Osmoregulation in a cell definition and examples in biology

Osmoregulation in cells is critical to the biology of every living thing. Its meaning in biology and some examples like osmoregulation in fish and humans will be discussed in this article.

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Osmoregulation definition in biology

Osmoregulation is defined as the process by which the human body maintains a proper electrolyte balance in the face of external factors such as temperature, diet, and weather.

What is osmoregulation?

Osmoregulation is the active regulation of the osmotic pressure of the body fluid of an organism.

This is achieved after the changes in osmotic pressure have been detected by osmoreceptors. The essence of osmoregulation is to maintain the homeostasis of the fluid content (water) of an organism. That is, it maintains fluid balance and electrolyte concentration (salts in solution, which in this case is represented by body fluid) to keep the body fluids from becoming too diluted or concentrated. This means homeostasis and osmoregulation occur together in organisms.

Although osmotic balance may vary hourly and daily, an animal is generally in an osmotic steady state over time.

Organisms in both aquatic and terrestrial environments must maintain the proper concentration of solutes and amount of water in their body fluids, which requires excretion (the removal of metabolic nitrogen wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood) via organs such as the skin and the kidneys.

Thermoregulation, osmoregulation, and excretion are all mechanisms that help moderate changes in the body.

Osmoregulation process

The process of osmoregulation deals with both osmosis (the movement of solvent molecules through a semipermeable membrane into an area with a higher solute concentration) and osmotic pressure (the external pressure required to prevent the solvent from crossing the membrane). This means that the concentration of solute particles affects osmotic pressure. The reason behind such a statement is that larger molecules (proteins and polysaccharides) and nonpolar or hydrophobic molecules (dissolved gases, lipids) do not cross a semipermeable membrane.

In the process of osmoregulation, water cannot be transported via active transport because there are no carrier proteins capable of binding and transporting it across cell membranes. But water can pass directly through membranes in response to ion concentration changes. Its movement is thus controlled indirectly by pumping ions such as sodium and potassium across cell membranes, resulting in a concentration gradient that causes water to follow via osmosis. For instance, water through osmosis follows sodium when it is being excreted from the body.

Osmotic regulation is typically accomplished by excretory organs, which also serve to dispose of metabolic wastes. As a result, urination serves as a mechanism for both waste excretion and osmoregulation. Contractile vacuoles, nephridia, antennal glands, malpighian tubules of invertebrates, salt glands, and kidneys of vertebrates, are examples of organelles and organs that regulate osmoregulation.

Types of osmoregulation

  1. Osmoregulators
  2. Osmoconformers

These are the two types of osmoregulatory mechanisms used by living things to maintain the balance between water and electrolytes.


Osmoregulators regulate internal osmotic pressure in order to keep conditions within a narrow range. Regardless of the salt concentrations in the environment, osmoregulators actively control salt concentrations.

Many animals, including vertebrates, are examples of osmoregulators. For instance, freshwater fish use mitochondria-rich cells in the gills to regulate internal osmotic pressure. By so doing, the gills actively take up salt from the environment thereby causing water to diffuse into the fish. This diffusion causes the freshwater fish to excrete very hypotonic urine in order to expel all of the excess water.


Osmoconformers use active or passive processes to complement their internal osmolarity to that of the environment. This is common in marine invertebrates, which have the same internal osmotic pressure inside their cells as the surrounding water. Despite the fact that the chemical composition of the solutes may differ.

Furthermore, there is definitely a difference between osmoconformers and osmoregulators. Osmoregulators refer to the animals that regulate internal osmotic pressure in order to keep conditions within a narrow range despite changes in their external environment, whereas osmoconformers refer to the animals whose body fluids are in osmotic balance with their environment.

Osmoregulation examples

  1. Osmoregulation in plants
  2. Osmoregulation in animals

The different examples of osmoregulation will be discussed under plants and animals.

Osmoregulation in plants

While there are no specific osmoregulatory organs in higher plants, there are some membrane-bound organelles and hormones that help in osmoregulation. These organelles and hormones are stomata, vacuole, and abscisic acid.

Stomata are important in regulating water loss through evapotranspiration, while vacuole is also important in regulating the concentration of solutes in the cytoplasm on a cellular level. Abscisic acid, on the other hand, is a key hormone in helping plants conserve water by closing stomata and stimulating root growth, allowing more water to be absorbed.

Plants and animals both struggle to obtain water, but unlike animals, water loss in plants is critical in creating a driving force to move nutrients from the soil to tissues. Certain plants have developed water uptake and saving strategies like xerophytes, hydrophytes, halophytes, and mesophytes which will be discussed below.


Xerophytes are plants that can survive in dry environments such as deserts and can withstand long periods of water scarcity.

Cacti, for example, store water in the vacuoles of large parenchyma tissues. Other plants, like the pine, have leaf modifications to reduce water loss, such as needle-shaped leaves, sunken stomata, and thick, waxy cuticles. Another leaf modification example is the rolled leaves of the sand-dune marram grass that have stomata on the inner surface.


Hydrophytes are plants that thrive in aquatic environments. They can be floating, submerged, or emergent, and they can grow in seasonal (rather than permanent) wetlands. Water absorption in these plants can occur through the entire surface of the plant, as in the case of water lily, or only through the roots, as in sedges. These plants do not face significant osmoregulatory challenges due to water scarcity.


Halophytes are known as plants that live in soils with high salt concentrations, such as salt marshes or alkaline soils in desert basins. They must absorb water from soil with a higher salt concentration and, as a result, develop a lower water potential (higher osmotic pressure). To deal with this situation, halophytes activate salts in their roots. As a result, the cells of the roots develop a lower water potential, allowing water to enter via osmosis.

Excess salt can be stored in cells or excreted through salt glands found on the leaves. Some species use the salt they secrete to trap water vapors in the air, which are then absorbed in liquid form by leaf cells. For plants such as glasswort and cordgrass, this is another method of obtaining additional water from the air.


Mesophytes are plants that grow in well-watered soil and live in temperate zones. They can easily make up for the water lost through transpiration by absorbing water from the soil. To prevent excessive transpiration, they created a waterproof external covering known as the cuticle.

Osmoregulation in animals

Animals use an excretory system to regulate the amount of water they lose to the environment and to keep osmotic pressure stable. The organs responsible for osmoregulation differ depending on the species.

Single-celled organisms

Osmoregulation occurs in single-celled organisms like paramecium and bacteria.

Paramecium osmoregulation

Because paramecium lives in freshwater, it faces the issue of water being transported into it via osmosis. Contractile vacuoles are present in this organism to prevent too much water from entering into it and thereby exploding the cell. The contractile vacuoles aid in the regulation of osmosis and thus aid in the process of osmoregulation.

The primary function of the contractile vacuoles found in the body of the paramecium is osmoregulation. The two contractile vacuoles, located at the anterior and posterior ends of the paramecium body, collect and expel excess water from the body. Water enters the cytoplasm of the cell via the semipermeable pellicle. This small amount of water that enters happens when the paramecium wants to take in food particles.

At regular intervals, the contractile vacuoles contract and expand accumulating excess water in the cytoplasm. The excess water from the cytoplasm is collected in endoplasmic reticulum tubules where it flows to the nephridial tubules. The nephridial tubules are branching internal tubules that function in osmoregulation. They send excess water to the collecting tubules of the radiating canals of the contractile vacuoles.

The water is then collected in the ampullas of the radiating canals and is discharged into the contractile vacuole when the ampulla fills up. The water collected in the vacuoles causes them to expand. When the pellicle reaches its maximum size, it contracts and expels water to the outside through the discharge canal, which opens to the outside via a pore in the pellicle.

Bacteria osmoregulation

Whenever the osmolarity of the environment increases, bacteria may use transport mechanisms to absorb electrolytes or small organic molecules. Osmotic stress causes genes in certain bacteria to be activated, resulting in the synthesis of osmoprotectant molecules.

Multicellular organisms

The examples of osmoregulation in multicellular organisms that will be discussed are fish and humans.

Osmoregulation in fishes
Osmoregulation diagram in freshwater and seawater fish
Osmoregulation diagram in fish

The above diagram is shown in order to compare and contrast osmoregulation in marine and freshwater fish.

Osmoregulation in freshwater fish and marine fish happens in different ways. The process of osmoregulation differs because the environments in which they live have varying levels of salinity.

Osmoregulation in freshwater fishes

In terms of osmoregulation, freshwater fishes are hypertonic to their surroundings, which means that the concentration of salt in their blood is higher than in the surrounding water. They absorb a specific amount of water via the mouth and gill membranes.

Because they drink so much water, they produce a lot of urine, which causes a lot of salt to be lost. The salt is replaced by mitochondria-rich cells in the gills. These cells absorb salt from the surrounding water and transport it into the bloodstream.

Osmoregulation in marine fish

When compared to freshwater fish, marine fish face the inverse issue. They have a higher concentration of water in their blood than the rest of the population. As a result, there is a tendency to lose water and absorb the salt.

To avoid this problem, marine fish drink a lot of water and limit their urination. Another source of osmoregulation in the marine environment is the expelling of salt from their bodies (through the gills).

Osmoregulation in humans
This is an osmoregulation in humans diagram.
An osmoregulation negative feedback diagram

Osmoregulation in humans is done primarily by the kidney. However, the skin also has pores that help humans lose water. In order to explain the role of the kidney in osmoregulation, there’s a need for an understanding that the kidney can reabsorb some substances like water, glucose, and amino acids or pass them out as waste to be excreted.

This way, the kidneys maintain blood electrolyte balance while also regulating blood pressure. The absorption of these substances is regulated through the secretion of three hormones namely, aldosterone, angiotensin II, and antidiuretic hormone (ADH).

The regulation of absorption happens when the osmoreceptors in the hypothalamus monitor the changes in water potential. For example, to explain osmoregulation in the kidney using the case of controlling thirst, ADH is secreted from the pituitary gland which stores it. When it is released, it targets the endothelial cells (these endothelial cells are distinct because they contain aquaporins) in the nephrons of the kidney. This means there is an essential role of the nephron in osmoregulation.

The reason for targeting these cells is because aquaporins allow water to pass directly through the cell membrane rather than through the lipid bilayer. The ADH hormone then allows water to flow through the aquaporins’ water channels.

Until the pituitary gland stops releasing ADH, the kidneys continue to absorb water and return it to the bloodstream. By so doing, it helps to quench or reduce thirst and this describes the role of ADH in human osmoregulation.

Mechanism of osmoregulation in humans

The mechanism of osmoregulation in humans includes:

  • Multiple bodies to brain signaling mechanisms
  • A neural network in the brain
  • Autonomic and endocrine reflexes
  • Mechanisms of behavior

Tonicity and osmoregulation

Tonicity is the ability of an extracellular solution to cause water to move into or out of a cell via osmosis. It is determined by the relative concentrations of solutes and the permeability of the cell membrane to those solutes.

It can be defined by three simple terms: hypertonic, hypotonic, and isotonic. These terms specify whether a solution causes water to move into or out of a cell.

Hypertonic solution

A solution is described as a hypertonic solution to a cell if its solute concentration is greater than the concentration inside the cell. Resulting in the solute being unable to cross the membrane and thus, remaining on the outside of the cell. Also, the net flow of water is also outwards, towards areas with more solute, causing the cell to lose volume and shrink.

Hypotonic solution

A solution is described as hypotonic to the cell if the solute concentration outside the cell is lower than the solute concentration inside the cell. This results in the inflow of water inside the cell where there is more solute, causing the cell to gain volume and expand.

Isotonic solution

A solution is said to be isotonic if the concentration of solutes outside and inside the cell is the same. As a result, there is no water flow inside or outside of the cell, keeping the cell volume constant.

The relationship between tonicity and osmoregulation is one that deals with the state of the solution surrounding a cell. The state will determine what type of strategy the cell will use to maintain the right amount of water content.

Excretion and osmoregulation

To support life functions, organisms must keep bodily fluids at a constant temperature and pH while maintaining specific solute concentrations. Material that cannot be used by the animal is excreted from the body.

For example, nitrogen is one of the most important types of waste in the body that must be excreted either directly or through conversion to urea or uric acid. This is because excess nitrogen produces toxic ammonia, which is not good for the body and must be disposed of.

Osmoregulation and excretion are frequently mediated by transport epithelia. These specialized cells transport solutes and can be found in excretory organs ranging from insect malpighian tubules to fish gills and vertebrate kidneys.

There is one main point to differentiate between excretion and osmoregulation. The point has to do with their definition. It states that excretion is the removal of metabolic toxic waste while osmoregulation is the balancing of osmotic pressure to maintain water in the body.

FAQ on osmoregulation

Why are the renal artery and vein critical to the process of osmoregulation in vertebrates?

The renal artery takes blood with nitrogenous waste to the kidney and the renal vein brings blood with less nitrogenous waste away from the kidneys.

Why is osmoregulation important?

Osmoregulation is important because it protects cells by stopping too much water from entering or leaving them by osmosis.

What is the meaning of osmoregulation?

The meaning of osmoregulation is the maintenance of constant osmotic pressure in the fluids of an organism by the control of water and salt concentrations.

What role do chloride cells play in osmoregulation of marine fish with bony skeletons?

They are involved in the excretion of excess salt.

What adaptations for osmoregulation are found in single-celled organisms?

They have intracellular tonicity that is nearly isotonic to water and maintained via a contractile vacuole.

What role do transport epithelia play in osmoregulation of marine fish with bony skeletons?

They are also involved in the excretion of excess salt like the chloride cells.

Define osmoregulation and excretion?

Osmoregulation is the maintenance of the right amount of solutes and water in the bodily fluids of an organism. A way for an organism to achieve such is through the excretion of unwanted solutes and water.

What does osmoregulation mean?

It means the way cells maintain water and electrolyte balance in the body.

What does the kidney do in osmoregulation?

The kidney regulates the osmotic pressure of the blood through purification and extensive filtration.

What is osmoregulation in biology?

In biology, osmoregulation is the maintenance of the internal water and dissolved solute content of the body irrespective of the external environmental conditions.

What are osmoregulation characteristics?

Osmoregulation characteristics are comprised of;
(1) Multiple body-to-brain signaling mechanisms that report the status of total body fluids and fluid distribution in the body.
(2) A neural network in the brain (the visceral neuraxis) that receives and integrates body fluid-related input.
(3) The visceral neuraxis controls and activates reflex (autonomic and endocrine) and behavioral (thirst- and sodium appetite-related behaviors) mechanisms.

How does osmoregulation in humans occur?

Osmoregulation is manifested in the body in two ways or cases: dehydration and waterlogging. In the event of dehydration, the hypothalamus signals the pituitary gland to secrete ADH (antidiuretic hormone).

In order to describe the role of ADH in human osmoregulation, it can be observed that the hormone increases water reabsorption in the renal tubules of the kidney, as well as blood volume and body fluid concentration.

There is a high level of fluids in the body when there is waterlogging. The hypothalamus signals the pituitary gland to stop ADH secretion, resulting in no water reabsorption in the kidney tubules.

How does AQP2 play a role in osmoregulation?

The AQP2 gene provides instructions for making a protein called aquaporin 2. The protein creates channels that carry water molecules across cell membranes.

How does osmoregulation work?

The process of osmoregulation deals with both osmosis (the movement of solvent molecules through a semipermeable membrane into an area with a higher solute concentration) and osmotic pressure (the external pressure required to prevent the solvent from crossing the membrane).

In the process of osmoregulation, water cannot be transported via active transport because there are no carrier proteins capable of binding and transporting it across cell membranes. But water can pass directly through membranes in response to ion concentration changes. Its movement is thus controlled indirectly by pumping ions such as sodium and potassium across cell membranes, resulting in a concentration gradient that causes water to follow via osmosis.

How does the paramecium maintain osmoregulation?

Paramecium transports ammonia and other excretory wastes from the cytoplasm to the cell membrane using the contractile vacuole, especially where the vacuole opens to the environment. Water is forced into the cytoplasm by osmotic pressure, while diffusion and active transport regulate the flow of water and electrolytes.

How does excretion and osmoregulation in reptiles occur?

Excretion and osmoregulation occur by modifying the ureteral urine, and by resorbing salt and water from feces in the lower intestine of reptiles.

What is squid osmoregulation?

This is the balancing of water and solute concentration by squids. This is done through having a plasma and cytoplasm that is isotonic with seawater.

What is shark rectal gland osmoregulation?

This is the use of an additional gland known as the rectal gland by sharks for salt regulation in the marine environment.

What is the function of endocrine hormones in osmoregulation?

The endocrine gland produces hormones like angiotensin II which affects multiple processes and increases blood pressure, aldosterone which prevents loss of sodium and water, and anti-diuretic hormone which prevents water loss. All these hormones function in osmoregulation.

What is the function of contractile vacuole in osmoregulation?

The contractile vacuole functions by controlling the intracellular water balance by accumulating and expelling excess water out of the cell.

How does earthworm osmoregulation occur?

Osmoregulation occurs in earthworms through the process of diffusion between the moist part of the skin and the environment.

What are some osmoregulation organs?

The kidney in humans, the contractile vacuole in unicellular organisms, and the skin in earthworms are some organs used in osmoregulation. The process of osmoregulation in kidneys is very vital for the functioning of the human body.

How does osmoregulation adaptations occur in organisms?

Osmoregulation occurs in organisms in various ways. For example, marine fishes have developed the habit of taking in more water and urinating less because of the excess salt content in their environment.

What is osmoregulation feedback loop?

Osmoregulation is an example of a negative feedback loop in homeostasis. Because it signals the need for more or less water in the body of an organism.

How does cnidaria osmoregulation occur?

osmoregulation in cnidaria happens through some specific and specialized cells. These cells are in charge of regulating the intake and release of water into the surrounding environment.

How does osmoregulation maintain homeostasis?

It helps maintain homeostasis when a signal is sent by osmoreceptors to regulate the osmotic pressure (water content) of an organism.

What is the relationship between osmoregulation and homeostasis?

Both osmoregulation and homeostasis work towards achieving balance in the body system of an organism.

How does osmoregulation in cells occur?

Osmoregulation in cells occurs via the diffusion of water or solutes.

Describe how saltwater fish deal with osmoregulation?

Saltwater fish deal with osmoregulation by expelling salt via their gills.

Where does osmoregulation occur in humans?

It occurs in the kidney. The function of the kidneys in osmoregulation is to filter blood and maintain the osmolarity of body fluids.

What adaptations for osmoregulation are found in single celled organisms?

They have intracellular tonicity that is nearly isotonic to water and maintained via a contractile vacuole.

What is the relationship between osmosis and osmoregulation?

Osmosis helps to increase or reduce osmotic pressure which in turn affects the rate at which osmoregulation takes place.

How does crayfish osmoregulation happen?

Terrestrial crustaceans like crayfish have reduced permeability, tolerate greater water loss, and osmoregulate behaviorally through the use of burrows.

How does osmoregulation in frogs occur?

Osmoregulation in frogs happen through their skin. They use their skin to regulate water and solute content via osmosis.

How does osmoregulation in aquatic invertebrates happen?

Osmoregulation in aquatic invertebrates occurs in their permeable bodies. It is controlled by the cerebral organs. Also, the mechanism of osmoregulation in invertebrates that are aquatic involves enzymatic systems that are connected with blood vessels and other various organs.

Where does osmoregulation occur in the kidney?

Osmoregulation in the kidney occurs in the medulla of the kidney. The kidney and osmoregulation go hand in hand in the human body.

What is the role of ADH in osmoregulation?

The primary role of Antidiuretic hormone (ADH) in osmoregulation is to regulate or control the amount of urine that is formed.

Why are the renal artery and vein critical to the process of osmoregulation in mammals?

The renal artery and vein are critical to the process of osmoregulation in mammals because the renal artery delivers blood with nitrogenous waste to the kidney. Another important point is that the renal vein brings blood with less nitrogenous wastes away from the kidneys.

Osmoregulation in humans diagram and explanation in a video