Advan. Physiol. Edu. 28: 210-212, 2004;
doi:10.1152/advan.00021.2004
1043-4046/04 $5.00
ADV PHYSIOL EDUC 28:210-212, 2004
© 2004 American Physiological Society
MISCONCEPTIONS
Addressing students’ misconceptions of renal clearance
Daniel Richardson and
Dexter Speck
Department of Physiology, University of Kentucky, College of Medicine, Lexington, Kentucky 40536-0298
Address for reprint requests and other correspondence: D. Richardson, Dept. of Physiology, College of Medicine, Univ. of Kentucky, MS 508 UKMC, Lexington, KY 40536-0298 (E-mail: drichar{at}uky.edu)
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Abstract
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Renal clearance is one of the more difficult concepts for students of physiology to learn. We hypothesized that this difficulty is rooted in a student’s misunderstanding of virtual volume. This was tested by having students select from several drawings the one they thought plasma would look like after a certain volume of it has been cleared of sodium by the kidneys. About half the participating students selected plasma pictured as having a certain volume of it devoid of sodium molecules. That is, their misconception of clearance seemed to be due to a lack of understanding about virtual volume, a deficiency which is reinforced by the classic definition of clearance. To address this misconception, a demonstration was devised in which a beaker of concentrated colored water was used to represent plasma before renal clearance, a beaker of the same concentrated colored water in which the top third had been replaced by clear mineral oil was used to represent what the definition of clearance said would happen to plasma after a third of it had been cleared of sodium, and a beaker of dilute colored water was used to represent what really happens to plasma when a certain volume of it is cleared of a solute. Incorporating this demonstration into discussions of renal clearance helped students to understand this concept, as evidenced by improved scores on related questions.
Key words: virtual volume
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Introduction
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THE RENAL SYSTEM has traditionally been one of the more difficult topics for students of physiology to learn (2). Within this context, the concept of renal clearance tends to be quite difficult for students to master, but possible pedagogical reasons for this are not clear. Since renal clearance is an example of the general model of mass balance in physiology (4), it is possible that the concept of mass balance is what is difficult for students, not renal clearance in particular. However, this notion is not strongly supported by data from Michael et al. (3). They measured the prevalence of 13 different misconceptions in cardiovascular physiology in a variety of courses in several institutions. The prevalence of the misconception representing mass balance averaged only 53% among all participating students. Although their study did not address renal clearance per se, these results do not provide unequivocal support for the notion that students’ difficulty in learning renal clearance is due to misconceptions about mass balance.
An alternative explanation for students’ difficulty with renal clearance comes from Nussbaum and Novick’s (5) results with teaching gas diffusion to 7th-grade students. They used a closed flask hooked to a hand vacuum pump to demonstrate suction by having the students feel the pulling effect of the pump. Then, after the students could see the pump move so as to pull air from the flask, they were asked to draw, on a sheet of paper, what the air inside the flask would look like after some of it has been pulled from the flask. Many of the students drew the flask with a certain space in it devoid of air molecules. These results are understandable given that these students had not as yet learned the laws of diffusion, nor had they been presented with the concept of a virtual volume, an artificial entity arising from an abstract concept, in this case that of a vacuum. In this context, a possible cause of physiology students’ difficulty with renal clearance is that they carry a misconception about virtual volume, which is reinforced by the definition of renal clearance itself, usually expressed as either "the volume of plasma from which a particular substance has been completely removed (cleared) by the kidneys per unit time" (7), or "the volume of plasma passing through the kidneys that has been totally cleared of a particular solute" (6). The obvious problem with such definitions is that they are misleading because they do not address the concept of a virtual volume. The definitions specifically state that there exists a volume of plasma devoid of a solute that has been excreted in the urine. Of course there can be no such volume, because its existence would violate the laws of free diffusion (i.e., entropy). Accordingly, the definition of renal clearance itself reinforces the students’ misconception of this concept. Our hypothesis is that students’ difficulty with understanding renal clearance is rooted in their lack of understanding about virtual volume; i.e., they interpret the definition of renal clearance literally, and do not recognize that blood leaving the kidneys is homogeneous.
Identification of misconception.
To determine whether students might be interpreting the definition of clearance literally, in 1994, a class of undergraduate students taking a first course in physiology at the University of Kentucky (Elementary Physiology–PGY 206) were asked to voluntarily and anonymously participate in a paper-and-pencil test after a lecture on renal clearance in which one of the aforementioned definitions was used.
For the test, students were shown a copy of the drawing shown in Fig. 1. As noted, the drawing consisted of four beakers each containing 400 ml of blood plasma, in which sodium molecules were represented as small dots and were evenly distributed such that they could be counted by the students. The beaker in the top left-hand corner represented plasma before being processed by the kidneys (i.e., the control situation). The three additional beakers represented plasma after processing by the kidneys (i.e., the experimental situation). In the first of these, the top 100 ml of plasma was pictured as being devoid of sodium, and the remainder of the beaker contained the same number of dots, representing sodium, as existed in the lower 300 ml of the control beaker. The second experimental beaker was similar to the first except that the bottom 100 ml of plasma was pictured as being devoid of sodium. The third experimental beaker contained the same number of dots as in 300 ml of the control beaker, but the dots were evenly distributed in the 400 ml of plasma.
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Fig. 1. Sketch of plasma samples with sodium ions represented as dots. The sample at the top represents plasma before flowing through the kidneys. The samples labeled 1, 2, and 3 represent three choices given to students for the distribution of sodium ions after 100 ml of plasma has been cleared of sodium.
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Students were asked to identify which of the three experimental beakers, presented in Fig. 1, represented what the plasma would look like after 100 ml of it had been cleared of sodium by the kidneys. A total of 23 students participated in the test, and of these, 11 (48%) selected either the first or second experimental beaker, the ones showing 100 ml of plasma devoid of sodium. That is, they selected what the definition of clearance said would happen.
Addressing the misconception.
To address students’ misunderstanding of renal clearance, subsequent lectures on the concept of clearance included a treatment of virtual volume. This was, and still is, done through the following demonstration (see Fig. 2).
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Fig. 2. Beakers of colored water used to demonstrate the concept of renal clearance. Beaker A contains a red dye and represents plasma before being processed by the kidneys. The bottom two-thirds of beaker B contains the same colored water as beaker A. The top one-third of this beaker contains clear mineral oil and represents what the clearance definition says would happen to plasma after one-third of it has been cleared of the red molecules in beaker A. Beaker C is a dilute solution of the colored water in beaker A and represents what really happens after a certain volume has been cleared of the red molecules in beaker A.
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A class is shown three beakers of water each representing a different plasma sample. The water in the first beaker (A) is made a dark red by adding several drops of food coloring. Students are told that this beaker represents plasma before being processed by the kidneys.
The bottom two-thirds of the second beaker (B) contains the same dark red liquid as in the first beaker, and the top third contains clear mineral oil. Students are told that this beaker represents what the clearance definition says would happen to plasma after a third of it has been cleared of the solute represented by the red dye.
The third beaker (C) contains a lighter red solution, obtained by diluting the solution in the first beaker. Students are told that this third beaker represents what really happens after a certain volume of plasma has been cleared of the solute represented by the red dye. That is, a certain number of molecules have been removed and the remaining molecules become evenly distributed in the solvent (plasma) in accordance to the laws of diffusion.
Testing for the correct concept of renal clearance.
A subsequent PGY 206 class was presented a lecture on renal clearance, which incorporated the concept of virtual volume. After this lecture the students were given a quiz consisting of two questions. Question 1 contained volume and sodium concentration information for a single plasma sample and several urine samples; while question 2 contained this information for a single urine sample and several plasma samples. Students were asked to identify which urine (Q1) and plasma (Q2) sample represents the highest renal sodium clearance. The correct response rates were, respectively, 71% and 72%. These are higher correct responses compared with the 52% percent of students in the original study who correctly identified which plasma sample represented the correct distribution of sodium ions after a certain volume of plasma had been cleared of sodium. Furthermore, in a recent section of PGY 206, in which renal clearance was taught without addressing the concept of virtual volume, students had only a 47% correct response on an examination question related to renal clearance while the class average for this examination as a whole was 64%. In our experience, average exam scores in PGY 206 typically range between 60 and 70%. Thus, on the basis of these results, classes of physiology students exposed to the concept of virtual volume within the context of a discussion on renal clearance would be expected to score above general examination averages on questions related to the concept of renal clearance.
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DISCUSSION
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The results of this study vis-a-vis those of Nussbaum and Novick (5) are consistent with the notion that students carry childhood misconceptions into later life and that these can interfere with the subsequent learning of correct concepts (1) (e.g., in college). In the present case, a misconception about virtual volume seems to be responsible for the difficulty college students have in learning renal clearance. We found that through the simple use of beakers of colored water (Fig. 2) the concept of virtual volume can be easily incorporated into a lecture on renal clearance. This demonstration is beneficial in helping students internalize (i.e., learn) this important, but difficult concept.
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REFERENCES
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- Driver R and Easley J. Pupils and paradigms: a review of literature related to concept development in adolescent science students. Stud Sci Educ 5: 61–84, 1978.
- Janssen HF. Teaching renal physiology concepts using a problem solving approach. Ann NY Acad Sci 701: 116–119, 1993.
- Michael JA, Wenderoth MP, Modell HI, Cliff W, Horwitz B, McHale P, Richardson D, Silverthorn D, Williams S, and Whitescarver S. Undergraduates understanding of cardiovascular phenomena. Adv Physiol Educ 26: 72–84, 2002.[Abstract/Free Full Text]
- Modell HI. How to help students understand physiology? Emphasize general models. Adv Physiol Educ 23: 101–107, 2000.[Abstract/Free Full Text]
- Nussbaum J and Novick S. Alternative frameworks, conceptual conflict and accommodation: toward a principled teaching strategy. Instruct Sci 11: 183–200, 1982.
- Silverthorn DU. Human Physiology: An Integrated Approach (3rd ed.). San Francisco: Cummings, 2004, p. 614.
- Widnaier EP, Raff H, and Strang KT. Vander, Sherman and Luciano’s Human Physiology: The Mechanisms of Body Function (9th ed.). Boston, MA: McGraw-Hill, 2004, p. 525.
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Copyright © 2004 by the American Physiological Society.
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Abstract
Introduction
