoregulation is the process by which an organism maintains the proper amount of water and dissolved solutes such as salts and electrolytes in its body fluids. By controlling their entry, movement and loss, the internal osmotic balance is maintained.

This process is necessary because continuous gain or loss of water can change the osmotic pressure of body fluids. Water enters through drinking and food, while it is lost through urine, sweating, respiration and other processes.

Osmoregulation protects the cells from excessive movement of water. In a hypotonic condition, excess water enters into the cells and causes swelling. In a hypertonic condition, water moves out from the cells and causes shrinking.

By maintaining water and electrolyte concentration, normal cellular activities, blood volume and internal homeostasis are maintained.

Characteristics of Osmoregulation

Maintenance of fluid and electrolyte balance – Osmoregulation maintains the amount of water and dissolved solutes in the body fluids. These solutes include electrolytes and non-electrolytes. Their movement takes place across the semipermeable membrane.
Protection of cells – It maintains the osmotic pressure of cells. In hypotonic condition, excess water enters into the cells and causes swelling or bursting. In hypertonic condition, water moves out and the cells become shrink.
Regulation of body fluid compartments – In mammals, it controls water and electrolyte concentration in three major fluid compartments. These are blood plasma, intracellular fluid and extracellular fluid.
Role of kidneys – The kidneys are the major organs of osmoregulation. It continuously filters the blood and removes excess water, electrolytes and toxic nitrogenous wastes from the body.
Hormonal regulation – Different hormones are involved in regulation of water and salts. The major hormones include antidiuretic hormone (ADH), angiotensin II and aldosterone. These hormones act according to the osmotic condition and blood volume of body.
Presence of specialised receptors – The change in solute concentration of blood is detected by osmoreceptors present in the hypothalamus. Baroreceptors detect the changes in blood pressure and blood volume.
Behavioural responses – Osmoregulation also produces different behavioural responses. Thirst causes intake of water, while salt appetite helps in the intake of sodium according to body requirement.
Requirement of energy – Osmoregulators use metabolic energy for maintaining their internal osmotic pressure. Humans and bony fishes maintain the internal osmotic condition different from their surrounding environment.

Principle of Osmoregulation

Osmoregulation is based on the movement of water and dissolved ions across a selectively permeable membrane. Water moves by osmosis, while ions are moved by passive or active transport.

The movement continues according to the concentration difference between the body fluid and surrounding medium. Different organs, cells and hormones control this movement. By this process, the osmotic balance of body fluids is maintained.

Movement of Water by Osmosis

Osmosis is the movement of water across a selectively permeable membrane. Water generally moves from a region having lower solute concentration toward a region having higher solute concentration.

In a hypotonic medium, the surrounding fluid contains less solute than the cell. Water enters into the cell and may cause swelling.

In a hypertonic medium, the surrounding fluid contains more solute than the cell. Water moves out from the cell and causes shrinking.

In an isotonic condition, the solute concentration remains almost equal on both sides. No major net movement of water takes place.

Active Transport of Ions

Ions such as sodium, potassium and chloride cannot always move according to their concentration gradient. Their movement against the gradient requires active transport.

Transport proteins and ion pumps are present in the cell membrane. These proteins use metabolic energy for moving ions from one side of the membrane to another.

The gills of fishes, kidneys of vertebrates and cell membrane of microorganisms contain different ion transport systems. These systems help in absorption or removal of salts.

Maintenance of Osmotic Balance

Water and salts continuously enter and leave the body. Water is obtained through drinking, food and metabolic reactions. It is lost through urine, respiration, sweating, evaporation and faeces.

Salts are obtained mainly through food and surrounding water. Excess salts are removed through kidneys, gills, salt glands and other excretory structures.

The rate of intake and removal is adjusted according to body requirement. By this regulation, the cells are protected from excessive gain or loss of water.

Types of Osmoregulation

The two major types of osmoregulation are osmoregulation and osmoconformation.

1. Osmoregulators

Osmoregulators maintain their internal osmotic concentration different from the surrounding environment. Water and salts are actively controlled inside the body.

This process requires metabolic energy. Humans, freshwater fishes, marine bony fishes and most terrestrial animals are common osmoregulators.

2. Osmoconformers

Osmoconformers maintain the osmotic concentration of body fluids nearly similar to the surrounding environment.

Most marine invertebrates are osmoconformers. Sharks, skates and rays also maintain their total body-fluid osmolarity close to seawater by storing urea and trimethylamine oxide.

Based on salinity tolerance, organisms are also divided into stenohaline and euryhaline organisms.

Stenohaline Organisms

Stenohaline organisms can tolerate only a narrow range of environmental salinity.

Most freshwater and marine bony fishes are stenohaline. A large change in salinity may disturb their osmotic balance.

Euryhaline Organisms

Euryhaline organisms can tolerate a wide range of salinity.

Their gills, kidneys and ion transport systems change according to the surrounding water. Salmon and molly are common examples.

Hormonal Control of Osmoregulation

The hormonal control of osmoregulation is mainly carried out by antidiuretic hormone, renin-angiotensin system, aldosterone and atrial natriuretic peptide.

These hormones regulate water reabsorption, sodium balance, blood volume and blood pressure.

Role of ADH

Antidiuretic hormone or ADH is also called vasopressin. It is released from the posterior pituitary gland.

The release of ADH is increased when blood osmolality becomes high or blood volume becomes low. Osmoreceptors present in the hypothalamus detect this change.

ADH acts on the collecting ducts of kidneys. It causes insertion of aquaporin-2 water channels into the membrane of collecting duct cells.

More water is reabsorbed into blood. The amount of urine becomes low and concentrated urine is formed.

Role of Renin-Angiotensin System

The renin-angiotensin system is activated during low blood pressure, low blood volume or reduced sodium delivery to the kidneys.

Renin is released from the juxtaglomerular cells of kidneys. It converts angiotensinogen into angiotensin I.

Angiotensin-converting enzyme converts angiotensin I into angiotensin II.

Angiotensin II causes constriction of blood vessels. It also stimulates thirst, ADH release and aldosterone secretion.

Role of Aldosterone

Aldosterone is a steroid hormone released from the adrenal cortex.

It acts mainly on the distal tubules and collecting ducts of kidneys. The activity of epithelial sodium channels and sodium-potassium pumps is increased.

Sodium is reabsorbed into blood. Water follows sodium by osmosis.

Potassium and hydrogen ions are removed through urine. Blood volume and blood pressure are increased.

Role of ANP

Atrial natriuretic peptide or ANP is released when the atria of heart become stretched due to high blood volume.

ANP inhibits renin and aldosterone secretion. It also decreases sodium reabsorption in the kidneys.

More sodium and water are removed through urine. Blood volume and blood pressure are decreased.

Osmoregulation Process

Osmoregulation is the process of maintaining water and electrolyte concentration of the body fluids. It takes place when the amount of water, salts or blood volume is changed. The process takes place in different steps, which are as follows-

Step 1- Detection of water and solute changes

During dehydration, sweating or bleeding, blood volume is decreased. The concentration of sodium and other solutes is now increased.

Osmoreceptors of hypothalamus detect the increased blood osmolality. Water moves out from these cells by osmosis and the cells become shrink. Signals are produced in this step.

At the same time, baroreceptors of aortic arch, carotid sinus and left atrium detect the low blood pressure and blood volume. Juxtaglomerular cells of kidneys detect low blood flow or decreased sodium delivery.

Step 2- Release and action of ADH

The signals from osmoreceptors and baroreceptors stimulate the posterior pituitary gland. Antidiuretic hormone (ADH) or vasopressin is released into blood.

ADH reaches the kidneys and combines with V2 receptors of collecting duct cells. The cAMP and protein kinase A (PKA) pathway is now activated.

Aquaporin-2 (AQP2) water channels are inserted into the apical membrane. Water moves from tubular fluid into blood.

Here, more water is reabsorbed. The amount of urine is decreased and concentrated urine is formed. Blood osmolality is also decreased.

Step 3- Activation of renin-angiotensin system

Low blood pressure and low sodium delivery stimulate the juxtaglomerular cells. Renin is now released into blood.

Renin converts angiotensinogen into angiotensin I.

Enzyme involved- Renin.

Angiotensin I is converted into angiotensin II.

Enzyme involved- Angiotensin-converting enzyme (ACE).

Angiotensin II causes constriction of blood vessels. Blood pressure is increased in this process.

Step 4- Release and action of aldosterone

Angiotensin II stimulates the zona glomerulosa of adrenal cortex. Aldosterone is released.

Aldosterone acts on the principal cells of distal tubules and collecting ducts. The epithelial sodium channels (ENaC) and sodium-potassium pumps are increased.

Sodium is reabsorbed from tubular fluid into blood. Potassium is removed into urine. Water follows sodium by osmosis.

The extracellular fluid volume is now increased. Blood volume and blood pressure are also increased.

Step 5- Thirst and salt intake

High blood osmolality and angiotensin II stimulate the thirst centre present in hypothalamus.

The feeling of thirst is produced and water is taken into the body. Salt appetite also causes intake of sodium.

During this process, the lost water and electrolytes are replaced. The high solute concentration of blood is decreased.

Step 6- Negative feedback

Water is reabsorbed by kidneys and also obtained through drinking. Blood volume is increased and plasma osmolality returns to about 280-300 mOsm/kg.

Osmoreceptor cells return to their normal size. Baroreceptors now detect the normal blood pressure.

The release of ADH and renin is decreased. Aldosterone secretion is also reduced.

During excess blood volume, the atria of heart become stretched. Atrial natriuretic peptide (ANP) is released.

ANP inhibits renin and aldosterone. It causes dilation of blood vessels and increases removal of sodium and water through urine.

The normal water and electrolyte condition is now restored. This is referred to as negative feedback regulation.

schematic diagram of Mechanism of Osmoregulation in Humans

Osmoregulation in Animals

Osmoregulation in animals is the process of maintaining the osmotic pressure of body fluids. It regulates water and dissolved salts such as sodium, potassium and chloride. This prevents excess swelling and shrinking of animal cells.

The process of osmoregulation in different animals is as follows-

1. Freshwater animals

Freshwater animals live in a hypotonic medium. Their body fluids contain more salts than the surrounding water.

Water continuously enters into the body through gills and body surface. At the same time, salts move out from the body.

These animals drink very little water. A large amount of dilute urine is removed.

Required salts are actively absorbed through the gills. Thus, excess water is removed and salts are retained.

2. Marine bony fishes

Marine bony fishes live in a hypertonic medium. The seawater contains more salts than their body fluids.

Water continuously moves out from the body. Salts enter through gills and also by drinking seawater.

The fishes drink large amount of seawater for replacing the lost water. Excess salts are removed through the gills.

A small amount of urine is formed, which helps in conservation of water.

3. Cartilaginous fishes

Sharks, skates and rays are cartilaginous fishes. They maintain high concentration of urea and trimethylamine oxide (TMAO) in blood.

These compounds increase the osmotic concentration of body fluids. The internal fluid becomes nearly isotonic with seawater.

Water loss from the body is reduced by this process. Excess salts are removed by specialised rectal glands.

4. Terrestrial animals

Terrestrial animals continuously lose water through respiration, evaporation, urine and faeces. Thus, prevention of dehydration is the major problem.

The feeling of thirst causes drinking of water. Water and sodium are also reabsorbed by kidneys.

Hormones such as antidiuretic hormone (ADH) and aldosterone are involved in this regulation. ADH increases water reabsorption, while aldosterone increases sodium reabsorption.

Concentrated urine is formed and loss of water is decreased.

Some water is also formed during oxidation of carbohydrates and fats. This is referred to as metabolic water.

5. Marine birds and reptiles

Marine birds and reptiles obtain large amount of salts through food and seawater. Their kidneys cannot remove all salts without excessive loss of water.

Specialised salt glands are present near the eyes or nasal region. These glands remove highly concentrated salt solution.

The salt solution may contain four to five times more salts than the body fluids. Thus, excess salts are removed with very less water loss.

Osmoregulation in Fish

Fish live in water having different concentration of salts. The water and salts continuously move through the gills and body surface. Thus, different fishes show different method for maintaining the osmotic condition of body fluids.

About 90% of bony fishes are stenohaline, they can tolerate only a narrow range of salinity. Some fishes such as salmon and molly are euryhaline and can survive in both freshwater and seawater.

The osmoregulation in different fishes are as follows-

1. Freshwater Bony Fish

Freshwater is hypotonic as compared to the body fluids of fish. The body fluid contains more salts than the surrounding water.

Water continuously enters into the body through gills and body surface. At the same time, salts are lost from the body.

Freshwater fishes drink very little water. Large amount of highly dilute urine is produced.

The required salts are taken from surrounding water by active transport through the gills. During this process, excess water is removed and the lost ions are replaced.

2. Marine Bony Fish

Seawater is hypertonic as compared to the body fluids of marine bony fish. The body fluid contains less salts than the surrounding seawater.

Water continuously moves out from the body. Excess salts enter through gills and also by drinking seawater.

Marine bony fishes drink seawater continuously for replacing the lost water. Excess Na⁺ and Cl⁻ are removed through specialised cells present in the gills.

A small amount of urine is formed. This helps in reducing the loss of water from the body.

3. Cartilaginous Fish

Sharks, skates and rays are included under cartilaginous fishes. Their osmoregulatory process is different from bony fishes.

The salt concentration of blood is lower than seawater. However, high concentration of urea and trimethylamine oxide (TMAO) are stored in blood.

Urea increases the osmotic concentration of body fluids and makes it almost isotonic with seawater. High amount of urea may damage the proteins and enzymes.

TMAO protects the proteins from harmful effect of urea. Thus, water loss from the body is prevented.

These fishes are also referred to as ureotelic animals. Excess salts are removed through a specialised rectal gland.

4. Euryhaline Fish

Euryhaline fishes can tolerate a wide variation of environmental salinity. Their gills and kidneys change their activity according to surrounding water.

In freshwater, they produce dilute urine and absorb salts through gills. In seawater, they drink water and remove excess salts.

Salmon moves from seawater to freshwater during breeding. Its osmoregulatory mechanism is changed during this movement.

Osmoregulation in Humans

Osmoregulation in humans maintains the water and electrolyte concentration of blood plasma, extracellular fluid and intracellular fluid.

The kidneys are the major osmoregulatory organs. They filter the blood and adjust the reabsorption or removal of water and electrolytes.

The hypothalamus detects changes in blood osmolality. The posterior pituitary releases ADH, while the adrenal cortex releases aldosterone.

Thirst increases water intake. The kidneys change the volume and concentration of urine according to the amount of water present in the body.

During dehydration, more water is reabsorbed and concentrated urine is formed. During excess water intake, less water is reabsorbed and dilute urine is formed.

Age, body composition, environmental temperature, physical activity, diet and health condition can change the total water requirement of the body.

Osmoregulation in Plants

Osmoregulation in plants maintains the balance of water and dissolved solutes inside the cells and tissues.

Water is mainly absorbed from the soil through root hairs. It moves from the roots toward the stem and leaves through xylem vessels.

The movement of water depends on water potential. Water generally moves from a region having higher water potential toward a region having lower water potential.

Vacuoles store water and dissolved solutes. They help in maintaining turgor pressure inside the plant cells.

Stomata control the loss of water through transpiration. During water deficiency, the stomata become closed and water loss is reduced.

Abscisic acid takes part in stomatal closure during drought condition.

Plants also accumulate soluble compounds such as sugars, amino acids and proline. These substances are called osmolytes.

Osmolytes decrease the water potential of cells and help in retaining water. This process is referred to as osmotic adjustment.

Halophytes show special adaptations for living in saline soil. Xerophytes show structural and physiological adaptations for reducing water loss under dry conditions.

schematic diagram of Osmoregulation in Plants

Osmoregulation in Amoeba and Paramecium

Amoeba and Paramecium commonly live in freshwater. The surrounding water is hypotonic compared to their cytoplasm.

Water continuously enters into the cell through the plasma membrane by osmosis.

The excess water is collected by contractile vacuoles. Small collecting canals or vesicles carry water toward the vacuole.

The contractile vacuole gradually becomes filled with water. It then contracts and releases the water outside the cell.

This process takes place repeatedly. It prevents excessive swelling and bursting of the cell.

The contractile vacuole is therefore the major osmoregulatory organelle in freshwater protozoans.

schematic diagram of Osmoregulation in Amoeba and Paramecium

Functions of Osmoregulation (importance of osmoregulation)

The main functions of osmoregulation include:

Maintaining the amount of water and dissolved solutes in the body fluids, including electrolytes and non-electrolytes.
It controls the movement of water and salts across the semipermeable membranes, hence maintaining the fluid balance of the body.
It also protects the cells from taking up excessive water and swelling. During excess loss of water, it prevents shrinking of the cells.
The regulation of water present in blood plasma has a role to play in maintaining the blood volume and blood pressure.
It is responsible for maintaining important ions such as Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cl⁻ in intracellular fluid, extracellular fluid and blood plasma.
It also provides a process for removal of excess water, electrolytes and harmful metabolic wastes through the kidneys, forming urine.
The body continuously receives water through food and drinking and loses water through urine, sweating and respiration, hence the internal fluid condition is maintained.
Osmoregulation also allows organisms to survive under different water and salt conditions of the environment. For example, fishes moving between freshwater and saltwater conditions.

Examples of Osmoregulation

Organism	Osmotic problem	Osmoregulatory mechanism
Freshwater fish	Water continuously enters and salts move out	Large amount of dilute urine is formed and salts are actively absorbed through gills
Marine bony fish	Water moves out and excess salts enter	Seawater is consumed, salts are removed through gills and a small amount of urine is formed
Shark	Water may move according to seawater concentration	Urea and TMAO maintain total body-fluid osmolarity close to seawater
Salmon	Environmental salinity changes during migration	Gill and kidney functions change between freshwater and seawater
Marine bird	Excess salts are obtained through food and seawater	Salt glands remove concentrated salt solution
Human	Water and electrolyte intake continuously change	Kidneys, ADH, aldosterone, RAAS and thirst maintain water and salt balance
Amoeba	Excess water enters from freshwater	Contractile vacuole removes the excess water
Plant	Water loss occurs through transpiration	Roots, vacuoles, stomata and osmotic adjustment maintain water balance

Osmoregulation vs Osmosis

Osmosis and osmoregulation are related processes, but they are not the same.

Basis	Osmoregulation	Osmosis
Definition	Regulation of water and dissolved solutes inside an organism	Movement of water across a selectively permeable membrane
Nature	Complete regulatory process	Passive physical process
Energy requirement	May require metabolic energy	Does not directly require metabolic energy
Components involved	Kidneys, gills, hormones, ion pumps and vacuoles	Water molecules and selectively permeable membrane
Main function	Maintains internal osmotic balance	Equalises water movement according to solute concentration
Level	Takes place at cellular and organismal level	Takes place across a membrane

Osmosis causes the movement of water. Osmoregulation controls this movement and adjusts the amount of water and salts inside the body.

Osmoregulation vs Excretion

Osmoregulation and excretion are closely connected. Both processes may involve the kidneys, gills and other excretory structures.

Basis	Osmoregulation	Excretion
Definition	Maintenance of water and solute balance	Removal of metabolic wastes from the body
Main purpose	Maintains osmotic pressure and fluid balance	Prevents accumulation of harmful waste products
Substances involved	Water, sodium, potassium, chloride and other solutes	Urea, uric acid, ammonia, carbon dioxide and excess substances
Major organs	Kidneys, gills, salt glands and contractile vacuoles	Kidneys, lungs, skin, gills and excretory tubules
Relation with urine	Controls urine volume and concentration	Removes nitrogenous wastes through urine
Role in homeostasis	Maintains fluid and electrolyte balance	Maintains chemical purity of body fluids

The kidneys take part in both processes. They regulate water and electrolytes during osmoregulation and remove nitrogenous wastes during excretion.

Osmoregulators vs Osmoconformers

Basis	Osmoregulators	Osmoconformers
Internal osmotic pressure	Internal osmotic pressure is maintained constant. It remains different from the surrounding environment.	Internal osmotic pressure changes according to the surrounding environment. It becomes nearly similar to external medium.
Regulation	Water and salts are actively regulated inside the body.	Body fluid concentration is allowed to change with environmental concentration.
Energy requirement	Continuous metabolic energy is required. Energy is used against the normal movement of water by osmosis.	Less metabolic energy is required for osmotic regulation. They do not continuously work against external osmotic pressure.
Effect of environmental change	Internal body condition remains nearly stable during change in external salinity.	Internal body condition changes with the external salinity. Large change may affect their normal activity.
Other name	They are also referred to as euryosmotic organisms.	They are also referred to as poikilosmotic organisms.
Salinity tolerance	Many osmoregulators can regulate under changing salinity conditions.	Most osmoconformers have limited tolerance to salinity change and are generally stenohaline.
Occurrence	It is the common method present in animals.	It is mainly present in marine animals.
Examples	Humans, freshwater fishes and marine bony fishes.	Most marine invertebrates, sharks, skates and rays.
Special condition in cartilaginous fishes	Generally not present.	Sharks and rays store high amount of urea and trimethylamine oxide (TMAO) in blood. This increases internal osmotic pressure and makes it nearly isotonic with seawater.