1. Regulation of Cell Volume
Dr. J
May 2012

2. Objectives
Identify major routes for water intake and loss, and predict how changes in intake or loss affect the distribution of total body water
Given the body weight, estimate the a) total body water, b) extracellular fluid volume, c) intracellular fluid volume, d) blood volume, and e) plasma volume. Identify normal extracellular fluid (plasma) and intracellular fluid (red blood cell) osmolarity.
Demonstrate the ability to use the indicator dilution method to measure plasma volume, extracellular fluid volume, intracellular fluid volume, interstitial fluid volume, and total body water, and identify compounds used to measure each volume.
Differentiate between the terms osmole, osmolarity, osmolality and tonicity. List the typical value and normal range for plasma osmolality.
Define the Gibbs-Donnan equilibrium and list the resulting characteristics.
Using the membrane as an example, define a reflection coefficient, and explain how the relative permeability of a cell to water and solutes will generate an osmotic pressure. Contrast the osmotic pressure generated across a cell membrane by a solution of particles that freely cross the membrane (osmotic) with that of a solution with the same osmolarity, but particles that cannot cross the cell membrane (tonic).
Given the composition and osmolarity of a fluid, identify it as hypertonic, isotonic, hypotonic, hyperosmotic, isosmotic, or hypoosmotic. Predict the change in transcellular fluid exchange that would be caused by placing a red blood cell in solutions with varying tonicities.
Using the volumes/compartments, contrast the movement between intracellular and extracellular compartments caused by increases or decreases in extracellular fluid osmolarity.
Reading Materials:
Required text book:
Cell Physiology, Landowne, Chapter 2
Supplemental (optional) reading materials:
Vander’s Physiology. 10th Ed., pg 5,
Medical Physiology. Boron and Boulpaep, Chapter 3
Medical Physiology. Rhoades and Tanner 2nd Ed., Chapter 2
Textbook of Medical Physiology. Guyton and Hall 11th Ed. Chapter 25

3. Over 60% of the body is water!
Why do we care?
Over 60% of the body is water!
Water gain
Drinking
Food
Intravenous Infusion
Water loss
Hemorrhage
Sweat
Feces
Urine
Vomit
Diarrhea
Respiration
I. Why do we care about fluid compartments?
Numerous factors can cause the extracellular and intracellular volumes to change, which will directly affect the total body water.
These factors include:
ingestion of water
intravenous (IV) infusion
dehydration
hemorrhage
respiration
loss of abnormal amounts of fluid through sweat, feces, urine, vomiting and diarrhea.

4. Membrane Permeable Molecules
Hydrophilic
water loving
Hydrophobic
water fearing
Therefore, our bodies have precise ways of regulating fluid volumes in our different compartments. Using methods discussed in this handout you will learn to calculate the changes in intracellular and extracellular fluid volumes and this will help you to later determine the type of therapy that should be instituted.
Note: Additional information regarding cell volume regulation and precise changes in osmolarity of body fluid compartments will be covered in the Renal module during 2nd semester.
oxygen
Water
carbon dioxide
steroid hormones
thyroid hormone

5. Body Fluid Compartments
Total body water
(TBW) =
Intracellular
fluid
(ICF)
Extracellular
fluid
(ECF) +
The Total body water (TBW) is the total amount of water within the body and is separated into two main compartments:
Intracellular fluid (ICF): the water contained within the cells.
Extracellular fluid (ECF): the water outside of the cells that is contained in the interstitial space (between the cells), in the blood, and in space completely surrounded by epithelial cells (transcellular). These three compartments are also defined:
Interstitial fluid volume (IFV)
Plasma volume (PV)
Transcellular fluid volume (TFV)
Water can move from compartment to compartment by diffusion (both directly through the membrane and through water pores or aquaporins within the membrane), which in the case of water is known as osmosis. Osmosis is an important means of moving water from one place to another within the body, and is also important in giving structure to cells by exerting osmotic or “hydrostatic” pressure.
Interstitial
Fluid (IF)
Plasma
Volume
Transcellular fluid

6. Body Fluid Compartments
Total body water. Total body water (TBW) is approximately 60% of total body weight in a young adult human male, approximately 50% of total body weight in a young adult human female, and 65-75% of total body weight in an infant. TBW accounts for a lower percentage of weight in females because they typically have more adipose tissue, and fat cells have a lower water content than muscle. Even if gender and age are taken into consideration, the fraction of total body weight contributed by water is not constant for all individuals under all conditions. For example, variability in the amount of adipose tissue can influence the fraction. Because water represents such a large fraction of body weight, acute changes in TBW can be detected simply by monitoring body weight. The prototypic 70 Kg male has approximately 42 L of TBW. Of these 42 L, approximately 25L (60%) is intracellular and 17 L (40%) is extracellular. Extracellular fluid is composed of blood plasma, interstitial fluid, and transcellular fluid.
Plasma volume. Of the approximately 17 L of ECF, only about 20% (~3 L) is contained within the cardiac chambers and blood vessels, that is, within the intravascular compartment. The total volume of this intravascular compartment is the blood volume, approximately 5.5 L. The extracellular 3 L of the blood volume is the plasma volume. The remaining 2.5 L consists of cellular elements of blood: erythrocytes, leukocytes, and platelets. The fraction of blood volume that is occupied by these cells is called the hematocrit. The hematocrit is determined by centrifuging blood that is treated with an anticoagulant and measuring the fraction of the total volume that is occupied by the packed cells.
Interstitial fluid. About 75% (~13 L) of the ECF is located outside the intravascular compartment, where it bathes the non-blood cells of the body. This is called the interstitial space. The barriers that separate the intravascular and interstitial compartments are the walls of capillaries. Water and solutes can move between the interstitium and blood plasma by crossing capillary walls and between the interstitium and cytoplasm by crossing cell membranes.
Transcellular fluid. The final approximately 1 L (5%) of ECF is trapped within spaces that are completely surrounded by epithelial cells. This transcellular fluid includes the synovial fluid within joints and the cerebrospinal fluid surrounding the brain and spinal cord. Transcellular fluid does not include fluids that are, strictly speaking, outside the body, such as the contents of the gastrointestinal (GI) tract or urinary bladder.
Boron and Boulpaep

7. Estimation of fluid volumes
Extracellular Fluid (ECF)
40% of TBW
Interstitial Fluid (IF)
75% of ECF
Intracellular Fluid (ICF)
60% of TBW
Plasma Volume
(PV)
20% of ECF
For educational purposes, we will use a generic example of a young adult male: Total body water (TBW) makes up about 60% of the total body mass. Intracellular fluid makes up 60% of the TBW (therefore 40% of BW), while extracellular fluid volume makes up 40% of TBW (20% of BW), 75% from interstitial fluid volume and 5% from plasma volume.
Plasma is blood with the cells and large solutes removed. Blood volume, plasma volume, red blood cell volume and hematocrit are all related to each other and can be calculated from each other by the following equations:
Blood Volume = Plasma Volume (PV) + Red Blood Cell Volume (RBCV)
Hematocrit (Hct) = Red Blood Cell Volume/Blood Volume (BV)
Blood Volume = Plasma Volume/1-Hct
(more information regarding blood will be covered in the Heme/lLmph module later in the semester.)
Approximate volume of the various fluid compartments can be calculated based on the percentage of the body weight, or percentage of fluid volume that they represent. A standard (average) patient weights 70 Kg, so we could use this to calculate the body fluid compartments as a percentage of body mass, a percentage of total body water, and as a percentage of extracellular fluid volume.
Transcellular Fluid
5% of ECF
Total Body Water = 60% of body mass (BW)

8. How to estimate body fluid volumes
Three methods:
By using a percent of the total body mass.
By using a percent of total body water.
By using a percent of extracellular fluid
It needs to be mentioned that these are ALL estimations and should be used as reference points for actual values to determine if the actual values are pathological or not.

9. Estimation of fluid volumes: 1. Based on total body mass (70 Kg)
Intracellular Fluid (ICF)
40% of BW
Interstitial Fluid (IF)
Transcellular Fluid
Plasma Volume
(PV)
Total Body Water = 60% of body mass (BW)
Extracellular Fluid (ECF)
20% of BW

10. Estimation of fluid volumes: 1. Based on total body water (42 L)
Intracellular Fluid (ICF)
60% of TBW
Interstitial Fluid (IF)
Transcellular Fluid
Plasma Volume
(PV)
Total Body Water = 42 L
Extracellular Fluid (ECF)
40% of TBW

11. Estimation of fluid volumes: 1. Based on ECF Volume (14 L)
Intracellular Fluid (ICF)
Interstitial Fluid (IF)
75% of ECF
Transcellular Fluid
5% of ECF
Plasma Volume
(PV)
20% of ECF
Total Body Water
Extracellular Fluid (ECF)
14 L

12. Body Fluid Compartments
Boron and Boulpaep
The actual anatomy and components of the body fluid compartments is illustrated above. Again, it needs to be mentioned that these are ALL estimations and should be used as reference points for actual values to determine if the actual values (determined using the indicator dilution method) are pathological or not.

13. The Indicator Dilution Method
How do we measure the amount of fluid
in these compartments?
The Indicator Dilution Method
Approximations are good, but clinically we need to have an accurate way of measuring fluid volume levels. The indicator dilution method is defined as a method for measuring blood volume. A known amount of a substance that dissolves freely in blood but does not leave the capillaries is injected intravenously. After a few minutes a sample of blood is withdrawn, and the volume of blood in the body is calculated from the concentration of the substance in the sample, the sample's volume, and the hematocrit. This method can be similarly utilized with indicators for volume compartments other than blood as well.

14. Indicator Dilution Method
Determination of compartmental volume
Place indicator A in the compartment B.
Allow it to disperse evenly though out the compartment’s fluid
Analyze the extent to which the substance is diluted

15. Indicator Dilution Method
Mass (mg) = volume (ml) x concentration (mg/ml)
Mass of indicator injected to compartment = Mass of indicator inside the compartment
Mass A (injected) = Mass B (inside)
Mass (mg) = volume (ml) x concentration (mg/ml)
The mass referred to in the indicator dilution method is the mass of the indicator (molecules) itself. The mass of the indicator before it is placed in the compartment is the same as the mass of the indicator in the compartment. Therefore:
A = the indicators volume and concentration before it is placed in the compartment.
B = the indicators volume and concentration after it has been dispersed in the compartment.
Volume (B) is the volume of the compartment that the indicator was placed in and therefore, the value that we want to solve for. By algebraic manipulation {dividing both sides of the equation by Concentration (B)} of the above equation we can solve for Volume (B).
Volume (A) x Concentration (A) = Volume (B) x Concentration (B)
Volume (B) = Volume (A) x Concentration (A)
Concentration (B)

16. Indicators for specific compartments
Total Body Water (TBW)
3H2O, antipyrine
Extracellular Fluid(ECF)
inulin, mannitol
Plasma Volume (PV)
125I-albumin, Evans blue dye
Extracellular Fluid (ECF)
Intracellular
Fluid
= TBW - ECF
Interstitial Fluid (IF)
Plasma Volume
(PV)
Note: transcellular fluid is often not calculated as part of the ECF. Additionally, there is not a specific indicator, or way to calculate its contents. Therefore, it will be omitted here.
Initial deposit is into the ECF through injection, so every indicator will start in the ECF (PV to be specific) and diffuse from there. The indicator substance, however, must only disperse in the compartment to be measured. An indicator that would move into the ICF would also have to disperse in the ECF (where it started from) and an indicator for the IF would also disperse in the PV. Therefore, there are no indicators to measure ICF and IF only, but these can be calculated.
Indicator Loss. This method assumes that there is no loss of the indicator by the body through the urine. However, this is not always true. To take into account any indicator that has been loss by urine, you must subtract the mass of indicator lost in the urine from the mass of indicator that was injected (Mass A). More regarding the indicator dilution method will be covered in renal physiology.
Interstitial
Fluid
Intracellular Fluid (ICF)
≈ ECF - PV
Transcellular Fluid
Total Body Water

17. 1) A 70 kg, 28 yr female presented after fainting while getting ready for work. 10 mL of an isotonic saline solution with the addition of 45 mg/mL of inulin was administered to her (‘Elizabeth’) through an arm vein. After a 4 hour equilibration period, a blood sample is withdrawn and the plasma is separated and analyzed for inulin content. A concentration of 0.05 mg/ml of inulin was found. What is the ECFV level of the patient?
14 L
11 L
9 L
22.5 L

18. Volume (A) x Concentration (A) = Volume (B) x Concentration (B)
Mass of indicator injected to compartment = Mass of indicator inside the compartment
Volume (A) x Concentration (A) = Volume (B) x Concentration (B)
Volume (B) = Volume (A) x Concentration (A)
Concentration (B)
Volume A =
Concentration (A) =
Concentration (B) =
Volume (B) =
What is the ECFV level of the patient?

19. Osmosis
Salt
Osmosis – is the net diffusion of WATER across a selectively permeable membrane from a region of high water concentration (i.e. low solute concentration) to a region lower water concentration (i.e. high solute concentration). When solutes are added to pure water this reduces the water concentration in the mixture and increases the solute concentration. Likewise, water diffuses from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration).
Osmole – measure of the total number of particles in a solution.
1 Osm = 1 mole (6.02 x 1023)
1 mOsm = 1 mM

20. Osmoles Glucose (C6H12O6) Salt (NaCl) 18 moles = 18 Osm
Osmole – the measure for the total number of particles in a solution. One osmole (Osm) equals one mole (6.02 x 1023) of solute particles. A solution that contains one mole of glucose in one liter of water has a concentration of 1 Osm/L. However, some molecules (i.e. sodium chloride) dissociate. A one molar solution of sodium chloride will have an osmolal concentration of 2 Osm/L

21. Osmolarity vs. Osmolality
Osmolarity – osmoles per liter of solution (Osm/L)
Osmolality – osmoles per kilogram of water (Osm/Kg)
Osmolarity – term used when osmolal concentration is expressed in osmoles per liter of solution (Osm/L).
Osmolality – term used when osmolal concentration is expressed in osmoles per kilogram of water (Osm/Kg). In general 1L=1Kg, consequently osmolarity and osmolality can almost be used synonymously. In clinical cases it is easier to express body fluid compartments in terms of liters of fluid (osmolarity) rather than kilograms of water (osmolality).

22. Osmotic Pressure
Salt
The osmotic pressure of a solution is the pressure necessary to stop the net movement of water across a selectively permeable membrane. When a membrane separates two solutions of different osmotic pressure (i.e. solutions of different osmolarity), water will move from the solution with the lower osmotic pressure (i.e. low solute concentration and high water concentration) to the solution of high osmotic pressure (i.e. high solute concentration and low water concentration). Think of osmotic pressure as a sucking force, the more particles (i.e. higher osmolal concentration) in a solution the more those particles are going to hold on to the water, therefore preventing it from moving across the membrane. A solution that has fewer particles and a lower osmotic pressure has an attenuated (decreased) ability to hold on to the water, therefore, the water will move across the membrane.

23. Calculating Osmotic Pressure
Van’t Hoff Equation
= nRTC
 = osmotic pressure
 = osmotic (reflection) coefficient
n = # of particles
R = universal gas constant
T = absolute temperature
C = concentration of the solute in mol/L
Van’t Hoff Equation: measures osmotic pressure
 = nRTC
 = osmotic pressure
 = osmotic (reflection) coefficient
n = # of particles
R = universal gas constant
T = absolute temperature
C = concentration of the solute in mol/L
The osmotic coefficient varies with the specific solute and its concentration and the values range from 0 to 1. An osmotic coefficient of 1 indicates that the solute is not permeable to the membrane while an osmotic coefficient of 0 indicates that the solute is readily permeable to the membrane. Values between 0 and 1 represent varying degrees of permeability.

24. = nRTC Osmotic Pressure  = 1 e.g. NaCl membrane 0 <  < 1
 = 0 e.g. urea
membrane
You do not have to memorize the Van’t Hoff equation.
However, what this equation tells you is:
The more permeable the membrane is for a solute (lower ), the lower the osmotic pressure.
The less permeable the membrane is for a solute (higher ), the higher the osmotic pressure.
In addition, inherent in this equation is the concept that an increase in the number of particles (n) or an increase in the concentration of the solutes (C) results in an increase in osmotic pressure. The converse is also true, a decrease in the number of particles or a decrease in the concentration of the solutes causes a decrease in the osmotic pressure. All of the variables in this equation (, n, T, C) are proportional to the osmotic pressure ().
= nRTC

25. Osmotic Pressure: GIBBS-DONNAN EFFECT
Cells
Large impermeant anions such as proteins attract small diffusible cations to maintain electrical neutrality.
Influx of cations increases osmotic concentration of the intracellular compartment. (Donnan Effect)
To counteract the influx of water, cells utilize the Na+/K+-ATPase pump (3 Na+ out versus 2 K+ in) to limit the number of osmotically active cations.
Gibbs-Donnan Effect on cellular osmotic pressure:
The cell membrane is impermeable to some negatively charged intracellular macromolecules (e.g. proteins and organic phosphates), therefore solutes like proteins and organic phosphates make up the majority of the anions in the intracellular environment, while bicarbonate and chloride (Cl-) are the primary anions in the extracellular environment. It is important to remember that there must also be cations (positively charged solutes) to counter the charge of the anions. The primary cation in the intracellular environment is potassium (K+) and in the extracellular environment sodium (Na+) is the primary cation.
Due to the impermeability of the cell membrane to the intracellular anions they are unable to move out of the cell despite the concentration gradient that would favor their movement out of the cell. However, Na+ (primary cation of the ECF) can move into the cell down its concentration gradient. If this were to occur, there would be more osmotically active particles in the intracellular environment, consequently, water would diffuse into the cell causing it to swell (i.e. Gibbs-Donnan Effect). The cell counters the Donnan effect by pumping out a net efflux of cations via the Na-K pump (pumps 3 Na+ out of cell and 2 K+ into the cell). The active extrusion of 3 Na+ in exchange for 2 K+ is balanced by the passive influx (i.e. movement down its concentration gradient) of Na+.

26. Gibbs Donnan Effect Extracellular Intracellular Sodium + + Potassium +- + + - - -
Protein + + + + + - - +
Sodium
Channel + + + + - + + + + - - + + - + -
Na+/K+ ATPase + + + + - + + + + + + + + + -
Water - + + + + - + - + + + + + + +
3Na+ - + - +
2K+ + + + + + + + + - +
Net movement of 1 cation out

27. Osmotic Pressure: GIBBS-DONNAN EFFECT
Plasma
The impermeable substances such as albumin exerts their own osmotic effect known as colloid osmotic pressure or plasma oncotic pressure.
Albumin repels permeable anions such as Cl-.
Plasma [Cl-] is slightly lower than interstitial [Cl-].
Gibbs-Donnon Effect on plasma osmotic pressure:
In the Plasma, impermeable substances such as albumin repel other permeable anions such as Cl-. The impermeable albumin exerts its own osmotic effect known as colloid osmotic pressure or plasma oncotic pressure. Due to this effect, the Plasma [Cl-] is slightly lower than interstitial [Cl-].

28. Colloid Osmotic Pressure
Chloride -
Interstitial Fluid
Plasma -
Protein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

29. Tonicity Similar to Osmolarity
Determined by the concentration of only those particles that are impermeable to the cell membrane, therefore it is referred to as the relative concentration of impermeable solutes
Refers to the osmolal (# of particles) concentration of:
Extracellular fluid compared to Intracellular Fluid
Plasma compared to Interstitial Fluid
Isotonic
Hypertonic
Hypotonic
Tonicity (a.k.a. effective osmolality) – is similar to osmolarity, but determined by the concentration of only those particles that do not enter (penetrate) the cell (i.e. impermeable to the cell membrane). When tonicity is discussed clinically if refers to the osmolal (number of particles) concentration of the extracellular fluid compared to intracellular or the plasma compared to the interstitial fluid.
Isotonic solution – has a concentration of impermeable solutes in the extracellular environment equal to the concentration of impermeable solutes in the intracellular environment. As a result there is no net movement of water across the plasma membrane because there is no concentration gradient to foster the net movement of water.
Hypertonic solution – has a higher concentration of impermeable solutes (particles) in the extracellular environment compared to the intracellular environment. The extracellular environment has a high solute concentration and a low water concentration, while the intracellular environment has a lower solute concentration (the solute concentration in the intracellular environment is normal, however, it is lower than that of the extracellular environment) and a higher water concentration. Therefore, water will move from the area of high water concentration (inside the cell) to the area of low water concentration and the cell placed in a hypertonic solution will shrink.
Hypotonic solution – has a lower concentration of impermeable solutes (particles) in the extracellular environment compared to the intracellular environment. Therefore, the water will move from an area of high water concentration (outside the cell) to the area of lower water concentration (inside the cell), consequently a cell place in a hypotonic solution will swell. If the concentration of the solution is too low it will cause the cell to lyse (burst).
Solutions commonly used clinically to
cause osmosis (water movement) to occur

30. Tonicity: in practice Isotonic Hypotonic Hypertonic 280 mOsm/L

31. Tonicity: in practice Isotonic No Change Hypotonic Hypertonic
280 mOsm/L
280 mOsm/L
Isotonic
No Change
240 mOsm/L
360 mOsm/L
Hypotonic
Hypertonic

32. Tonicity: in practice Isotonic Isotonic No Change Hypotonic Hypertonic
280 mOsm/L
280 mOsm/L
Isotonic
Isotonic
No Change
240 mOsm/L
360 mOsm/L
Hypotonic
Hypertonic
Cell Shrinks

33. Tonicity: in practice Isotonic Isotonic No Change Hypotonic Hypertonic
280 mOsm/L
280 mOsm/L
Isotonic
Isotonic
No Change
240 mOsm/L
360 mOsm/L
Hypotonic
Hypertonic
Cell Swells
Cell Shrinks

34. Osmotic vs. Tonic
Solution of ALL molecules in a solution; including those freely permeable to the plasma membrane AND those not freely permeable
Isosmotic
Hyperosmotic
Hyposmotic
Solution of molecules that are NOT freely permeable to the plasma membrane (ex. saline solution)
Isotonic
Hypertonic
Hypotonic
Osmotic vs. Tonic:
A molecule that is freely permeable to the plasma membrane (i.e. epithelial layer of blood vessels) will be described in terms of iso, hypo, hyperosmotic, while a molecule that is not freely permeable to the plasma membrane (i.e. epithelial layer of the blood vessel) will be described as iso, hypo, hypertonic.
Isosmotic refers to a solution with an osmolarity (i.e. solute concentration) that is the same as the intracellular environment with no regard to whether the solute can penetrate the cell membrane. Isotonic is with regard to solute permeability and isosmotic is without regard to permeability.
Hyperosmotic refers to a solution with a higher osmolarity or solute concentration than intracellular environment with no regard to the permeability of the solute to the plasma membrane.
Hypo-osmotic refers to a solution with a lower extracellular solute concentration (osmolarity) compared to solute concentration inside the cell, with no regard to the solute permeability of the membrane.

35. 2) ‘Elizabeth’, the same patient as previously analyzed, was also administered 30 mL of tritiated water (3H2O) as an isotonic saline solution (300 mOsm/L) into an arm vein. Similarly, after a 2 hr equilibration period, a blood sample is withdrawn and the plasma is separated and analyzed for 3H2O. A concentration of mOsm/L 3H2O was found. Based on this information, and that found previously, what is the ICFV of the patient?
37 L
28 L
46 L
38 L

36. Volume (A) x Concentration (A) = Volume (B) x Concentration (B)
Mass of indicator injected to compartment = Mass of indicator inside the compartment
Volume (A) x Concentration (A) = Volume (B) x Concentration (B)
Volume (B) = Volume (A) x Concentration (A)
Concentration (B)
Volume A =
Concentration (A) =
Concentration (B) =
Volume (B) =
What fluid compartment does tritiated water measure?
How can you use TBW and ECFV to calculate ICFV?
TBW = ECFV + ICFV
TBW – ECFV = ICFV
What is the ICFV level of the patient?

37. Problem with ‘Elizabeth’?
Based on what we have learned today about average volumes, is there a possible problem that you may see with ‘Elizabeth’? Why did Elizabeth faint?
What can be done about it?
Important to note that the indicators used to measure fluid volumes are not perfect and that you frequently loose significant amounts of solution during the tests…for example Inulin gives a low approximation of ECFV levels. However, given the numbers you came up to today, what could possibly be a problem with Elizabeth?

38. Effect of adding various saline solutions to the extracellular fluid
2 Basic Principles
Most cells are permeable to water (via water channels/pores)
Most cells are impermeable to most solutes
Effects of adding various saline solutions to the extracellular fluid
Keep in mind these two basic principles:
Most cell membranes are permeable to water allowing water to move rapidly across the membrane, therefore, when the osmolarity of one compartment changes water movement quickly equilibrates the osmolarity. This allows the osmolarity of the intracellular and extracellular compartments to remain almost exactly equal to each other.
Cell membranes are impermeable to most solutes, therefore the number of osmoles in the extracellular or intracellular fluid generally remain constant unless solutes are directly added to or lost from the extracellular compartment.
Solutions are added to the extracellular fluid space.

39. Normal State Diagram ICFV ECFV Osmolarity Volume (L)
Normal State (70kg person)
300
200
ICFV
ECFV
Osmolarity
100 10 20 30 40
Volume (L)

40. How does the diagram change if you administer intravenous (IV) Isotonic Saline to this patient?
Volume
ECFV
ICFV
Osmolarity
Add Isotonic Saline
300
No change
200
ICFV
ECFV
Addition of an Isotonic solution to the extracellular fluid compartment:
Because the fluid is isotonic (same concentration of osmotically active particles) there is no change in the osmolarity of the extracellular fluid, therefore, there is no movement of water (i.e. osmosis) across the cell membranes. The only change that occurs is an increase in the extracellular fluid volume. The osmotically active particles in the isotonic solution (e.g. sodium chloride) remain in the extracellular compartment because the cell membrane behaves as if it were virtually impermeable to these particles.
Osmolarity
No change
100
No change 10 20 30 40
Volume

41. How does the diagram change if you administer intravenous (IV) Hypertonic Saline to this patient?
Volume
ECFV
ICFV
Osmolarity
Add Hypertonic Saline
300
200
Osmolarity
Addition of a hypertonic solution to the extracellular fluid compartment:
A hypertonic solution causes the extracellular osmolarity to increase which makes the extracellular fluid hypertonic compared to the intracellular compartment. Water will move down its concentration gradient from the intracellular compartment (high water concentration) to the extracellular compartment (hypertonic – low water concentration, high solute concentration). Therefore, the increase in extracellular osmolarity leads to a decrease in intracellular fluid volume and an increase in extracellular fluid volume. The decrease in volume of the intracellular compartment causes the intracellular osmolarity to increase. Water will continue to move out of the intracellular compartment into the extracellular compartment until both compartments have an equal osmolarity.
ICFV
ECFV
100 10 20 30 40
Volume

42. How does the diagram change if you administer intravenous (IV) Hypotonic Saline to this patient?
Volume
ECFV
ICFV
Osmolarity
Add Hypotonic Saline
300
200
Addition of a hypotonic solution to the extracellular environment:
A hypotonic solution causes the extracellular osmolarity to decrease resulting in an extracellular fluid that is hypotonic to the intracellular fluid. Therefore, water from the extracellular fluid diffuses into the intracellular compartment until both compartments have the same osmolarity. As a result both the intracellular and extracellular volumes are increased, however, the intracellular volume will be increased to a greater extent. This also causes both compartment to experience a decrease in their osmolarity.
Osmolarity
ICFV
ECFV
100 10 20 30 40
Volume

43. Problem with ‘Elizabeth’?
Based on what we have learned today about average volumes, is there a possible problem that you may see with ‘Elizabeth’? Why did Elizabeth faint?
What could be done about it?
Important to note that the indicators used to measure fluid volumes are not perfect and that you frequently loose significant amounts of solution during the tests…for example Inulin gives a low approximation of ECFV levels. However, given the numbers you came up to today, what could possibly be a problem with Elizabeth?

44. How does the diagram change if you administer intravenous (IV) Isotonic Saline to this patient?
Volume
ECFV
ICFV
Osmolarity
Add Isotonic Saline
300
No change
200
ICFV
ECFV
Osmolarity
No change
100
No change 10 20 30 40
Volume

45. Clinical Correlation Mannitol Stroke Edema
Neurological procedures and cerbrovascular accidents often result in the accumulation of interstitial fluid in the brain (i.e., edema) and swelling of neurons. Because the brain is enclosed within the skull, edema can raise intracrainial pressure and thereby disrupt neuronal function, eventually leading to coma and death. The blood-brain barrier, which separates the cerebrospinal fluid and brain interstitial fluid from blood, is freely permeable to water but not to most other substances. As a result, excess fluid in brain tissue can be removed by imposing an osmotic gradient across the blood-brain barrier. Mannitol can be used for this purpose. Mannitol is a sugar (molecular weight of 182 g/mol) that does not readily cross the blood-brain barrier and membranes of cells (neurons, as well as other cells of the body). Therefore, mannitol is an effective osmole, and intravenous infusion results in the movement of fluid from the brain tissue by osmosis.
Edema
Pulls water out of brain tissue through osmosis
Coma and possibly death

46. Thought Questions Open-ended questions to help you study the material
How can the concentration of water in a solution be decreased?
If two solutions with different osmolarities are separated by a water-permeable membrane, why will a change occur in the volumes of the two compartments if the membrane is impermeable to the solutes, but no change in volume will occur if the membrane is permeable to solute?
What is the approximate osmolarity of the extracellular fluid? Of the intracellular fluid?
What will happen to cell volume if a cell is placed in each of the following solutions? Are these solutions isotonic, hypertonic, hypotonic, isoosmotic, hyperosmotic, or hypoosmotic?
150 mM NaCl (nonpenetrating) and 100 mM Urea (penetrating)
100 mM NaCl and 150 mM Urea
200mM NaCl and 100 mM Urea
100 mM NaCl and 50 mM Urea
Under what conditions could a hyperosmotic solution be isotonic?
Could a solution be hypoosmotic and isotonic?
Given the following solutions, which has the lowest water concentration? Which two have the same osmolarity?
Solution Glucose Urea NaCl CaCl2
A 20 mM 30 mM 150 mM 10 mM
B 10 mM 100 mM 20 mM 50 mM
C mM 200 mM 10 mM 20 MM
D 30 mM 10 mM 60 mM 100 mM
If you took a cell with a normal ICF osmolarity of 300 mM/L (nonpenetrating solutes) and transferred it to an ECF containing only 200 mM/L of nonpenetrating solutes, what would happen?
Into two beakers containing 1 liter of pure water you add: 2 moles of glucose to beaker A, and 1 mole of salt (NaCl) to beaker B. What is the osmolarity of each solution? If you then pour the contents of both beakers into beaker C, what would be the osmolarity of that beaker?
Assume that a membrane separating two compartments is permeable to urea but not permeable to NaCl. If compartment 1 contains 200 mmol/L of NaCl and 100 mmol/L of urea, and compartment 2 contains 100 mmol/L of NaCl and 300 mMol/L of urea, which compartment will have increased volume when osmotic equilibrium is reached?
Initially, a cell is floating in a 1 liter flask full of ECF, and it is in osmotic equilibrium at 300 mM/L. If you add a nonpenetrating solute to the solution in the flask, will the variables below increase, decrease, or stay the same? How about if you instead added a penetrating solute to the solution in the flask, how will the variables change? Explain.
Osmolarity of the ECF
Volume of the ICF
Osmolarity of the ICF
The terms isotonic, hypertonic, and hypotonic are sometimes used to describe extracellular fluid; define them, and differentiate between those terms and isoosmotic, hyperosmotic, and hypoosmotic.
What changes would occur to your intracellular and extracellular compartments if a person were to drink 4L of distilled water? What if they were to drink 1L of a 300 mosM sports beverage?
BONUS: By what mechanism does the active transport of sodium lead to the osmotic flow of water across an epithelium?

47. Practice Problems Multiple-choice questions to help you study the material
Turningpoint Question Answers: 2 3
1. A membrane separating two compartments is permeable to urea but not permeable to NaCl (normal physiological conditions). If compartment 1 contains 200 mmol/L of NaCl and 100 mmol/L of urea, and compartment 2 contains 100 mmol/L of NaCl and 300 mmol/L of urea, which of the following will occur?
Practice Problems:
A. Urea will move from compartment 1 to compartment 2
B. NaCl will move from compartment 1 to compartment 2
E. Water and Urea will move from compartment 2 to compartment 1
D. NaCl and Urea will move from compartment 2 to compartment 1
C. Water will move from compartment 1 to compartment 2
2. If a small amount of urea were added to an isoosmotic saline solution containing cells, what would be the result?
C. The cells would swell and remain that way
B. The cells would first shrink, but then be restored to normal volume after a brief period of time
A. The cells would shrink and remain that way
E. The urea would have no effect, even transiently
D. The cells would first swell, but then be restored to normal volume after a brief period of time
B mM NaCl
A mM CaCl2
3. A red blood cell will swell the most when it is placed in a solution containing which of the following?
C mM KCl
4. Assuming complete dissociation of all solutes, which of the following solutions would be hyperosmotic to 1 mM NaCl?
E mM Mannitol
D mM Urea
B mM glucose
A. 1 mM glucose
E. 1 mM KCl
D. 1 mM sucrose
C. 1 mM CaCl2
5. A red blood cell will shrink the most when its placed in which of the following solutions?
C mM potassium chloride
B mM sodium chloride
A mM calcium chloride
E mM mannitol
D mM urea
B. It can only cross the membrane through the lipid bilayer
A. It will not permeate the membrane
6. Which of the following statements best characterizes a molecule whose reflection coefficient to a membrane is zero?
C. It causes water to flow across the membrane
7. In error, a patient is infused with large volumes of a solution that causes large increases in volume and thereby lysis of his red blood cells (RBCs). The solution was most likely which of the following?
E. It can only cross the membrane by a carrier-mediated transport
D. It is as diffusible through the membrane as water
B. Isotonic KCl
A. Isotonic NaCl
E. Hypertonic KCl
D. Hypotonic NaCl
C. Hypertonic NaCl
8. Solutions A and B are separated by a semipermeable membrane. Solution A contains 1mM sucrose and 1mM urea. Solution B contains 1mM sucrose. The reflection coefficient for sucrose is one and the reflection coefficient for urea is zero. Which of the following statements regarding these solutions is correct?
C. Solution A and B are isoosmotic
B. Solution A is hypertonic with respect to solution B
A. Solution A is hypotonic with respect to solution B
E. Solution A is hypoosmotic with respect to solution B, and the solutions are isotonic
D. Solution A is hyperosmotic with respect to solution B, and the solutions are isotonic
9. Assuming complete dissociation of all solutes, which of the following solutions would be hyperosmotic to 1 mM NaCl?
B mM glucose
10. As part of an experimental study, a volunteer agrees to have 10g of mannitol injected intravenously. After sufficient time for equilibration, blood is drawn, and the concentration of mannitol in the plasma is found to be 65 mg/100mL. Urinalysis reveals that 10% of the mannitol had been excreted into the urine during this time period. What is the approximate extracellular fluid volume for this volunteer?
A. 10 L
D. 30 L
C. 22 L
B. 15 L
11. In a person weighing 75 kg, the volumes of total body water (TBW), intracellular fluid, and extracellular fluid are approximately what respectively?
E. 42 L
C. 45L, 35L, 10L
B. 45L, 30L, 15L
A. 40L, 30L, 10L
E. 50L, 35L, 15L
D. 50L, 25L, 25L
Correct answers: D B E C