Effect of Isotonic Hypertonic and Hypotonic Solutions on Red Blood Cells Peer Reviewed Articls
INTRODUCTION
Truly understanding osmolarity and tonicity is ane of the more than challenging endeavors undertaken by students of the natural sciences.1 In 2012, Cheng and Durairajanayagam (3) reported that 41% of students participating in the International Intermedical School Physiology Quiz were unable to predict correctly what would happen to the book of an erythrocyte when placed in an isosmotic solution of urea. And then how can we teach osmolarity and tonicity more effectively? One strategy for improving student agreement is to allow them feel the effects of different solutions on erythrocyte hemolysis, a archetype experiment in physiology [encounter, for example, the recent Sourcebook paper by Goodhead and MacMillan (v)]. But teaching osmolarity and tonicity in a classroom setting tin be tough. Notwithstanding, mastering these concepts is essential for those interested in pursuing careers in medicine, physiology research, and other related fields. For example, health professionals need to understand that not all isosmotic intravenous (Four) fluids are isotonic (xv).
The arroyo we present here has been simplified for teaching osmolarity and tonicity to undergraduate health professions students. Information technology is specifically designed to assist them in the proper selection of Four fluid therapy in clinical settings. As with whatever simplification of a circuitous physiological topic, the nuances of the topic are glossed over to facilitate student comprehension. The option of appropriate IV solutions is non an exact science, and information technology does not require understanding or application of the van't Hoff equation or reflection coefficients (1, 4, 18). The clinician needs to choose Iv solutions based on their ability to increase effective circulating book, to expand or contract jail cell volume, and to affect osmolarity. Having the skills to calculate approximations of these changes using broad assumptions helps preclude iatrogenic errors in clinical settings.
The difficulties students and teachers alike encounter while learning osmolarity and tonicity are at least partially accounted for by the differing approaches taken by many physiology texts. A lack of consistent terminology and detail often leaves room for confusion and misconceptions (Table 1).
| Michael D. Johnson, Human Biology: Concept and Current Bug (8th Edition) ( nine ). "Tonicity refers to the relative concentrations of solutes in two fluids. Extracellular fluid that is isotonic has the same solute concentration as the intracellular fluid. When cells are placed in a hypertonic solution, one with a concentration of solutes higher than the intracellular fluid, water diffuses out of the cells and the cells shrink." |
| Cindy L. Stanfield, William J. Germann, Principles of Human Physiology (third Edition) ( 17 ). "Whereas a solution'due south osmolarity is based solely on its full solute concentration, its tonicity is adamant past how information technology affects cell book, which depends not only on the solute concentration but as well on the solute permeability of cell membranes. A solution is said to be isotonic when it does not modify cell book. This refers to the cell's last volume. Under certain conditions a cell may compress or swell initially, even if the solution is isotonic." |
| Richard W. Hill, Gordon A. Wyse, Margaret Anderson, Brute Physiology ( 7 ). "If a jail cell membrane or epithelium is impermeable to a solute and the solute is more concentrated on one side than the other, the solute creates a persistent difference of osmotic pressure level across the prison cell membrane or epithelium." |
| Arthur C. Guyton, John E. Hall, Medical Physiology (10th Edition) ( 6 ). "If a cell is placed into a hypotonic solution that has a lower concentration of impermeant solutes, water will diffuse into the cell, causing information technology to swell; h2o volition continue to diffuse into the cell, diluting the intracellular fluid while as well concentrating the extracellular fluid until both solutions take most the same osmolarity." |
| Walter F. Boron, Emile Boulpaep, Medical Physiology: A Cellular and Molecular Arroyo (2nd Edition) ( ii ). "The difference between furnishings of mannitol and urea on the concluding cell book illustrates the need to distinguish betwixt total osmolality and effective osmolality (also known as tonicity)." |
| Kenneth S. Saladin, Anatomy & Physiology: The Unity of Form and Function (fourth Edition) ( xiv ). "Tonicity is the power of a solution to affect the fluid volume and force per unit area in a cell. If a solute cannot pass through a plasma membrane, merely remains more concentrated on ane side of the membrane than on the other, it causes osmosis." |
Because osmolarity and tonicity are fundamental and ofttimes confusing, nosotros thought it important to review the topic for instructors, every bit well as outline some teaching approaches nosotros accept found effective in the classroom. We start with our definitions of the 2 terms, so give you a hazard to check your understanding of the basics of osmolarity and tonicity through a few brusque questions and a cursory review. We so present several aspects of this topic that our students find well-nigh disruptive. Finally, we outline our approach to education osmolarity and tonicity and give examples of questions that we have used to develop our students' understanding of the topic.
Clarifying definitions.
The varying presentations of tonicity in Table 1 are tricky to understand at first glance. Permit us first by clarifying the definitions of osmolarity and tonicity. Our definitions apply to biologically relevant situations simply, and so we are ignoring solutions such as concentrated acids and bases. One note on units: in this paper, milliosmole is expressed in units of mosmol, whereas milliosmolar (mosmol/l) is expressed as mosM. (Note the capitalizations.)
Osmolarity is a measure of the concentration of osmotically active particles in a solution. It is sometimes chosen a "colligative" property of the solution past chemists because it depends on the number of particles in a volume of solution rather than the identity of the particles. Osmolarity is closely related to molarity, a concept that most students learn in introductory chemistry. Ane mole of whatsoever substance has Avogadro's number (half dozen.02 × 1023) of particles. Molarity (M) of a solution is an expression of concentration, with one mole of solute per liter of solution.
Yet, the molarity of a solution is not always the same as the solution's osmolarity. This is because some solutes, such every bit ionic compounds similar NaCl, dissociate into split up particles (eastward.chiliad., Na+ and Cl−) when dissolved in water. For an ideal solution, the solution's osmolarity equals its molarity times a dissociation gene, the number of ions formed from i particle of the solute when placed in water:
Every solute has a dissociation factor, or osmotic coefficient (5). For whatever substance that does not dissociate into ions when dissolved in water, such as glucose or urea, the dissociation factor is 1 and the solution's osmolarity is the same every bit its molarity (e.g., 1 M glucose = 1 osM glucose).
The dissociation factor for an platonic solution of NaCl is 2, given that the chemical compound is made up of two ions. However, many ionizable substances do not completely dissociate in h2o. For instance, the particles formed when NaCl dissolves in water include Na+, Cl−, and some undissociated NaCl. The calculated dissociation factor for NaCl at 25°C is one.8 particles per NaCl (13). This means that a 1 mM solution of NaCl would be a i.eight mosM solution. We adopt to apply the term "dissociation factor" when teaching, considering students understand the relationship betwixt dissociation of ionic compounds and osmolarity.
You can measure the osmolarity of a solution using a machine called an osmometer. The most common commercial osmometers measure either freezing betoken depression or vapor pressure of a sample of solution (12, sixteen). An educational activity for student laboratories with access to an osmometer is to have the students calculate how to make a 150 mM solution of NaCl, arrive using good laboratory technique, then measure out the osmolarity of the resultant solution. (Answer: Use 8.775 g NaCl/liter of solution. Predicted osmolarity using the dissociation gene of i.eight would be 270 mosM.)
Osmolarity is an of import property of any biological solution. But just as knowing the molarity of a solution does not tell us its osmolarity, knowing the osmolarity of a solution does not tell us its tonicity. This is because tonicity is a term that requires two compartments: the solution being described and a cell. In add-on to knowing the concentration (osmolarity) of the solution, you must know the composition of the solution: what the solutes are and whether or not they can enter the cell. Solutes that enter a jail cell by whatsoever means (simple improvidence, protein-mediated transport, and and so on) are said to exist "penetrating" solutes. Solutes that do not enter the prison cell are said to be "nonpenetrating" solutes.
The tonicity of a solution predicts the effect of the solution on prison cell volume at equilibrium and depends on the relative concentrations of nonpenetrating solutes in the prison cell and the solution. At equilibrium, water movement into the compartment with the higher starting concentration of nonpenetrating solutes volition increase that compartment's book. Net h2o movement stops when the concentrations of nonpenetrating solutes in the cell and solution are equal.
The cell's book change in response to the solution tells us the tonicity of the solution:
| • | If jail cell volume at equilibrium has increased, the solution is said to be hypotonic to the cell. | ||||
| • | If prison cell volume at equilibrium has decreased, the solution is said to be hypertonic to the cell. | ||||
| • | If cell volume at equilibrium has not inverse, the solution is said to be isotonic to the prison cell. | ||||
In other words,
| • | If the cell has a college initial concentration of nonpenetrating solutes than the solution, at equilibrium water will have moved into the cell. The solution is hypotonic to the cell. | ||||
| • | If the solution has a higher initial concentration of nonpenetrating solutes than the cell, at equilibrium water will take moved out of the cell. The solution is hypertonic to the jail cell. | ||||
| • | If the jail cell and solution have equal concentrations of nonpenetrating solutes, at equilibrium in that location volition be no internet movement of water. The solution is isotonic to the cell. | ||||
Because clinicians are usually selecting 4 fluids based on their result on intracellular fluid (ICF) and extracellular fluid (ECF) volumes, describing tonicity in terms of cell volume change is the about useful approach for pedagogy health professions students.
Freely penetrating solutes in a solution tin can exist ignored for the purpose of determining tonicity. They will contribute to the osmolarity of the solution simply distribute throughout the cell-solution system every bit if the cell membrane were non nowadays. They, therefore, do not ultimately contribute to water movement between compartments. This is discussed more below.
There are several of import points to go on in mind about tonicity:
| • | Tonicity has no units like mM or mosM. It is a comparative term that predicts changes in cell volume at equilibrium afterwards exposure of the prison cell to a solution. | ||||
| • | Past convention, we speak of the tonicity of a solution relative to a given cell. Nosotros never speak of the cell being hypo-/hyper-/isotonic to the solution. | ||||
| • | With one exception, knowing but the osmolarity of the solution tells you lot aught about the tonicity of the solution. The exception is that all hyposmotic solutions are hypotonic. | ||||
| • | Tonicity describes what has happened to cell volume at equilibrium but does non tell you what happens to cell book on the fashion to equilibrium. We will return to this concept afterward. | ||||
| • | It is important to be articulate about the cell that is the frame of reference when discussing tonicity. Solutes that can enter one type of cell may not exist able to enter a different type of cell. Ane instance where this is true involves the disaccharide sucrose (tabular array sugar). Mammalian cells do not accept transporters for sucrose, so sucrose is a nonpenetrating solute for mammals. Simply plant cells do have sucrose transporters, making sucrose a penetrating solute for plants. | ||||
When teaching health professions students, we presume that the idealized cell is a typical human cell, and nosotros utilise urea and NaCl as our prototypical solutes. Urea is the classic example of a penetrating solute. Information technology freely crosses most jail cell membranes, especially the erythrocyte membrane, via diffusion through urea transporters and (to a small degree) through the phospholipid bilayer (x). NaCl is the primary nonpenetrating solute of the ECF. Its ions acquit a accuse, making information technology hard for them to freely laissez passer through the phospholipid bilayer of the jail cell membrane. Equally noted later, while some Na+ does leak into cells, Na+-Thousand+-ATPases pump Na+ out at roughly the aforementioned rate, making this solute functionally impermeable. We also make the assumption that all solutes in the prison cell are nonpenetrating and volition non get out the prison cell equally long as the jail cell membrane is intact.
Test your understanding.
Beneath is a gear up of basic questions meant to bank check your understanding of osmolarity and tonicity. For each of the following questions, there are v assumptions nosotros volition brand to simplify the thing:
| one. | Our idealized cell has an internal concentration of 300 mosM. (We volition go on this assumption through the rest of the text, unless otherwise noted.) | ||||
| 2. | The solutes present in the prison cell are assumed to be nonpenetrating and cannot exit the cell. | ||||
| 3. | NaCl is a functionally nonpenetrating solute that behaves as if it cannot cross the cell membrane. (As noted above, in reality, Na+ that leaks into the cell is removed past the Na+-Grand+-ATPase at a rate that closely matches the rate of leakage in.) | ||||
| 4. | Urea is a freely penetrating solute that easily crosses the cell membrane. Because of this, urea volition distribute throughout body compartments until its concentration is equal in all compartments. | ||||
| 5. | Water freely crosses all jail cell membranes, dividing the intracellular and extracellular compartments. | ||||
Try to answer these questions based on your understanding of osmolarity and tonicity.
| ane. | You place a cell with an internal concentration of 300 mosM in a 400 mosM solution. What happens to the volume and osmolarity of the cell at equilibrium? Answer: The showtime part of this question asks about cell volume and, therefore, addresses tonicity of the solution. Tonicity depends on the relative concentrations of nonpenetrating solutes in the solution and cell. In that location is no way to tell what will happen to the cell'southward volume from this question every bit written, because all you are told is that the solution is 400 mosM. To predict tonicity, you must know if the solution solutes are penetrating or nonpenetrating, and the question did not give you that information. The 2d part of the question asks about the osmolarity of the prison cell at equilibrium. This is a question near concentration, not cell volume. The 400 mosM solution is hyperosmotic to our arcadian cell. Thus you can predict that the cell will become more than concentrated, either because water moves out into the solution, or because penetrating solute moves into the cell. Y'all do not know the solutes in the solution, and you practise not know the relative volumes of the prison cell and the solution. Without this data, you lot cannot say what the actual osmolarity volition be at equilibrium. You tin simply say that osmolarity has increased. | ||||||||||||||||
| 2. | Which would take a greater effect on the volume of a jail cell at equilibrium: solution A equanimous of 300 mosM NaCl and 200 mosM urea, or solution B composed of 200 mosM NaCl and 300 mosM urea? Respond: This question asks about cell volume and is, therefore, addressing tonicity, which depends on the concentrations of nonpenetrating solutes. The nonpenetrating solute in these solutions is NaCl, so we compare the NaCl concentrations in the solution to the prison cell's osmolarity, assumed to exist due but to nonpenetrating solutes. At equilibrium, water will have moved to equalize the nonpenetrating solute concentrations. We can ignore the urea, which is penetrating and will not cause water to shift between compartments once equilibrium is reached.
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| iii. | Given the same solutions as in the previous question (300 mosM NaCl/200 mosM urea or 200 mosM NaCl/300 mosM urea), which solution would take a greater effect on the osmolarity of the cell at equilibrium? Answer: This question, like the 2nd function of question 1, asks nearly the osmolarity of the jail cell at equilibrium. If you lot place a 300 mosM jail cell in a 500 mosM solution, you know that the final concentration of the prison cell volition increase. Considering solutions A and B have equal osmolarities, y'all know that they will take the same effect on the concluding concentration of the cell. | ||||||||||||||||
One important take-home message from these three questions is that a student should be able to look at the concentrations of penetrating and nonpenetrating solutes in a solution, effigy out its relative osmolarity and tonicity compared with a jail cell, and recognize how the solution will bear upon the volume and osmolarity of a cell at equilibrium. For case, given our hypothetical 300 mosM prison cell, describe the osmolarity and tonicity of solution C with 200 mosM NaCl and 500 mosM urea. The answer is that solution C at 700 mosM is hyperosmotic just hypotonic, because the solution has 200 mosM nonpenetrating solutes vs. 300 mosM in the cell.
How tin a hypotonic solution like C increase the osmolarity of the cell? If the cell takes up h2o and swells, you might expect that the prison cell'south internal concentration would decrease (concentration = solute/book). However, this explanation fails to account for the penetrating solute urea, which is also able to become into the prison cell. Urea, spreading by diffusion throughout the solution and cell, volition ultimately add together to the overall solute in the jail cell. Thus the cell in solution C will peachy, just also become more full-bodied due to movement of urea into the cell downwardly its concentration gradient.
An important concept to emphasize with students is that the cell'south last concentration will follow that of the osmolarity of the solution, regardless of the solution'south effect on cell volume. If you add a cell to a hyperosmotic solution, its osmolarity volition increase, regardless of whether the jail cell swells, shrinks, or stays the same volume. The cell'south concentration volition always follow that of the solution. Placing a cell in a hyposmotic solution will cause the cell'south concentration to decrease. Placing a jail cell in an isosmotic solution will not change the jail cell'due south concentration.
Student points of defoliation.
Because that understanding osmolarity and tonicity seem to crave more effort on the part of the student, we thought it important to uncover what aspects students perceive equally most complicated or confusing. We asked the students who had completed a course in Animal Physiology at the Academy of Belgrade (Serbia) during the last 4 academic years (2014–2017) to submit a short essay explaining what they found most challenging nearly osmolarity and tonicity and what in-class activities proved most useful to them. After analyzing essays submitted past over lxxx students, we found that confusion stems from ane or more of the three main problems:
| 1. | Distinguishing tonicity from osmolarity | ||||
| 2. | Determining tonicity based on the solution'due south limerick | ||||
| 3. | Distinguishing units expressing amount vs. concentration of solutes (eastward.g., osmol vs. osmol/fifty or osM) | ||||
Given these very mutual points of confusion, we designed a fix of instruction activities that enable students to fully empathize and utilize the concepts of osmolarity and tonicity.
A starting bespeak in the classroom.
Beginning with clear and concise definitions of the terms establishes a distinction between osmolarity and tonicity. Whereas about students are quick to sympathize that osmolarity is a concentration, nigh of them struggle with differentiating osmolarity from tonicity. Osmolarity and tonicity are certainly related, but they are not the same matter as noted above, and this should exist stressed in the classroom. According to our students' reports, they found it helpful to call up that tonicity is defined by the effect a solution has on jail cell volume at equilibrium, and that tonicity is determined by comparison the concentrations of nonpenetrating solutes in the solution and the prison cell. In these essays, students also mentioned they found it particularly difficult to recall that tonicity describes how the solution affects cell book at equilibrium. Furthermore, the fact that a hyperosmotic solution can be hypo-, iso-, or hypertonic proved perplexing to many of them.
One of the tasks that seems very effective at clarifying these two dirty points is request students to graph the time-dependent effects of three hyperosmotic solutions, all with the same concentration of 400 mosM, on the volume of an idealized prison cell:
| • | Solution A contains 200 mosM NaCl and 200 mosM urea (400 mosM solute) | ||||
| • | Solution B contains 300 mosM NaCl and 100 mosM urea (400 mosM solute) | ||||
| • | Solution C contains 400 mosM NaCl (400 mosM solute) | ||||
Nosotros ask students to draw the cell'south change in volume from the moment of placing the cell in the solution until osmotic equilibrium is reached. The complexity of this task is reflected in the fact that, although the three solutions have the same osmolarity (400 mosM), they exhibit different tonicities due to varying concentrations of nonpenetrating and penetrating solutes (Fig. i).
Fig. 1.The time-dependent furnishings of 3 hyperosmotic solutions of dissimilar tonicities. Solution A contains 200 mosM NaCl and 200 mosM urea (400 mosM solute); solution B has 300 mosM NaCl and 100 mosM urea (again, 400 mosM solute); and solution C is 400 mosM NaCl (once over again, 400 mosM solute). Because that water molecules motility faster across the membrane than solutes, all of the solutions will initially cause a cell to compress. However, once the osmotic equilibrium is reached, the cell's volume volition increase in hypotonic solution A, stay the same in isotonic solution B, and decrease in hypertonic solution C.
To simplify the affair, students are instructed to assume that the cell's osmolarity is 300 mosM and that the cytosol only contains nonpenetrating solutes. To successfully complete this task, students take to demonstrate that they understand the following points:
| 1. | The solution's tonicity depends on the solution's concentration of nonpenetrating solutes (e.g., NaCl) relative to that in the prison cell, not on the full osmolarity of the solution. | ||||
| two. | Tonicity provides the data on how the solution affects the jail cell'due south volume in one case osmotic equilibrium is reached; that is, when the osmolarities of the solution and the cytosol become equal. | ||||
| 3. | H2o molecules move faster through the cell membrane than particles of solute do. Therefore, a cell placed in a hyperosmotic solution will always compress initially, regardless of the solution'southward tonicity. | ||||
This blazon of trouble-solving can be equally equally constructive when presented in opposite. For example, students can be asked to predict a solution'southward osmolarity and tonicity based on the effect it has on the volume of a cell, as depicted in a graph (Fig. ii). To successfully complete these assignments, students must simultaneously consider both the osmolarity and tonicity of a solution.
Fig. 2.The task that requires students to predict a solution's osmolarity and tonicity based on how the solution affects the cell's volume before reaching osmotic equilibrium. Considering that the jail cell's volume did not modify after beingness placed in solution A, this solution is isosmotic and isotonic. In the 2d scenario, the jail cell's volume increased one time the osmotic equilibrium was reached, indicating the solution is hypotonic. Since the cell's volume was continuously increasing, solution B could be either hypo- or isosmotic.
Osmolarity and tonicity in context.
To ensure that students completely understand the significance of osmolarity and tonicity in the light of systemic homeostasis, a lecturer might design tasks that put these processes in dissimilar physiological contexts while giving them relevance. For example, it is essential that pre-health professions students understand that the advisable selection of 4 or oral fluid therapy depends on the particular clinical scenario they will encounter.
An in-class activity that lends itself well to showing how different solutions influence osmotic concentration and compartment volumes is to have students calculate numerical values after calculation a solution to the body. To successfully complete such an assignment, nosotros inquire our students to do the following:
| 1. | Compare the solution'due south osmolarity to the osmolarity of the body or jail cell. | ||||||||||||||||||||||||||||
| two. | Decide the solution'due south tonicity. | ||||||||||||||||||||||||||||
| three. | Predict how adding this solution will touch the volume and osmolarity of body fluids.
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| 4. | Perform calculations that mathematically demonstrate how the solution affects the torso volumes and osmolarity (described more below). | ||||||||||||||||||||||||||||
| v. | Cheque whether their theoretical predictions (step 3) are in accordance with their calculations (step iv). | ||||||||||||||||||||||||||||
Let us take a solution and add it to an idealized body with simple volumes and concentration to piece of work through these steps. For our example, students are asked to describe the effects of adding 1 liter of a 100 mosM NaCl and 300 mosM urea solution to a person whose normal total body volume is 30 liters, with two-thirds of body fluid volume (twenty liters) inside the cells. We work through each of the to a higher place steps in sequence.
| one. | The solution is hyperosmotic to the torso considering the solution has a concentration of 400 mosM, and our idealized body has a concentration of 300 mosM. | ||||
| two. | The solution is hypotonic to the body because the solution has a concentration of nonpenetrating solutes of 100 mosM, and the concentration of nonpenetrating solutes in the body is 300 mosM. | ||||
| 3. | The concentration of all body compartments volition increment, because the solution is hyperosmotic. This is of import for the student to empathise. The body compartments are in osmotic equilibrium. Thus increasing the concentration of the body with a hyperosmotic solution will increase the concentration of all compartments. | ||||
| four. | Total body book volition increase, because we are adding volume to the body. | ||||
| 5. | ECF volume volition increase because nosotros are adding volume to the body and because the NaCl will remain in the ECF, keeping some water with it. ECF osmolarity goes up considering we added a hyperosmotic solution to the body. | ||||
| 6. | ICF volume will increment considering the solution is hypotonic. ICF osmolarity as well goes upwards considering we added a hyperosmotic solution to the body and it contained a penetrating solute. | ||||
It might initially exist hard for students to think their way through steps 1–3 to a higher place, equally some of the predictions seem at outset to make no sense. How can the ICF concentration increase if the book of the ICF is increasing? Mathematically describing how the body compartments are affected (stride 4) can demonstrate these processes in a concrete manner. Nosotros call the series of tables used for the calculations in stride four "box problems," and they are shown in Tables two–vii.
The first step is filling out a chart that contains the initial values of an idealized and simplified body (Table two). Columns include full torso fluid (TBF) every bit well every bit the ECF and ICF compartments. Each column contains rows for solute corporeality, compartment volume, and compartment osmolarity. Unknown values can be calculated by using solute corporeality/volume = osmolarity.
| Starting Table | Total Body Fluid (TBF) | Extracellular Fluid (ECF) | Intracellular Fluid (ICF) |
|---|---|---|---|
| Amount of solutes (mosmol) | 30 liters × 300 mosmol/l = nine,000 mosmol | 10 liters × 300 mosmol/50 = 3,000 mosmol | 20 liters × 300 mosmol/l = 6,000 mosmol |
| Volume (liters) | thirty liters | i/iii × 30 liters = x liters | 2/iii × xxx liters = 20 liters |
| Osmolarity (mosM = mosmol/l) | 300 mosM | 300 mosM | 300 mosM |
The starting tabular array uses volume and osmolarity values as they are before the solution is added. Our example assumes that normal total torso volume is thirty liters, and that two-thirds of the volume (20 liters) are within the cells. The initial trunk osmolarity is 300 mosM. For students, using numbers like 300 mosM and volumes of 10, xx, and 30 liters simplifies the piece of work.
As students fill up in the chart, they must differentiate between the amount of solute and the concentration of solute besides as the units for amount (mosmol) and concentration (mosM). Students are instructed to e'er fill in the TBF column get-go.
The problem asks the effect of administering 1 liter of a solution composed of 100 mosM NaCl and 300 mosM urea. In the second step (Tabular array 3), students add together the volume of the solution and the amount (mosmoles) of nonpenetrating solute in the solution to the starting conditions. (The milliosmoles of urea are held in reserve and added subsequently, once h2o has redistributed.) This step requires students to summate the amount of solute to add, based on the solution's concentration and volume. In this example, i liter of a solution with 100 mosM NaCl was added. Students calculate that ane liter of 100 mosmol/50 NaCl = 100 mosmol NaCl.
| Add together Volume and Nonpenetrating Solutes | Full Body Fluid (TBF) | Extracellular Fluid (ECF) | Intracellular Fluid (ICF) |
|---|---|---|---|
| Solutes (mosmol) | nine,000 + 100 = 9,100 | ||
| Volume (liters) | 30 + 1 = 31 | ||
| Osmolarity (mosM) |
Students use the changed book and solute values to calculate the new TBF osmolarity (Tabular array iv). They utilise their understanding of osmotic equilibrium in the torso past recognizing that the calculated total torso osmolarity will be the same every bit the osmolarities of both the ECF and ICF compartments.
| Summate New Osmolarity | Total Body Fluid (TBF) | Extracellular Fluid (ECF) | Intracellular Fluid (ICF) |
|---|---|---|---|
| Solutes (mosmol) | 9,100 | ||
| Volume (liters) | 31 | ||
| Osmolarity (mosM) | ix,100/31 = 293.five | 293.5 | 293.5 |
Students and so need to figure out that the added 100 mosmol of NaCl stay entirely in the ECF because NaCl is nonpenetrating and cannot enter cells (Tabular array 5). The amount of solute in the ICF remains unchanged at this bespeak in the calculations.
| Add together NaCl to ECF | Full Body Fluid (TBF) | Extracellular Fluid (ECF) | Intracellular Fluid (ICF) |
|---|---|---|---|
| Solutes (mosmol) | 9,100 | 3,000 + 100 = iii,100 | vi,000 |
| Volume (liters) | 31 | ||
| Osmolarity (mosM) | 293.5 | 293.v | 293.five |
The revised values for the ECF and ICF volumes can now be calculated by dividing the corporeality of solute each compartment contains by its osmolarity (Table half dozen).
| Calculate New ECF and ICF Volumes | Total Body Fluid (TBF) | Extracellular Fluid (ECF) | Intracellular Fluid (ICF) |
|---|---|---|---|
| Solutes (mosmol) | nine,100 | three,100 | vi,000 |
| Volume (liters) | 31 | 3,100/293.five = 10.six liters | half dozen,000/293.five = 20.iv liters |
| Osmolarity (mosM) | 293.5 | 293.5 | 293.five |
The concluding function of the trouble is to add together the amount of the penetrating solute independent in the solution to the torso. In this case, the 1 liter of the solution had 300 mosM urea, in addition to the 100 mosM NaCl. This means that 300 mosmol of urea (300 mosmol/50 × one liter) were added to the torso. Because urea is a freely penetrating solute, it volition non cause water to shift betwixt the ECF and ICF compartments. The urea contributes to the osmolarity of the solution but not its tonicity.
Students first add the urea to the total body solute prison cell and calculate the new total body osmolarity (Table 7). The total trunk osmolarity can one time over again be copied to the ECF and ICF columns because the ii compartments are always in a state of osmotic equilibrium. Considering that the entire volume of the solution was already added in the preceding pace, the values for the TBF, ECF, and ICF volume accept non changed and may be copied from the previous nautical chart.
| Add Urea to Get Terminal Osmolarity | Full Trunk Fluid (TBF) | Extracellular Fluid (ECF) | Intracellular Fluid (ICF) |
|---|---|---|---|
| Solutes (mosmol) | ix,100 + 300 = nine,400 | ||
| Volume (liters) | 31 | 10.6 | twenty.4 |
| Osmolarity (mosM) | 9,400/31 = 303.2 | 303.2 | 303.ii |
Finally, based on the ECF and ICF volumes and new osmolarity, the amount of solutes contained in each of the two subcompartments tin exist calculated. If concentration = corporeality/volume, then amount = volume × concentration. Depending on the student population, this final calculation can exist skipped considering clinically nosotros are usually only interested in the compartment volumes and their osmolarity. Once the tables are complete, students check whether their theoretical predictions (step iii) are in line with the calculations (step 4). The predictions were that osmolarity of all compartments would increment and that volume of all compartments would increase. By comparing Tabular array 2 with Table 7, a student should notice the post-obit:
| 1. | Torso osmolarity increased while the ICF volume increased. This is in accord with the initial predictions: the solution added to the body was hyperosmotic and hypotonic. Because the solution was hypotonic, the ICF volume increased. (In our tables, nosotros run across ICF volume increase from twenty to 20.iv liters). It might seem that this would dilute the ICF, causing the ICF concentration to decrease; nonetheless, the solution contained enough urea to make it hyperosmotic. Urea, a penetrating solute, passes through cell membranes and adds to the solute in the cells, increasing the overall concentration of the ICF from 300 to 303.2 mosM. | ||||
| 2. | The mosmoles of penetrating solute can exist distributed between ECF and ICF in proportion to their corresponding compartment volumes afterward the volumes changed from the add-on of nonpenetrating solute. This puts an equal concentration of urea into each compartment. Tell students to avoid thinking "distributes equally," because that makes them inclined to divide the urea corporeality into ii equal batches rather than distributing the urea proportionately according to compartment book. | ||||
| 3. | A change in total body osmolarity is reflected in a similar modify in ECF and ICF osmolarity. ECF and ICF will always exist in the state of osmotic equilibrium (in other words, the osmolarity of all compartments are equal). | ||||
Insisting that students consistently re-create the symbols for units (mosmol, l, mosM) in each of the tables helps them memorize the units and prevents them from mistaking the amount for the concentrations of solute.
The complexity of these box problem assignments tin be taken to the next level past creating a variety of real-life scenarios in which a person is subjected, for instance, to sweat loss from increased physical action followed by the inappropriate option of a rehydration drink. In addition to following the usual set of instructions for filling in the charts, students demonstrate the ability to calculate the osmotic concentration of a solution and deduce how the incorrect choice of sport potable can pb to a disturbance in osmotic homeostasis (Fig. 3).
Fig. iii.This is the case of a box trouble consignment that helps students sympathize the importance of osmolarity and tonicity within the context of systemic homeostasis. This particular task illustrates how the inappropriate choice of a rehydration beverage following sweat loss from increased concrete activity may disturb osmotic homeostasis. Students follow the steps laid out in the unlike parts of Table 2. For simplicity, sweat was assumed to include merely nonpenetrating solute lost from the ECF. In reality, sweat is made mostly of sodium with smaller components of chloride, potassium, lactate, and urea. This question could exist made more complex by including these substances in the question stem.
Glucose as a solute.
The examples above used NaCl as the paradigm nonpenetrating solute and urea equally the freely penetrating solute. But what about glucose, clinically known as dextrose? Many IV solutions incorporate glucose, either as the sole solute (for case, 5% dextrose in water, or D5Westward) or in combination with NaCl (for example, 5% dextrose in one-half-normal saline). As information technology turns out, glucose equally a solute must be treated as a special case for the purpose of working problems.
Students will state that glucose is a penetrating solute because they know that it enters cells. Nonetheless, it is not freely penetrating because, in virtually cells, glucose that enters is immediately phosphorylated by hexokinase to glucose-6-phosphate (G-vi-P). Phosphorylated compounds are not able to exit the jail cell, so the glucose that enters adds to the jail cell's pool of nonpenetrating solutes when information technology becomes M-6-P.
In a normal person given a load of glucose by mouth or IV, over a period of several hours 100% of the administered load will enter the cells. In addition, through normal aerobic metabolism, much of the administered glucose will be processed from Chiliad-six-P to CO2 and water. Thus the net outcome of giving a pure glucose Iv over time is the aforementioned equally giving manifestly h2o, making glucose Four solutions hypotonic, fifty-fifty if they are isosmotic. Failure to recognize hypotonicity and the metabolism of glucose to water can effect in iatrogenic hyponatremia when DfiveWest is used as a routine maintenance solution (8).
When administering a solution containing glucose in the box bug described higher up, nosotros use a "freeze frame" approach. Students are asked to calculate changes in ECF and ICF volumes and osmolarities at the time when X% of glucose has entered the cell. They must recognize that 100 −X% of the glucose has remained in the ECF, and they must distribute the appropriate amounts of glucose into each compartment to exercise their calculations.
One variant of glucose every bit a solute involves the distribution of glucose in a patient with type 1 diabetes mellitus, where absence of insulin prevents glucose uptake into virtually cells. In these situations, glucose remains in the blood and effectively becomes a nonpenetrating solute. A similar situation occurs in hyperglycemic hyperosmolar states (11). These 2 scenarios help students link changes in tonicity to physiologically pregnant disruptions of normal function.
Extending the concept.
The box bug described above are a uncomplicated example of the principle of mass balance: take what was in the body at the start, then add together and subtract solutes and volume to find what is in the torso now. The box trouble approach tin can exist extended to include computing the concentration of specific components of the unlike compartments. For example, a patient comes in dehydrated with an elevated plasma potassium concentration. If you lot replace his volume loss with an Four of isotonic NaCl, what will happen to his plasma K+ concentration? Or a patient comes in with acute hyponatremia from water intoxication. How will administering a pocket-sized volume of hypertonic saline alter both her plasma Na+ concentration and the distribution of h2o between the ECF and ICF?
Answering these questions requires using the box trouble strategy with the addition of a set of boxes that are specific to the ion (Table eight). In the ion-specific tables, students summate the corporeality of ion in the ECF from the plasma ion concentration and the ECF volume. This is a skilful place to reinforce that we tin can substitute plasma ion concentration as a surrogate for ECF ion concentration (unless pregnant amounts of ion are bound to plasma proteins, as is the case for Ca2+).
| Initial ECF Conditions | ECF Conditions Post-obit 1 liter of 100 mosM NaCl and 300 mosM Urea | |
|---|---|---|
| Amount of Yard+ (meq) | 4.5 meq/l × 10 liters = 45 | 45 |
| Volume (liters) | 10 (from Table two) | 10.6 (from Table 7) |
| Plasma [M+] (meq/l) | 4.5 | iv.two |
The K+ instance shown in Table 8 is based on the discipline beingness given 1 liter of 100 mosM NaCl and 300 mosM urea from the gear up of Tables 2–7 calculations. Students should recognize that the corporeality of Grand+ in the ECF does not change postal service-Iv. But there is a new ECF volume after giving the Four, so the plasma K+ concentration will change.
In the instance of hyponatremia treated with a bolus of hypertonic saline, afterward students work out the effects of the saline 4 on the body, they use a 2d set of Na+ boxes to add together Na+ from the IV to the ECF Na+ corporeality. The revised Na+ amount divided by the post-Four ECF book yields the new ECF Na+ concentration.
Word
Our students felt that several different approaches to osmolarity and tonicity were effective in overcoming misconceptions with this ofttimes disruptive field of study. Approaches included simple definitions, real-earth situations, and the simultaneous consideration of both osmolarity and tonicity. Although we have non conducted formal research into the efficacy of our arroyo, nosotros have observed over the years that students achieve different levels of mastery. Some students learn the algorithm for working the box problems and tin answer quantitative problems successfully, only are unable to transfer their agreement to answer conceptual questions, such as: "Given a 300 mosM cell, design a solution using NaCl and urea that is isosmotic and hypotonic." A few students never master the task of converting percent solutions to molarity and then to osmolarity. Our evidence of efficacy is successful completion of test questions similar to the ones in Fig. three and Table 8 and questions request students to predict changes in ECF and ICF book and osmolarity in scenarios they accept never studied. For more examples of box issues that tin can be used to test students' understanding, readers tin can navigate to Osmolarity and Tonicity at SYSTEMSphys.org (http://test.systemsphys.org), a website put together by the authors.
The awarding of osmolarity and tonicity is key to understanding physiology. It is also crucial for the selection of appropriate IV fluid therapy in different situations, such as dehydration, hemorrhage, and hyponatremia. A thoughtful approach to teaching osmolarity and tonicity promotes the development of problem-solving skills, builds student self-confidence for answering bug they have never seen earlier, and enables students to reason their way through a variety of other physiological processes. We promise the arroyo we take to teaching osmolarity and tonicity as described in this newspaper may evidence useful to physiology lecturers who are looking for new means of introducing this complicated topic to their students.
DISCLOSURES
No conflicts of interest, financial or otherwise, are alleged past the authors.
Author CONTRIBUTIONS
P.Five., Grand.C., and D.U.Southward. conceived and designed research; P.Five., K.C., and D.U.S. performed experiments; P.V., 1000.C., and D.U.S. analyzed data; P.Five., M.C., and D.U.South. interpreted results of experiments; P.5., M.C., and D.U.S. prepared figures; P.V., G.C., and D.U.S. drafted manuscript; P.V., K.C., and D.U.S. edited and revised manuscript; P.V., Yard.C., and D.U.S. approved terminal version of manuscript.
ACKNOWLEDGMENTS
The authors thank Dr. Jan Machart (Higher of Natural Sciences, University of Texas at Austin) for permission to adapt her "hot yoga" problem to the version shown in Fig. iii.
Present address: M. Chirillo, Internal Medicine-Pediatrics Residency Program, Department of Internal Medicine, University of Utah School of Medicine, Salt Lake City, UT.
FOOTNOTES
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Source: https://journals.physiology.org/doi/full/10.1152/advan.00094.2018
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