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3.2: Osmoregulation and Osmotic Balance - Biology

3.2: Osmoregulation and Osmotic Balance - Biology



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Skills to Develop

  • Define osmosis and explain its role within molecules
  • Explain why osmoregulation and osmotic balance are important body functions
  • Describe active transport mechanisms
  • Explain osmolarity and the way in which it is measured
  • Describe osmoregulators or osmoconformers and how these tools allow animals to adapt to different environments

Osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body’s fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte, in contrast, doesn’t dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (such as the pleural, serous, and cell membranes) are semi-permeable membranes. Semi-permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. As seen in Figure (PageIndex{1}), a cell placed in water tends to swell due to gain of water from the hypotonic or “low salt” environment. A cell placed in a solution with higher salt concentration, on the other hand, tends to make the membrane shrivel up due to loss of water into the hypertonic or “high salt” environment. Isotonic cells have an equal concentration of solutes inside and outside the cell; this equalizes the osmotic pressure on either side of the cell membrane which is a semi-permeable membrane.

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.

Need for Osmoregulation

Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, extracellular fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Because blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.

Transport of Electrolytes across Cell Membranes

Electrolytes, such as sodium chloride, ionize in water, meaning that they dissociate into their component ions. In water, sodium chloride (NaCl), dissociates into the sodium ion (Na+) and the chloride ion (Cl). The most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium (Na+), potassium (K+), calcium (Ca+2), magnesium (Mg+2), and the anions chloride (Cl-), carbonate (CO3-2), bicarbonate (HCO3-), and phosphate(PO3-). Electrolytes are lost from the body during urination and perspiration. For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.

Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into their component ions, they, in essence, add more solute particles into the solution and have a greater effect on osmotic pressure, per mass than compounds that do not dissociate in water, such as glucose.

Water can pass through membranes by passive diffusion. If electrolyte ions could passively diffuse across membranes, it would be impossible to maintain specific concentrations of ions in each fluid compartment therefore they require special mechanisms to cross the semi-permeable membranes in the body. This movement can be accomplished by facilitated diffusion and active transport. Facilitated diffusion requires protein-based channels for moving the solute. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.

Concept of Osmolality and Milliequivalent

In order to calculate osmotic pressure, it is necessary to understand how solute concentrations are measured. The unit for measuring solutes is the mole. One mole is defined as the gram molecular weight of the solute. For example, the molecular weight of sodium chloride is 58.44. Thus, one mole of sodium chloride weighs 58.44 grams. The molarity of a solution is the number of moles of solute per liter of solution. The molality of a solution is the number of moles of solute per kilogram of solvent. If the solvent is water, one kilogram of water is equal to one liter of water. While molarity and molality are used to express the concentration of solutions, electrolyte concentrations are usually expressed in terms of milliequivalents per liter (mEq/L): the mEq/L is equal to the ion concentration (in millimoles) multiplied by the number of electrical charges on the ion. The unit of milliequivalent takes into consideration the ions present in the solution (since electrolytes form ions in aqueous solutions) and the charge on the ions.

Thus, for ions that have a charge of one, one milliequivalent is equal to one millimole. For ions that have a charge of two (like calcium), one milliequivalent is equal to 0.5 millimoles. Another unit for the expression of electrolyte concentration is the milliosmole (mOsm), which is the number of milliequivalents of solute per kilogram of solvent. Body fluids are usually maintained within the range of 280 to 300 mOsm.

Osmoregulators and Osmoconformers

Persons lost at sea without any fresh water to drink are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90 percent of all bony fish are restricted to either freshwater or seawater. They are incapable of osmotic regulation in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life in fresh water and part in sea water. Organisms like the salmon and molly that can tolerate a relatively wide range of salinity are referred to as euryhaline organisms. This is possible because some fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in fresh water, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure (PageIndex{1})a. In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very dilute urine, and they achieve electrolyte balance by active transport of salts through the gills. When they move to a hypertonic marine environment, these fish start drinking sea water; they excrete the excess salts through their gills and their urine, as illustrated in Figure (PageIndex{1})b. Most marine invertebrates, on the other hand, may be isotonic with sea water (osmoconformers). Their body fluid concentrations conform to changes in seawater concentration. Cartilaginous fishes’ salt composition of the blood is similar to bony fishes; however, the blood of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). This does not mean that their electrolyte composition is similar to that of sea water. They achieve isotonicity with the sea by storing large concentrations of urea. These animals that secrete urea are called ureotelic animals. TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would occur in other animals exposed to similar levels of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in osmoregulation.

Career Connection: Dialysis Technician

Dialysis is a medical process of removing wastes and excess water from the blood by diffusion and ultrafiltration. When kidney function fails, dialysis must be done to artificially rid the body of wastes. This is a vital process to keep patients alive. In some cases, the patients undergo artificial dialysis until they are eligible for a kidney transplant. In others who are not candidates for kidney transplants, dialysis is a life-long necessity.

Dialysis technicians typically work in hospitals and clinics. While some roles in this field include equipment development and maintenance, most dialysis technicians work in direct patient care. Their on-the-job duties, which typically occur under the direct supervision of a registered nurse, focus on providing dialysis treatments. This can include reviewing patient history and current condition, assessing and responding to patient needs before and during treatment, and monitoring the dialysis process. Treatment may include taking and reporting a patient’s vital signs and preparing solutions and equipment to ensure accurate and sterile procedures.

Summary

Solute concentrations across a semi-permeable membranes influence the movement of water and solutes across the membrane. It is the number of solute molecules and not the molecular size that is important in osmosis. Osmoregulation and osmotic balance are important bodily functions, resulting in water and salt balance. Not all solutes can pass through a semi-permeable membrane. Osmosis is the movement of water across the membrane. Osmosis occurs to equalize the number of solute molecules across a semi-permeable membrane by the movement of water to the side of higher solute concentration. Facilitated diffusion utilizes protein channels to move solute molecules from areas of higher to lower concentration while active transport mechanisms are required to move solutes against concentration gradients. Osmolarity is measured in units of milliequivalents or milliosmoles, both of which take into consideration the number of solute particles and the charge on them. Fish that live in fresh water or saltwater adapt by being osmoregulators or osmoconformers.

Review Questions

When a dehydrated human patient needs to be given fluids intravenously, he or she is given:

  1. water, which is hypotonic with respect to body fluids
  2. saline at a concentration that is isotonic with respect to body fluids
  3. glucose because it is a non-electrolyte
  4. blood

B

The sodium ion is at the highest concentration in:

  1. intracellular fluid
  2. extracellular fluid
  3. blood plasma
  4. none of the above

B

Cells in a hypertonic solution tend to:

  1. shrink due to water loss
  2. swell due to water gain
  3. stay the same size due to water moving into and out of the cell at the same rate
  4. none of the above

A

Free Response

Why is excretion important in order to achieve osmotic balance?

Excretion allows an organism to rid itself of waste molecules that could be toxic if allowed to accumulate. It also allows the organism to keep the amount of water and dissolved solutes in balance.

Why do electrolyte ions move across membranes by active transport?

Electrolyte ions often require special mechanisms to cross the semi-permeable membranes in the body. Active transport is the movement against a concentration gradient.

Glossary

electrolyte
solute that breaks down into ions when dissolved in water
molality
number of moles of solute per kilogram of solvent
molarity
number of moles of solute per liter of solution
mole
gram equivalent of the molecular weight of a substance
non-electrolyte
solute that does not break down into ions when dissolved in water
osmoconformer
organism that changes its tonicity based on its environment
osmoregulation
mechanism by which water and solute concentrations are maintained at desired levels
osmoregulator
organism that maintains its tonicity irrespective of its environment
osmotic balance
balance of the amount of water and salt input and output to and from a biological system without disturbing the desired osmotic pressure and solute concentration in every compartment
osmotic pressure
pressure exerted on a membrane to equalize solute concentration on either side
semi-permeable membrane
membrane that allows only certain solutes to pass through

Connection for AP ® Courses

Much of the information in this chapter is not within the scope for AP ® . However, the chapter is filled with illustrative examples that are applicable to concepts we’ve explored previously, including chemistry, structure of the plasma cell membrane, and movement of molecules across membranes. With this in mind, it is helpful to have a general understanding how the human body, specifically the excretory system, maintains osmotic homeostasis despite the influence of external factors like temperature, diet, and varying environmental conditions. For example, if we drink eight to ten glasses of water per day, the human body excretes that water via urination, defecation, sweating, and, to a small extent, respiration.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.B Growth, reproduction and dynamic homeostasis require that cell create and maintain internal environments that are different form their external environment.
Essential Knowledge 2.B.1 Cell membranes are selectively permeable due to their structure.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.11 The student is able to construct models that connect the movement of molecules across membranes with membrane structure and function.
Essential Knowledge 2.B.2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.
Science Practice 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Learning Objective 2.12 The student is able to use representations and models to analyze situations or solve problems qualitatively and quantitatively to investigate whether dynamic homeostasis is maintained by the active movement of molecules across membranes.

Osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body’s fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte, in contrast, doesn’t dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body, such as the pleural, serous, and cell membranes are semi-permeable membranes. Semi-permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. As seen in Figure 32.2, a cell placed in water tends to swell due to gain of water from the hypotonic or low salt environment. A cell placed in a solution with higher salt concentration, on the other hand, tends to make the membrane shrivel up due to loss of water into the hypertonic or high salt environment. Isotonic cells have an equal concentration of solutes inside and outside the cell this equalizes the osmotic pressure on either side of the cell membrane which is a semi-permeable membrane.

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.


Need for Osmoregulation

Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, extracellular fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Because blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.


Biology - annotated exemplar Level 3 AS91604

This annotated exemplar is intended for teacher use only. The student work shown does not always represent a complete sample of what is required. Selected extracts are used, focused on the grade boundaries, in order to assist assessors to make judgements at the national standard.

Low Excellence

For Excellence, the student needs to demonstrate comprehensive understanding of how an animal maintains a stable internal environment.

This involves linking biological ideas, see Explanatory Note (EN) 4, about maintaining a stable internal environment in an animal, by at least one of:

  • a discussion of the significance of the control system (EN3) in terms of its adaptive advantage
  • an explanation of the biochemical and/or biophysical processes underpinning the mechanism
  • an analysis of a specific example of how external and/or internal environmental influences result in a breakdown of the control system.

This student has described and explained the purpose (1), components (2) and the mechanism (3) of osmoregulation in Chinook salmon.

Biological ideas are used to explain how a specific disruption results in responses to re-establish a stable internal environment (4).

Some biological ideas linking the significance of osmotic balance in terms adaptive advantage are discussed (5).

For a more secure Excellence, the student could:

  • discuss the significance of osmotic balance in terms of its adaptive advantage in more depth, or
  • analyse how external and/or internal influences result in a breakdown of osmoregulation in more detail.

High Merit

For Merit, the student needs to demonstrate in-depth understanding of how an animal maintains a stable internal environment.

This involves using biological ideas, see Explanatory Note (EN) 4, to explain how or why an animal maintains a stable internal environment. This includes explaining how a specific disruption results in responses within a control system (EN3) to re-establish a stable internal environment.

This student has described and explained the purpose (1), components (2) and the mechanism (3) of osmoregulation in Chinook salmon.

Biological ideas are used to explain how a specific disruption results in responses to re-establish a stable internal environment (4).

An attempt is made to link biological ideas on the significance of osmoregulation in terms of its adaptive advantage (5).

To reach Excellence, the student could link biological ideas further to:

  • discuss the significance of osmotic balance in terms of its adaptive advantage in more detail, or
  • analyse how external and/or internal influences result in a breakdown of osmoregulation, or
  • explain the biochemical and/or biophysical processes underpinning the mechanism of osmoregulation.

Low Merit

For Merit, the student needs to demonstrate in-depth understanding of how an animal maintains a stable internal environment.

This involves using biological ideas, see Explanatory Note (EN) 4, to explain how or why an animal maintains a stable internal environment. This includes explaining how a specific disruption results in responses within a control system (EN3) to re-establish a stable internal environment.

This student has described and explained the purpose (1), components (2) and the mechanism (3) of osmoregulation in Chinook salmon.

Some biological ideas are used to describe and explain how a specific disruption results in responses to re-establish a stable internal environment (4).

For a more secure Merit, the student could use biological ideas to give a more in-depth explanation of:

  • the mechanism of osmoregulation in Chinook salmon
  • how a specific disruption results in responses within a this control system to re-establish a stable internal environment.

High Achieved

For Achieved, the student needs to demonstrate understanding of how an animal maintains a stable internal environment.

This involves using biological ideas, see Explanatory Note (EN) 4, to describe a control system (EN3) by which an animal maintains a stable internal environment. Annotated diagrams or models may be used to support the description.

This student has described and partly explained the purpose (1), components (2) and the mechanism (3) of osmoregulation in Chinook salmon.

Some biological ideas are used to describe and partly explain how a specific disruption results in responses to re-establish osmotic balance (4).

To reach Merit, the student could use biological ideas to explain in more depth:

  • how or why an animal maintains a stable internal environment to regulate osmotic balance
  • how a specific disruption results in responses within this control system to re-establish a stable internal environment.

Low Achieved

For Achieved, the student needs to demonstrate understanding of how an animal maintains a stable internal environment.

This involves using biological ideas, see Explanatory Note (EN) 4, to describe a control system (EN3) by which an animal maintains a stable internal environment. Annotated diagrams or models may be used to support the description.

This student has described the purpose (1) and components (2) of osmoregulation in Chinook salmon.

Some biological ideas are used to describe the mechanism of (3), and disruption to this control system (4).

For a more secure Achieved, the student could use biological ideas to give a more detailed description of the:

  • feedback mechanism of osmoregulation in Chinook salmon.
  • potential effect of disruption to this control system by internal or external influences.

High Not Achieved

For Achieved, the student needs to demonstrate understanding of how an animal maintains a stable internal environment.

This involves using biological ideas, see Explanatory Note (EN) 4, to describe a control system (EN3) by which an animal maintains a stable internal environment. Annotated diagrams or models may be used to support the description.

This student has demonstrated limited understanding by describing the purpose (1) and components (2) of osmoregulation in Chinook salmon.

Some biological ideas are briefly considered to describe the mechanism of (3), and disruption to this system (4).

To reach Achieved, the student could use biological ideas to give a more thorough description of:


Persons lost at sea without any fresh water to drink are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90 percent of all bony fish are restricted to either freshwater or seawater. They are incapable of osmotic regulation in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life in fresh water and part in sea water. Organisms like the salmon and molly that can tolerate a relatively wide range of salinity are referred to as euryhaline organisms. This is possible because some fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in fresh water, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure 22.3 a . In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very dilute urine, and they achieve electrolyte balance by active transport of salts through the gills. When they move to a hypertonic marine environment, these fish start drinking sea water they excrete the excess salts through their gills and their urine, as illustrated in Figure 22.3 b . Most marine invertebrates, on the other hand, may be isotonic with sea water ( osmoconformers). Their body fluid concentrations conform to changes in seawater concentration. Cartilaginous fishes’ salt composition of the blood is similar to bony fishes however, the blood of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). This does not mean that their electrolyte composition is similar to that of sea water. They achieve isotonicity with the sea by storing large concentrations of urea. These animals that secrete urea are called ureotelic animals. TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would occur in other animals exposed to similar levels of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in osmoregulation.

Figure 22.3. Fish are osmoregulators, but must use different mechanisms to survive in (a) freshwater or (b) saltwater environments. (credit: modification of work by Duane Raver, NOAA)


215 Osmoregulation and Osmotic Balance

By the end of this section, you will be able to do the following:

  • Define osmosis and explain its role within molecules
  • Explain why osmoregulation and osmotic balance are important body functions
  • Describe active transport mechanisms
  • Explain osmolarity and the way in which it is measured
  • Describe osmoregulators or osmoconformers and how these tools allow animals to adapt to different environments

Osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance ( osmotic balance ) across membranes within the body’s fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte , in contrast, doesn’t dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (such as the pleural, serous, and cell membranes) are semi-permeable membranes . Semi-permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. As seen in (Figure), a cell placed in water tends to swell due to gain of water from the hypotonic or “low salt” environment. A cell placed in a solution with higher salt concentration, on the other hand, tends to make the membrane shrivel up due to loss of water into the hypertonic or “high salt” environment. Isotonic cells have an equal concentration of solutes inside and outside the cell this equalizes the osmotic pressure on either side of the cell membrane which is a semi-permeable membrane.


The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.

Need for Osmoregulation

Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, extracellular fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Because blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.

Transport of Electrolytes across Cell Membranes

Electrolytes, such as sodium chloride, ionize in water, meaning that they dissociate into their component ions. In water, sodium chloride (NaCl), dissociates into the sodium ion (Na + ) and the chloride ion (Cl – ). The most important ions, whose concentrations are
very closely regulated in body fluids, are the cations sodium (Na + ), potassium (K + ), calcium (Ca +2 ),
magnesium (Mg +2 ), and the anions chloride (Cl – ), carbonate (CO3 -2 ), bicarbonate (HCO3 – ), and phosphate(PO3 – ). Electrolytes are lost from the body during urination and perspiration. For this
reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.

Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to
the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into their component ions, they, in essence, add more solute particles into the solution and have a greater effect on osmotic pressure, per mass than compounds that do not dissociate in water, such as glucose.

Water can pass through membranes by passive diffusion. If electrolyte ions could passively diffuse across membranes, it would be impossible to maintain specific concentrations of ions in each fluid compartment therefore they require special mechanisms to cross the semi-permeable membranes in the body. This movement can be accomplished by facilitated diffusion and active transport. Facilitated diffusion requires protein-based channels for moving the solute. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.

Concept of Osmolality and Milliequivalent

In order to calculate osmotic pressure, it is necessary to understand how solute concentrations are measured. The unit for measuring solutes is the mole . One mole is defined as the gram molecular weight of the solute. For example, the molecular weight of sodium chloride is 58.44. Thus, one mole of sodium chloride weighs 58.44 grams. The molarity of a solution is the number of moles of solute per liter of solution. The molality of a solution is the number of moles of solute per kilogram of solvent. If the solvent is water, one kilogram of water is equal to one liter of water. While molarity and molality are used to express the concentration of solutions, electrolyte concentrations are usually expressed in terms of milliequivalents per liter (mEq/L): the mEq/L is equal to the ion concentration (in millimoles) multiplied by the number of electrical charges on the ion. The unit of milliequivalent takes into consideration the ions present in the solution (since electrolytes form ions in aqueous solutions) and the charge on the ions.

Thus, for ions that have a charge of one, one milliequivalent is equal to one millimole. For ions that have a charge of two (like calcium), one milliequivalent is equal to 0.5 millimoles. Another unit for the expression of electrolyte concentration is the milliosmole (mOsm), which is the number of milliequivalents of solute per kilogram of solvent. Body fluids are usually maintained within the range of 280 to 300 mOsm.

Osmoregulators and Osmoconformers

Persons lost at sea without any freshwater to drink are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90 percent of all bony fish are restricted to either freshwater or seawater. They are incapable of osmotic regulation in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life in freshwater and part in seawater. Organisms like the salmon and molly that can tolerate a relatively wide range of salinity are referred to as euryhaline organisms. This is possible because some fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in freshwater, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in (Figure)a. In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very dilute urine, and they achieve electrolyte balance by active transport of salts through the gills. When they move to a hypertonic marine environment, these fish start drinking seawater they excrete the excess salts through their gills and their urine, as illustrated in (Figure)b. Most marine invertebrates, on the other hand, may be isotonic with seawater ( osmoconformers ). Their body fluid concentrations conform to changes in seawater concentration. Cartilaginous fishes’ salt composition of the blood is similar to bony fishes however, the blood of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). This does not mean that their electrolyte composition is similar to that of seawater. They achieve isotonicity with the sea by storing large concentrations of urea. These animals that secrete urea are called ureotelic animals. TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would occur in other animals exposed to similar levels of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in osmoregulation.


Dialysis Technician Dialysis is a medical process of removing wastes and excess water from the blood by diffusion and ultrafiltration. When kidney function fails, dialysis must be done to artificially rid the body of wastes. This is a vital process to keep patients alive. In some cases, the patients undergo artificial dialysis until they are eligible for a kidney transplant. In others who are not candidates for kidney transplants, dialysis is a life-long necessity.

Dialysis technicians typically work in hospitals and clinics. While some roles in this field include equipment development and maintenance, most dialysis technicians work in direct patient care. Their on-the-job duties, which typically occur under the direct supervision of a registered nurse, focus on providing dialysis treatments. This can include reviewing patient history and current condition, assessing and responding to patient needs before and during treatment, and monitoring the dialysis process. Treatment may include taking and reporting a patient’s vital signs and preparing solutions and equipment to ensure accurate and sterile procedures.

Section Summary

Solute concentrations across semi-permeable membranes influence the movement of water and solutes across the membrane. It is the number of solute molecules and not the molecular size that is important in osmosis. Osmoregulation and osmotic balance are important bodily functions, resulting in water and salt balance. Not all solutes can pass through a semi-permeable membrane. Osmosis is the movement of water across the membrane. Osmosis occurs to equalize the number of solute molecules across a semi-permeable membrane by the movement of water to the side of higher solute concentration. Facilitated diffusion utilizes protein channels to move solute molecules from areas of higher to lower concentration while active transport mechanisms are required to move solutes against concentration gradients. Osmolarity is measured in units of milliequivalents or milliosmoles, both of which take into consideration the number of solute particles and the charge on them. Fish that live in freshwater or saltwater adapt by being osmoregulators or osmoconformers.

Review Questions

When a dehydrated human patient needs to be given fluids intravenously, he or she is given:

  1. water, which is hypotonic with respect to body fluids
  2. saline at a concentration that is isotonic with respect to body fluids
  3. glucose because it is a non-electrolyte
  4. blood

The sodium ion is at the highest concentration in:

  1. intracellular fluid
  2. extracellular fluid
  3. blood plasma
  4. none of the above

Cells in a hypertonic solution tend to:

  1. shrink due to water loss
  2. swell due to water gain
  3. stay the same size due to water moving into and out of the cell at the same rate
  4. none of the above

Critical Thinking Questions

Why is excretion important in order to achieve osmotic balance?

Excretion allows an organism to rid itself of waste molecules that could be toxic if allowed to accumulate. It also allows the organism to keep the amount of water and dissolved solutes in balance.

Why do electrolyte ions move across membranes by active transport?

Electrolyte ions often require special mechanisms to cross the semi-permeable membranes in the body. Active transport is the movement against a concentration gradient.

Glossary


Osmoregulation and Osmotic Balance

Cells placed in a hypertonic environment tend to shrink due to loss of water. In a hypotonic environment, cells tend to swell due to intake of water. The blood maintains an isotonic environment so that cells neither shrink nor swell. (credit: Mariana Ruiz Villareal)

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.


Osmoregulation

Kidneys are responsible for maintaining the process of excretion and osmoregulation in humans. Alongside the kidneys, there is a whole excretory system that is responsible for maintaining the osmoregulation of the body. The kidney, ureter, urinary bladder, and urethra are a part of the excretory system.

Osmoregulation meaning that it is the maintenance of the concentration of salts and water in the body is known as osmoregulation. In different organisms, different organs are responsible for this process. Osmoregulation meaning in biology or chemistry is the same as it is mentioned above. We will learn more about what is osmoregulation and the roles of various organs that help us to achieve this process.

Kidneys

Understanding the kidneys will help us understand what is osmoregulation. They are reddish-brown in colour and are two in number in human beings. They are in the shape of a bean and are situated in the abdominal cavity. The right kidney is slightly lower than the left kidney because of the liver occupying a great amount of space. Inside the kidneys, we have the nephrons that are mainly responsible for the formation of urine and also for maintaining the osmotic balance or osmoregulation of the body. The kidneys are covered by three layers that are:

Renal Capsule: It is the innermost layer and is the tough protective covering. It is made up of white fibrous connective tissue. Some elastic fibres and muscles are also present.

Adipose Capsule: It is the middle covering and as the name suggests it has the presence of adipose tissue. It acts as a shock-absorbing layer.

Renal Fascia: It is the outermost layer and it helps to link the abdominal wall.

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Nephron

Here we will understand more about the phenomenon of what is osmoregulation. It is known as the structural and functional unit of the kidney. The glomerulus and renal tubule are the two parts of the nephron. The glomerulus is formed by the afferent arteriole. Efferent arterioles are formed by the glomerular capillaries. The renal tube has a bowman’s capsule and distal convoluted tubule and proximal convoluted tubule. The concentration of salts and water is performed by the nephrons at its distal end. This is the actual osmoregulation meaning. It also helps in the formation of urine.

Types of Osmoregulation

After having an idea about what is osmoregulation, you should also know that it is divided into two major types –

Those organisms that strictly regulates their body osmolarity, and their overall internal conditions stays same despite the osmotic condition of external environment, are known as osmoregulator. Mostly freshwater and marine organisms fall under this category.

Osmoconformer are those that adapt their body’s osmolarity according to the outside environment. This may be done either actively or passively. Marine organisms are mostly osmoconformer.

Functions of the Tubules

After understanding the kidneys, we will understand what is osmoregulation in biology and the structures that are responsible for its maintenance.

Proximal Convoluted Tubule: Cuboidal brush border epithelium cells line this tubule. This helps in increasing their surface area for the reabsorption of salts and water. About 70-80% of the electrolytes are absorbed in this tubule. More than 80% of water is reabsorbed at this segment. The Proximal convoluted tubule is very helpful in maintaining the ionic balance of the body fluids and also the pH balance of the body. This is done by selective secretion of hydrogen ions and ammonia and the absorption of hydrogen carbonate ions from it.

Loop of Henle: Minimum reabsorption takes place in this segment. But it plays a major role in maintaining the high osmolarity of the medullary interstitial fluid. The descending loop is permeable to water and impermeable to electrolytes. The ascending loop is permeable to electrolytes and impermeable to water.

Distal Convoluted Tubule: A selective reabsorption of sodium and water ions takes place here. Also, it is capable of selectively secreting hydrogen and potassium ions and ammonia so as to maintain the pH and the osmotic balance in the body.

Collecting Duct: As the name suggests, this duct helps in collecting all the water and concentrates that have been filtered out by the tubules so far. It plays an important role in maintaining the pH and mineral concentration of the blood.

Regulation of Kidney Function

After answering the question of what do you mean by osmoregulation, now we will learn about the hormones that are responsible for its maintenance. There is hormonal feedback that is responsible for the maintenance of kidney functions. The hypothalamus, the juxtaglomerular apparatus, and the heart are responsible for the regulation of kidney functions. There are certain osmoreceptors that are activated when there are changes in the volume of the blood. When there is excessive loss of fluid from the body then these receptors are activated. The anti-diuretic hormone is released from the neurohypophysis. It is also known as vasopressin. It then facilitates the reabsorption of water from the distal convoluted tubule. When things are normalized, then the secretion of this hormone is stopped. This helps us to understand osmoregulation meaning and the ways by which it is maintained.


Watch the video: Osmoregulation bei Süß- und Salzwasserfischen und Landtieren Biologie, Oberstufe (August 2022).