renal clearance, that is, the volume of blood cleared of a substance in a particular time period, is commonly recognized as one of the most difficult concepts in physiology (9). This difficulty may in part reflect the quantitative nature of renal clearance since many life sciences majors perceive that mathematics is irrelevant to their discipline (1, 12). Moreover, students must apply the general model of mass balance to events occurring within the kidney (5). Wenderoth et al. (11) observed that most (75%) students were able to correctly answer a question about mass balance, but only 40% of students were able to apply this model to a situation in renal physiology. Richardson and Speck (7) attributed the problem to misconceptions about virtual volume. Students may infer from the clearance definition that, in blood leaving the kidney, the blood volume corresponding to the clearance will be completely free of the substance, but the remainder of the blood volume will contain the original concentration. The authors use a simple demonstration to show that blood leaving the kidney is homogeneous.
Thus, renal clearance is difficult for students to master both qualitatively and quantitatively. A variety of excellent case studies have been developed to address the quantitative aspect; these provide opportunities to practice calculating clearance and other indicators of renal handling (3, 4, 6). However, although students may become proficient at slotting clinical values into memorized formulae, personal experience suggests that these activities do not always result in conceptual understanding, even when coupled with well-designed diagrams and videos. This activity was developed to improve student understanding by modeling the renal handling of different solutes in a quantifiable manner. Using clay droplets to represent water volumes (50 ml/droplet) and beads to represent solutes (10 mg/bead), students trace the path of solutes and water as they enter the glomerulus, are exchanged between the tubule and peritubular capillaries, and eventually leave the kidney in urine or in the renal vein. The simulation thus allows students to visualize and enact dynamic processes in a way that cannot be done with data sets, images, or videos.
The study population consisted of 44 students in their third or fourth year of a B.Sc. program specializing in biology, neuroscience, or biochemistry. The course was the second in a series of two animal physiology courses required of the biology and biochemistry students but optional for neuroscience students and had two cell biology courses as prerequisites. The class format was somewhat “flipped” in that students were first exposed to the material via a directed reading (8) and an online video and quizzing engine (Mastering A & P Dynamic Study Modules; Pearson Education). Students completed these out-of-class activities during the 4-day period separating the final lecture of one week and the first lecture of the subsequent week. They were thus acquainted with renal structure and the basics of renal handling. Although both the reading and the online materials discussed renal clearance, students were very vocal in their request that class time be devoted to this concept.
The first half of the class (40 min) utilized the peer instruction method to discuss renal handling; students used a personal response system (“clicker”) to record an answer to each multiple choice question (10). If a significant portion of the class chose an incorrect answer, students were instructed to discuss their answer with a classmate prior to repeating the answer selection process. Each of the five questions was followed by an instructor's explanation of the correct and incorrect response options. Clicker questions addressed the equation “excretion rate (E) = filtration rate (F) + secretion rate (S) − reabsorption rate (R),” the cellular mechanism of sodium reabsorption, substances transported by secondary active transport, the mechanism of potassium reabsorption in the proximal tubule, and the distinction between tubular maximum and the renal threshold. For instance, the last concept was addressed by this question: “Which of these statements is true about the tubular maximum but not the renal threshold?” Answer choices included the following:
A. It is measured in mg/min.
B. It reflects the number of carriers available for a particular solute.
C. Above this value, the substance begins to appear in the urine.
D. None of these statements correctly answers the question.
The clearance simulation activity took up the rest of the class (∼40 min), including a 10-min debriefing. The activity was performed too late in the semester to be included in the general in-class survey of class activities administered in the 8th wk of term. Instead, students were asked to complete an optional online survey one week after the last class of the semester. Student perceptions of the clearance activity were compared with those of two previously validated activities examined in the in-class survey: peer instruction (10) and a role-playing simulation in which students acted out the events of inhalation and exhalation (2). Results are presented as means (SD).
Student mastery of the concepts of renal clearance and handling was examined using quantitative questions on the final exam. The questions asked students to calculate renal clearance based on renal blood flow and the solute concentrations in the renal artery and the renal vein; the solute filtration rate and the glomerular filtration rate based on the plasma solute concentration, the solute excretion rate, and the solute reabsorption rate; and renal clearance from the urine production rate and the solute concentrations in urine and plasma.
The study was approved by the Research Ethics Board of Bishop's University.
Description of the activity.
This simulation is informed by the clearance figure, Fig. 19.13 from the course textbook (see Fig. 1) (8), but refers to a “hypothetical mammal” because the concentrations and filtration fraction differ significantly from those of real animals. Student pairs were provided with a full-page game board large enough to allow student pairs to work together, a small amount of blue modeling clay to represent water, and four beads to represent solute (Fig. 2). The activity instructions were provided on a separate sheet. Both the full-page game board and the student handouts are available upon request from the author.
First, students were asked to label the renal tubule, peritubular capillaries, glomerulus, and renal capsule and to add labeled arrows representing filtration (F), reabsorption (R), and secretion (S). They then created two flattened teardrop shapes from the modeling clay that were large enough to accommodate two beads each yet small enough to fit on the game board's blood vessel and renal tubule. The parameters of the simulation were outlined as follows:
The game board represents all of the nephrons of the kidney of a hypothetical mammal.
Each bead represents 10 mg of solute, and each water droplet represents 50 ml of water.
It takes 1 min for the two droplets (representing 100 ml) to pass from one side to the other (i.e., through the kidney).
50% of the fluid arriving at the glomerulus is filtered (i.e., 1 droplet).
Based on these parameters, students were asked to determine the glomerular filtration rate (GFR; 50 ml/min) and the renal blood flow (100 ml/min). They were then asked to model renal handling of water by following these steps:
1. Ignore the solute for now (put your beads aside).
2. Place your two droplets in the blood.
3. As the droplets enter the glomerulus, one droplet is filtered into the tubule (remember, 50% filtration). The other continues in the blood.
4. As the droplet passes through the tubule, most of it gets reabsorbed into the blood. The amount varies widely, but in our simulation we will reabsorb 48 ml of the droplet. Break off a small piece of the filtered droplet to represent the small amount of water that forms the urine, flowing to the renal pelvis and then to the bladder.
5. Calculate the urine flow rate (V; 2 ml/min). What percentage of the filtered water is reabsorbed (96%)?
Before proceeding with the other trials, the instructor modeled the movement of water on the board and confirmed that all students understood the basic principles of the simulation and arrived at the correct value for V.
The additional trials addressed various types of solutes. For each, students were asked to model what will happen to the solute (beads) and the water (droplets) and then to use the model to determine the clearance (Table 1). They were reminded that the GFR (50 ml/min) and the urine flow rate (2 ml/min) are the same in all trials. If desired, examples can be given of solutes handled in each manner.
A. The solute is filtered, completely reabsorbed, and not secreted (clearance = 0 ml/min).
B. The solute is too big to be filtered. It is not reabsorbed or secreted (clearance = 0 ml/min).
C. The solute is filtered and not reabsorbed nor secreted (clearance = 50 ml/min).
D. The solute is filtered and not reabsorbed. Any nonfiltered solute is secreted (clearance = 100 ml/min).
E. The solute is filtered and 50% reabsorbed but not secreted (clearance = 25 ml/min).
This activity could theoretically strengthen the misconception involving virtual volume discussed earlier (7), since the solute may be cleared of the filtered water droplet but not the other. Students should thus be reminded that solutes diffuse freely through a volume of fluid and that they should redistribute the solute after the calculation, but before the blood or urine leaves the kidney. Preceding the activity with the thought problems outlined by Richardson and Speck (7) may also help to prevent this misconception.
Following the activity, the instructor modeled each situation on the board. Students were then asked to predict which trial could be used to measure renal blood flow (trial D), and which could be used to measure GFR (trial C).
RESULTS AND DISCUSSION
Instructor and student observations.
The students divided themselves into 12 groups of three or four students each. All students effectively engaged in the activity, and there was a considerable amount of lively discussion both within groups and between groups. Requiring only minimal instructor intervention, all student groups were able to predict the clearance of each solute using the model. Moreover, students were confronted with and were largely able to correct several misconceptions. For example, the simulation only produced correct results if students realized that 1) water is handled differently from solute (that is, the droplets and the solute do not always move together), 2) each solute is handled differently, and 3) the GFR applies to all solutes and water passing through the kidney at a particular time.
Twenty-one students completed the optional online survey of the activity, and 42 students completed the earlier in-class survey of the other class activities. In response to the question “Did the activity using play dough and beads help you understand the concept of renal clearance?”, only three students ranked the activity “not useful” or “a bit useful”; the mean score was 3.43 (SD 1.00) (Fig. 3). For comparison, the average scores for peer instruction and the ventilation simulation were 3.9 (SD 1.02, n = 42) and 3.38 (SD 1.13; n = 40), respectively. Some student comments were very positive, such as, “I loved it! I didn't understand clearance at all, and then after the activity, I felt like a master of clearance.” Students mentioned that “it was very helpful to be able to see a concept and not just imagine it,” and that they liked the “visual and hands-on aspect.” Four students noted that including additional problems to solve using the game sheet would have rendered the activity more useful, and two students stated that they understood clearance already and would have preferred to spend the time doing more difficult problems. The extension activity described below was designed to address these comments. In response to the question “Was the clearance activity interesting and/or enjoyable?”, all students ranked the activity at 3 or higher, with a mean score of 4.10 (SD 0.61). For comparison, the average scores for peer instruction and the ventilation simulation were 3.88 (SD 0.95, n = 42) and 3.23 (SD 1.23, n = 40), respectively.
Free response quantitative problems relating to renal clearance and handling constituted 8% of the final exam. The average overall score on these questions was 78.7% compared with 73% for the entire exam.
If students will be expected to use laboratory values to calculate clearance and other measures of renal handling, the model can be used quantitatively to prepare students for this task. For each trial, ask students to determine the urine solute concentration (US) by counting the solute beads in urine (each representing 10 mg) and dividing this number by the urine volume (2 ml). Multiplying this number by 100 results in the most useful unit of measurement (mg/100 ml) for later calculations. Students can use this calculated value for US along with the values for plasma solute concentration (PS; always 40 mg/100 ml) and V (always 2 ml/min) to calculate clearance (C = UsV/Ps) and to compare their calculated value with their theoretical value (see Table 1).
Students can also use the model to determine the filtered load and the excreted load for each solute by determining the number of beads filtered and excreted (respectively) per minute. They can compare values obtained using the model with those obtained with formulae (F = PS × GFR; E = Us × V). See Table 1 for a summary of the expected values for each trial. Other measures of renal handling can also be explored. For instance, students can calculate the fractional tubular reabsorption (TRP; for trials A and E) by dividing the number of beads “reabsorbed” by the number of beads that were “filtered” and compare this observed value with the calculated value [TRP = (F − E/F) × 100%].
The power of this renal clearance model is its ability to link conceptual understanding with quantitative problems, especially if time permits the inclusion of the extension activity. Many students consider the clearance formula as a black box; they input clinical values and magically obtain the clearance. By modeling the renal handling of various substances using quantifiable amounts of clay and beads, students can logically deduct the value for clearance (as well as other measures of renal handling) and compare their “observed” values with their calculated values.
This work was supported by the Senate Research Committee of Bishop's University.
K. Hull is a contributing author to Anatomy and Physiology textbooks published by Wolters Kluwer Health, but the article discusses a concept that is not discussed in the author's textbooks.
K.H. conception and design of research; K.H. performed experiments; K.H. analyzed data; K.H. interpreted results of experiments; K.H. prepared figures; K.H. drafted manuscript; K.H. edited and revised manuscript; K.H. approved final version of manuscript.
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