LEC # 6 FLUID EXCHANGE AND OSMOTIC EQUILIBRIUM BETWEEN INTRACELLULAR AND EXTRACELLULAR FLUID PAGE # 314 CH # 25 GUYTON PHYSIO: 15Eth Ed:

• A common problem in seriously ill patients is maintaining enough fluid in the body.
• The fluid shortage may occur in the intracellular fluid (ICF) compartment, the extracellular fluid (ECF) compartment, or both.
• The amount of extracellular fluid present in the plasma and interstitial spaces is not always the same.
• The distribution of extracellular fluid between plasma and interstitial fluid is mainly controlled by forces acting across the capillary membranes.
• These forces are hydrostatic pressure and colloid osmotic (oncotic) pressure.
• The balance between these two forces determines how much fluid stays in the plasma and how much moves into the interstitial space.
• • The distribution of fluid between intracellular fluid (ICF) and extracellular fluid (ECF) is mainly controlled by osmotic forces.
• • These osmotic forces are produced by small dissolved substances called solutes.
• • The most important solutes are sodium, chloride, and other electrolytes.
• • These solutes act across the cell membrane.
• • The cell membrane is highly permeable to water.
• • However, the cell membrane is relatively impermeable to sodium, chloride, and other small ions.
• • Because water can move easily across the cell membrane, it moves rapidly between ICF and ECF.
• • As a result, the intracellular fluid and extracellular fluid remain isotonic, meaning they have the same overall osmotic concentration.

BASIC PRINCIPLES OF OSMOSIS AND OSMOTIC PRESSURE

Osmosis and Osmotic Pressure

• The principles of osmosis and osmotic pressure are important for understanding body fluid volume regulation.
• Here, only the key concepts related to volume regulation are discussed.
• Cell membranes are relatively impermeable to most solutes.
• In contrast, cell membranes are highly permeable to water.
• When one side of the cell membrane contains a higher concentration of impermeable solutes, a concentration difference is created.

Effect of Sodium Chloride on Water Movement

• If a solute such as sodium chloride (NaCl) is added to the extracellular fluid (ECF), the solute concentration in ECF increases.
• Water then moves rapidly out of the cells through the cell membrane and into the extracellular fluid.
• This movement continues until the water concentration becomes equal on both sides of the membrane.
• Conversely, if sodium chloride is removed from the extracellular fluid, the solute concentration in ECF decreases.
• Water then moves from the extracellular fluid into the cells through the cell membrane.
• As a result, the cells gain water and their volume increases.

Key Concept

👉 Adding solute to ECF → Water moves out of cells.

👉 Removing solute from ECF → Water moves into cells.

👉 Water always moves to maintain equal concentrations on both sides of the cell membrane.

Osmolality and Osmolarity

• The concentration of osmotically active particles in a solution is called osmolal concentration.
• It is called osmolality when the concentration is expressed as osmoles per kilogram of water.
• It is called osmolarity when the concentration is expressed as osmoles per liter of solution.
• In dilute solutions such as body fluids, the difference between osmolality and osmolarity is very small.
• Therefore, these two terms are often used almost interchangeably.
• Most clinical calculations use osmolarity rather than osmolality.
• The calculations discussed in later chapters are also mainly based on osmolarity.

Key Concept

👉 Osmolality = Osmoles/kg of water

👉 Osmolarity = Osmoles/L of solution

👉 In body fluids, the difference is very small, so they are often considered nearly the same.

Calculation of the Osmolarity and Osmotic Pressure of a Solution

Calculation of Osmotic Pressure Using van ’t Hoff Law

What Does van ’t Hoff Law Do?

• van ’t Hoff law is used to calculate the potential osmotic pressure of a solution.
• This calculation assumes that the cell membrane does not allow the solute to pass through it.

Example: 0.9% Sodium Chloride (NaCl) Solution

Step 1: Understand the Meaning of 0.9% Solution

• A 0.9% NaCl solution contains 0.9 g of sodium chloride in 100 mL of solution.
• This is equal to 9 g of sodium chloride in 1 liter (1000 mL) of solution.

Key Point:

👉 0.9% NaCl = 9 g/L

Step 2: Calculate the Molarity

• Molecular weight of sodium chloride (NaCl) = 58.5 g/mol.
• Molarity is calculated by dividing grams per liter by molecular weight.
• Therefore, the molarity of the solution is approximately 0.154 mol/L.

Key Point:

👉 Molarity = 0.154 mol/L

Step 3: Convert Molarity to Osmolarity

• One NaCl molecule separates into:

o 1 sodium ion (Na⁺)

o 1 chloride ion (Cl⁻)

• Therefore, 1 NaCl molecule produces about 2 osmoles.

Calculation:

• Osmolarity = 0.308 Osm/L

Step 4: Express Osmolarity in mOsm/L

• 1 Osm/L = 1000 mOsm/L
• Therefore:

Key Point:

👉 Osmolarity of 0.9% NaCl = 308 mOsm/L

Step 5: Calculate Potential Osmotic Pressure

• Each mOsm/L produces approximately 19.3 mm Hg of osmotic pressure.

Calculation:

• Potential osmotic pressure = 5944 mm Hg

Final Summary

• 0.9% NaCl = 9 g/L
• Molarity = 0.154 mol/L
• Osmolarity = 0.308 Osm/L
• Osmolarity = 308 mOsm/L
• Potential Osmotic Pressure = 5944 mm Hg

Remember

👉 NaCl dissociates into 2 particles (Na⁺ and Cl⁻).

👉 More dissolved particles = higher osmolarity.

👉 Higher osmolarity = greater osmotic pressure.

👉 A 0.9% NaCl solution has a potential osmotic pressure of 5944 mm Hg.

Correction of Osmolarity Calculation

• The previous osmolarity calculation is only an approximation.
• Sodium and chloride ions do not behave completely independently in solution.
• This is because there is attraction between sodium and chloride ions.
• As a result, the actual osmotic effect is slightly lower than predicted by Van ’t Hoff’s law.
• To correct this difference, a factor called the osmotic coefficient is used.
• For sodium chloride (NaCl), the osmotic coefficient is approximately 0.93.
• Therefore, the actual osmolarity of a 0.9% NaCl solution is calculated by multiplying 308 mOsm/L × 0.93.
• The corrected osmolarity is approximately 286 mOsm/L.
• In many practical and clinical calculations, the osmotic coefficients of solutes are often ignored for simplicity.
• Therefore, osmolarity and osmotic pressure are frequently estimated without applying this correction factor.

Key Concept

👉 Theoretical osmolarity of 0.9% NaCl = 308 mOsm/L

👉 Osmotic coefficient of NaCl = 0.93

👉 Actual osmolarity = 308 × 0.93 = 286 mOsm/L

👉 Ion attraction slightly reduces the actual osmotic effect.

Osmolarity of Body Fluids.

• Table 25.2 ( PAGE # 312 15th: Ed: guyton) shows the approximate osmolarity of different osmotically active substances in plasma, interstitial fluid, and intracellular fluid.
• In plasma and interstitial fluid, about 80% of the total osmolarity is due to sodium and chloride ions.
• Therefore, sodium and chloride are the major contributors to extracellular fluid osmolarity.
• In intracellular fluid, almost half of the total osmolarity is due to potassium ions.
• The remaining intracellular osmolarity is produced by many other substances present inside the cells.

Key Concept

👉 ECF (Plasma + Interstitial Fluid): Osmolarity mainly depends on sodium (Na⁺) and chloride (Cl⁻).

👉 ICF (Inside Cells): Osm

• As shown in Table 25.2, the total osmolarity of plasma, interstitial fluid, and intracellular fluid is about 300 mOsm/L.
• The osmolarity of plasma is about 1 mOsm/L higher than that of interstitial fluid and intracellular fluid.
• This difference is very small.
• The slight increase in plasma osmolarity is caused by the osmotic effect of plasma proteins.
• Plasma proteins create an osmotic pressure that pulls water into the capillaries.
• Because of these proteins, the pressure inside the capillaries is about 20 mm Hg higher than in the surrounding interstitial spaces.

Key Concept

👉 Plasma osmolarity ≈ 300 mOsm/L

👉 Interstitial fluid osmolarity ≈ 300 mOsm/L

👉 Intracellular fluid osmolarity ≈ 300 mOsm/L

👉 Plasma is slightly more concentrated because of plasma proteins.

👉 Plasma proteins help retain fluid inside the blood vessels by creating osmotic pressure.

Corrected Osmolar Activity of Body Fluids.

• Table 25.2 shows the corrected osmolar activities of plasma, interstitial fluid, and intracellular fluid.
• These values are corrected because the actual osmotic activity is slightly different from the calculated osmolarity.
• Positively charged ions (cations) and negatively charged ions (anions) attract each other in solution.
• This attraction is called interionic attraction.
• Because of interionic attraction, dissolved ions do not produce their full osmotic effect.
• As a result, the actual osmotic activity of the solution decreases slightly.

Key Concept

👉 Cations (+) and anions (−) attract each other.

👉 This attraction slightly reduces the osmotic effect of dissolved particles.

👉 Therefore, corrected osmolar activity is slightly lower than the calculated osmolarity.

Osmotic Equilibrium Between Intracellular and Extracellular Fluids

( Osmotic Pressure Across the Cell Membrane)

• Even small changes in the concentration of solutes in the extracellular fluid (ECF) can create very high osmotic pressures across the cell membrane.
• For every 1 mOsm/L concentration difference of an impermeable solute across the cell membrane, about 19.3 mm Hg of osmotic pressure is produced.
• An impermeable solute is a substance that cannot pass through the cell membrane.
• If a cell is surrounded by pure water, a very large osmotic pressure can develop across the cell membrane.
• If the intracellular fluid osmolarity is 282 mOsm/L, the potential osmotic pressure across the membrane can exceed 5400 mm Hg.
• This shows that osmotic forces can be extremely powerful.
• These forces can move large amounts of water across the cell membrane.
• This movement occurs when the intracellular fluid (ICF) and extracellular fluid (ECF) are not in osmotic equilibrium.
• Therefore, even small changes in the concentration of impermeable solutes in the extracellular fluid can cause large movements of water.
• As a result, cell volume can increase or decrease significantly.

Key Concept

👉 1 mOsm/L difference = 19.3 mm Hg osmotic pressure

👉 Osmotic forces across cell membranes are very powerful.

👉 Small changes in ECF solute concentration can cause large changes in water movement and cell volume.

Isotonic, Hypotonic, and Hypertonic Fluids

• Different concentrations of impermeable solutes in the extracellular fluid can affect cell volume.
• If a cell is placed in a solution with an osmolarity of 282 mOsm/L, the cell will not shrink or swell.
• This is because the water concentration is equal inside and outside the cell.
• The solutes cannot enter or leave the cell.
• Such a solution is called an isotonic solution.
• An isotonic solution does not cause cells to shrink or swell.
• A 0.9% sodium chloride (NaCl) solution is an example of an isotonic solution.
• This solution is very important in clinical medicine.
• It can be infused into the blood without disturbing the osmotic balance between intracellular fluid and extracellular fluid.

Key Concept

👉 Isotonic solution = No change in cell size

👉 ICF osmolarity = ECF osmolarity

👉 0.9% NaCl is isotonic

👉 Cells neither gain nor lose water in an isotonic solution.

Hypotonic Solution and Cell Swelling

• If a cell is placed in a hypotonic solution, the concentration of impermeable solutes outside the cell is less than 282 mOsm/L.
• In a hypotonic solution, the extracellular fluid contains fewer solutes than the intracellular fluid.
• Water diffuses into the cell through the cell membrane.
• As water enters, the cell swells.
• Water continues to move into the cell until the osmolarity inside and outside the cell becomes nearly equal.
• The incoming water dilutes the intracellular fluid.
• At the same time, movement of water out of the extracellular fluid makes the extracellular fluid more concentrated.
• Eventually, both compartments reach approximately the same osmolarity.
• Sodium chloride solutions with a concentration less than 0.9% are hypotonic solutions.
• These hypotonic solutions cause cells to swell.

Key Concept

👉 Hypotonic solution = Lower solute concentration outside the cell

👉 Water moves into the cell

👉 Cell swells (increases in volume)

👉 NaCl < 0.9% = Hypotonic solution.

Hypertonic Solution and Cell Shrinkage

• If a cell is placed in a hypertonic solution, the concentration of impermeable solutes outside the cell is higher than inside the cell.
• Water moves out of the cell and into the extracellular fluid.
• As water leaves the cell, the intracellular fluid becomes more concentrated.
• At the same time, the extracellular fluid becomes more diluted.
• The cell shrinks because it loses water.
• Water continues to move until the osmolarity inside and outside the cell becomes approximately equal.
• Sodium chloride solutions with a concentration greater than 0.9% are hypertonic solutions.
• Hypertonic solutions cause cells to shrink.

Key Concept

👉 Hypertonic solution = Higher solute concentration outside the cell

👉 Water moves out of the cell

👉 Cell shrinks (decreases in volume)

👉 NaCl > 0.9% = Hypertonic solution

Isosmotic, Hyperosmotic, and Hypo-Osmotic Fluids.

• The terms isotonic, hypotonic, and hypertonic describe whether a solution will change the volume of a cell.
• The tonicity of a solution depends on the concentration of impermeable solutes.
• Some solutes can pass through the cell membrane.
• Solutions that have the same osmolarity as the cell are called isosmotic solutions.
• A solution can be isosmotic whether or not its solutes can cross the cell membrane.
• A 5% glucose solution is an example of a nearly isosmotic solution.
• This solution can be given intravenously (IV) without causing red blood cells to shrink or swell.
• After entering the body, glucose is transported into cells and metabolized.

Key Concept

👉 Tonicity depends on impermeable solutes and determines whether cells change size.

👉 Isosmotic means the solution has the same osmolarity as the cell.

👉 5% glucose solution is nearly isosmotic, so it does not initially cause red blood cells to shrink or swell.

• Hyperosmotic solutions have a higher osmolarity than normal extracellular fluid.
• Hypo-osmotic solutions have a lower osmolarity than normal extracellular fluid.
• These terms compare a solution’s osmolarity with normal extracellular fluid.
• These terms are used without considering whether the solute can cross the cell membrane.
• Some substances, such as urea, can pass through the cell membrane very easily.
• These highly permeable substances can cause temporary (transient) movement of water between intracellular fluid (ICF) and extracellular fluid (ECF).
• With enough time, the concentration of these substances becomes equal inside and outside the cells.
• Once equilibrium is reached, these substances have very little effect on intracellular fluid intracellular volume under steady-state conditions.

Osmotic Equilibrium Between Intracellular and

Extracellular Fluids Is Rapidly Attained.

• Water moves across the cell membrane very rapidly.
• Any difference in osmolarity between intracellular fluid (ICF) and extracellular fluid (ECF) is usually corrected within seconds or a few minutes. S
• This rapid water movement does not mean that the whole body reaches osmotic equilibrium immediately.
• Complete equilibrium between intracellular and extracellular fluids throughout the body takes longer.
• This is because fluid first enters the body through the gastrointestinal tract (gut).
• The absorbed fluid must then be transported by the blood to all tissues of the body.
• Only after this distribution can complete osmotic equilibrium occur throughout the body.
• After drinking water, it usually takes about 30 minutes for osmotic equilibrium to be reached everywhere in the body.
• Effects of Different Solutions on Cell Volume
• Isotonic Solution (282 mOsm/L)
• Cell volume does not change.
• Hypotonic Solution (200 mOsm/L)
• Water enters the cell.
• The cell swells.
• Hypertonic Solution (360 mOsm/L)
• Water leaves the cell.
• The cell shrinks.
• Key Concept
• 👉 Water crosses cell membranes very quickly.
• 👉 Hypotonic → Cell swells
• 👉 Isotonic → No change in cell size
• 👉 Hypertonic → Cell shrinks
• 👉 Whole-body osmotic equilibrium takes about 30 minutes after drinking water.

VOLUME AND OSMOLALITY OF EXTRACELLULAR AND INTRACELLULAR FLUIDS IN ABNORMAL STATES.

Causes of Changes in Body Fluid Volumes

• Many factors can cause major changes in extracellular fluid (ECF) and intracellular fluid (ICF) volumes.
• Excess water intake can increase body fluid volumes.
• Retention of water by the kidneys can also increase body fluid volumes.
• Dehydration can decrease body fluid volumes.
• Intravenous (IV) infusion of different solutions can change the volumes of ECF and ICF.
• Loss of large amounts of fluid from the gastrointestinal tract can alter body fluid volumes.
• Excessive fluid loss through sweating can change body fluid volumes.
• Excessive fluid loss through the kidneys can also affect body fluid volumes.

Understanding Fluid Volume Changes

• Changes in intracellular and extracellular fluid volumes can be estimated by understanding some basic principles.
• These principles also help determine the appropriate treatment (therapy) for fluid imbalances.

Key Concept

👉 Water gain or water loss changes ECF and ICF volumes.

👉 Common causes include:

• Excess water intake
• Kidney water retention
• Dehydration
• IV fluid administration
• Gastrointestinal fluid loss
• Excess sweating
• Excess kidney fluid loss

👉 Understanding these changes helps in choosing the correct treatment.

• Water moves very rapidly across cell membranes.
• Therefore, the osmolarity of intracellular fluid (ICF) and extracellular fluid (ECF) remains almost the same.
• A difference in osmolarity may occur only for a few minutes after a change in one of the compartments.

Basic Principle 2

• Cell membranes are almost completely impermeable to many solutes.
• Examples of these solutes are sodium and chloride ions.
• Therefore, the number of osmoles in the extracellular fluid usually remains relatively constant.
• The number of osmoles in the intracellular fluid also usually remains relatively constant.
• Significant changes occur only when solutes are added to or removed from the extracellular fluid.

Application of These Principles

• These basic principles help us understand abnormal changes in body fluids.
• Using these principles, we can analyze changes in extracellular fluid volume.
• We can also analyze changes in intracellular fluid volume.
• These principles help explain changes in the osmolarity of both fluid compartments.

Key Concept

👉 Water moves freely across cell membranes → ICF and ECF osmolarities become equal.

👉 Sodium and chloride do not cross cell membranes easily → Osmoles usually stay in their own compartment.

👉 These principles are used to understand and treat fluid and electrolyte disorders.

ind: 1. Water moves rapidly across cell membranes; there￾fore, the osmolarities of intracellular and extracellu￾lar fluids remain almost exactly equal to each other, except for a few minutes after a change in one of the compartments. 2. Cell membranes are almost completely imperme￾able to many solutes, such as sodium and chloride; therefore, the number of osmoles in the extracellu￾lar or intracellular fluid generally remains relatively constant unless solutes are added to or lost from the extracellular compartment. With these basic principles in mind, we can analyze the effects of different abnormal fluid conditions on extra￾cellular and intracellular fluid volumes and osmolarities. CONVERT ONLY PROVIDED TEXT OF EACH AND EVERY SENTENCE IN EASIEST CONCEPTUAL ENGLISH IN POWER POINT STYLE

Basic Principle 1

• Water moves rapidly across cell membranes.
• Therefore, the osmolarity of intracellular fluid (ICF) and extracellular fluid (ECF) remains almost exactly the same.
• A temporary difference may occur for a few minutes after a change in one of the fluid compartments.

• Cell membranes are almost completely impermeable to many solutes.
• Basic Principle 2
• Examples of these solutes include sodium and chloride ions.
• Therefore, the number of osmoles in the extracellular fluid usually remains relatively constant.
• The number of osmoles in the intracellular fluid also usually remains relatively constant.
• This number changes mainly when solutes are added to or lost from the extracellular fluid.

Applying These Principles

• These basic principles help us understand different abnormal fluid conditions.
• Using these principles, we can analyze changes in extracellular fluid volume.
• We can also analyze changes in intracellular fluid volume.
• These principles help explain changes in the osmolarity of both fluid compartments.

Effect of Adding Saline Solution to the Extracellular Fluid.

• When isotonic saline is added to the extracellular fluid (ECF), the osmolarity of the extracellular fluid does not change.
• The main effect of isotonic saline is an increase in extracellular fluid volume.
• The sodium and chloride ions in isotonic saline remain mainly in the extracellular fluid.
• This happens because the cell membrane is almost impermeable to sodium and chloride ions.
• Therefore, sodium and chloride do not easily enter the cells.
• As a result, the added fluid stays mainly in the extracellular compartment.

Key Concept

👉 Isotonic saline does not change osmolarity.

👉 ECF volume increases.

👉 ICF volume remains unchanged.

👉 Na⁺ and Cl⁻ stay in the extracellular fluid because they cannot easily cross the cell membrane.

• When a hypertonic solution is added to the extracellular fluid (ECF), the osmolarity of the extracellular fluid increases.
• The increased extracellular osmolarity causes water to move out of the cells by osmosis.
• Water moves from the intracellular fluid (ICF) into the extracellular fluid (ECF).
• Almost all of the added sodium chloride remains in the extracellular compartment.
• Sodium and chloride do not easily enter the cells.
• Water continues to move from the cells into the extracellular space until osmotic equilibrium is reached.
• The extracellular fluid volume increases by more than the volume of fluid added.
• This extra increase occurs because water is pulled from the cells into the extracellular fluid.
• The intracellular fluid volume decreases because cells lose water.
• The osmolarity of both the intracellular and extracellular compartments increases.

Key Concept

👉 Hypertonic solution increases ECF osmolarity.

👉 Water moves out of cells into ECF.

👉 ECF volume increases.

👉 ICF volume decreases (cells shrink).

👉 Osmolarity increases in both ECF and ICF.

• When a hypotonic solution is added to the extracellular fluid (ECF), the osmolarity of the extracellular fluid decreases.
• Some of the water in the extracellular fluid moves into the cells by osmosis.
• Water continues to enter the cells until the intracellular fluid (ICF) and extracellular fluid (ECF) reach the same osmolarity.
• Both the intracellular fluid volume and extracellular fluid volume increase after adding a hypotonic solution.
• However, the intracellular fluid volume increases more than the extracellular fluid volume.

Key Concept

👉 Hypotonic solution decreases ECF osmolarity.

👉 Water moves from ECF into cells.

👉 Both ECF and ICF volumes increase.

👉 ICF volume increases the most because cells gain water and swell.

Calculation of Fluid Shifts and Osmolarities After

Infusion of Hypertonic Saline Solution.

• Figure 25.6 shows the effects of adding isotonic, hypertonic, and hypotonic solutions to the extracellular fluid.
• The solid lines and shaded areas represent the normal state.
• The dashed lines show the changes that occur after fluid is added and osmotic equilibrium is reached.
• The horizontal axis (X-axis) shows the volumes of the intracellular fluid (ICF) and extracellular fluid (ECF) compartments.
• The vertical axis (Y-axis) shows the osmolarity of these fluid compartments.

Calculating Fluid Changes

• We can calculate the step-by-step effects of infusing different solutions on ICF volume, ECF volume, and osmolarity.
• These calculations help predict how body fluid compartments change after fluid administration.

Example

• Suppose 2 liters of hypertonic 3.0% sodium chloride solution are infused into the extracellular fluid compartment.
• The patient weighs 70 kg.
• The patient’s initial plasma osmolarity is 280 mOsm/L.
• After the infusion, osmotic equilibrium will eventually be established between the intracellular and extracellular compartments.
• The goal is to determine the approximate intracellular fluid volume, extracellular fluid volume, and osmolarity after equilibrium is reached.
• The first step is to calculate the initial conditions of the body fluid compartments.
• These initial conditions include the volume, concentration, and total milliosmoles in each compartment.
• Assume that the extracellular fluid (ECF) volume is 20% of body weight.
• Assume that the intracellular fluid (ICF) volume is 40% of body weight.
• Using these assumptions, the initial fluid volumes can be calculated.
• Using these assumptions, the initial osmolar concentrations can also be calculated.
• Key Concept
• 👉 Before analyzing any fluid infusion, first determine:
• ECF volume
• ICF volume
• Osmolarity
• Total milliosmoles in each compartment
• 👉 These values serve as the starting point for all subsequent calculations.

Calculating the Osmoles Added by Hypertonic Saline

• The next step is to calculate the total milliosmoles added to the extracellular fluid by the infused solution.
• A 3.0% sodium chloride (NaCl) solution contains 3.0 g of NaCl in 100 mL of solution.
• This is equal to 30 g of NaCl per liter.
• The molecular weight of sodium chloride is approximately 58.5 g/mol.
• Therefore, 1 liter of 3.0% NaCl contains about 0.5128 mole of NaCl.
• Since 2 liters are infused, the total amount of NaCl is about 1.0256 moles.
• Each mole of sodium chloride produces approximately 2 osmoles because it dissociates into sodium (Na⁺) and chloride (Cl⁻) ions.
• Therefore, the infusion of 2 liters of 3.0% NaCl adds about 2.051 osmoles of particles.
• This is equal to 2051 milliosmoles (mOsm).
• Thus, the net effect of the infusion is the addition of 2051 mOsm of sodium chloride to the extracellular fluid.

Key Concept

👉 3.0% NaCl = 30 g/L

👉 2 liters contain 1.0256 moles of NaCl

👉 1 mole NaCl = 2 osmoles

👉 Total added osmoles = 2051 mOsm

👉 These osmoles remain mainly in the extracellular fluid, increasing its osmolarity.

Step 2: Immediate Effect of Adding Hypertonic Saline

• In Step 2, the immediate effect of adding 2051 milliosmoles of sodium chloride and 2 liters of fluid to the extracellular fluid is calculated.
• At this moment, there is no change in intracellular fluid (ICF) volume.
• At this moment, there is no change in intracellular fluid osmolarity.
• Osmotic equilibrium has not yet been established.
• In the extracellular fluid (ECF), 2051 additional milliosmoles of solute are added.
• As a result, the total amount of solute in the extracellular fluid becomes 5971 milliosmoles.
• The extracellular fluid volume increases to 16 liters.
• The new extracellular fluid concentration is calculated by dividing 5971 mOsm by 16 liters.
• This gives an extracellular fluid osmolarity of approximately 373 mOsm/L.
• These changes occur immediately after the solution is infused, before water has time to move between compartments.

Key Concept

👉 Immediately after infusion:

• ICF volume = No change
• ICF osmolarity = No change
• No osmotic equilibrium yet

👉 ECF volume increases to 16 L

👉 ECF osmolarity rises to about 373 mOsm/L

👉 Water movement between ECF and ICF occurs later to restore osmotic equilibrium.

Step 3: After Osmotic Equilibrium is Reached

• In the third step, the fluid volumes and concentrations are calculated after osmotic equilibrium develops.
• Osmotic equilibrium is usually reached within a few minutes.
• At equilibrium, the osmolarity of intracellular fluid (ICF) and extracellular fluid (ECF) becomes equal.
• The common osmolarity is calculated by dividing the total body milliosmoles (13,811 mOsm) by the total body fluid volume (44 L).
• This calculation gives a final osmolarity of approximately 313.9 mOsm/L.
• Therefore, all body fluid compartments will have an osmolarity of 313.9 mOsm/L after equilibrium.
• These calculations assume that no water or solute has been lost from the body.
• They also assume that sodium chloride remains in the extracellular fluid and does not enter the cells.

Calculation of Intracellular Fluid Volume

• The intracellular fluid contains 7840 milliosmoles of solute.
• Intracellular fluid volume is calculated by dividing 7840 mOsm by 313.9 mOsm/L.
• The final intracellular fluid volume is approximately 24.98 liters.

Calculation of Extracellular Fluid Volume

• The extracellular fluid contains 5971 milliosmoles of solute.
• Extracellular fluid volume is calculated by dividing 5971 mOsm by 313.9 mOsm/L.
• The final extracellular fluid volume is approximately 19.02 liters.

Key Concept

👉 After osmotic equilibrium:

• ICF osmolarity = ECF osmolarity = 313.9 mOsm/L
• ICF volume decreases from 28 L to about 25 L
• ECF volume increases from 14 L to about 19 L

👉 Water moves from ICF → ECF because the hypertonic saline remains in the extracellular fluid.

👉 The added sodium chloride stays in ECF and pulls water out of the cells until osmotic equilibrium is achieved.

Effect of Hypertonic Sodium Chloride Solution

This example shows what happens when 2 liters of hypertonic sodium chloride solution are added to the body.

• The extracellular fluid (ECF) volume increases by more than 5 liters.
• At the same time, the intracellular fluid (ICF) volume decreases by almost 3 liters.

Importance of These Calculations

This method can be used to calculate changes in intracellular and extracellular fluid volumes and osmolarities.

It can be applied to almost any clinical problem involving fluid volume regulation.

Students and healthcare professionals should become familiar with these calculations.

Understanding the mathematical relationship between ICF and ECF osmotic equilibrium is very important.

This knowledge helps explain most body fluid disorders.

It also helps in understanding the treatment of fluid and electrolyte abnormalities.

Key Concept

👉 Hypertonic saline pulls water out of cells.

👉 ECF volume increases greatly.

👉 ICF volume decreases.

👉 Understanding osmotic equilibrium is essential for diagnosing and treating fluid balance disorders.