A Simple Model for Demonstrating the Factors Affecting Glomerular Filtration Rate

IlluminationsA simple model for demonstrating the factors affecting glomerular filtration rateAnand Bhaskar and Vinay OommenAnand BhaskarDepartment of Physiology, Christian Medical College, Vellore, Tamil Nadu, India and Vinay OommenDepartment of Physiology, Christian Medical College, Vellore, Tamil Nadu, IndiaPublished Online:15 May 2018https://doi.org/10.1152/advan.00195.2017MoreSectionsSupplemental MaterialPDF (824 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat INTRODUCTIONGlomerular filtration, an important part of renal function, helps in eliminating waste materials and in maintaining fluid and electrolyte balance. The rate of filtration, or the glomerular filtration rate (GFR), is determined by the equation GFR = Kf × net filtration pressure, where Kf is the filtration coefficient. The Kf is directly proportional to the surface area of the filtering membrane and its hydraulic conductivity. The net filtration pressure is determined by the balance of the Starling forces (the hydrostatic pressure and the oncotic pressure within the glomerular capillaries and Bowman's capsule). The glomerular capillary hydrostatic pressure is affected by the afferent and efferent arteriolar resistance and the renal artery pressure (3). An increase in the afferent arteriolar diameter (decrease in resistance) causes an increase in the glomerular capillary hydrostatic pressure and an increase in GFR. A decrease in the diameter of the afferent arteriole has the opposite effect. An increase in the efferent arteriolar diameter (decrease in resistance) causes a decrease in the glomerular capillary hydrostatic pressure and a decrease in GFR. A decrease in the diameter of the efferent arteriole has the opposite effect. An increase in renal arterial pressure (or renal blood flow) causes an increase in GFR. A reduction in renal arterial pressure (or renal blood flow) will have the opposite effect (1).Pictures are often used to teach the concept of glomerular filtration in a didactic setting, and students have to visualize the various factors affecting glomerular filtration. Animations with user-changeable parameters have been recently described to teach the regulation of glomerular filtration (2). Physical models that demonstrate the factors affecting GFR are not available. Renal physiology is also an area where practical experimentation is not common. We felt that a simple physical model would further assist students in understanding this concept, as they could directly observe the effect of changing factors affecting the GFR. This article describes the construction of such a model, its presentation, and the assessment of the student feedback regarding the usefulness of this activity in a laboratory session in the first year of medical training.Construction of the ModelThe model was constructed using commonly available plumbing supplies given below (Fig. 1).Fig. 1.The constructed model connected to a tap. The shower head (representing the glomerular capillaries) is connected to two ball valves through a T-junction. Valve 1 represents the afferent arteriole. Valve 2 represents the efferent arteriole. Water filtered through the shower head represents the glomerular filtrate and is collected in beaker 1, representing Bowman's capsule. Water exiting the model represents the flow through the peritubular capillaries and is collected in beaker 2.Download figureDownload PowerPointTwo plastic ball valves (0.5-in. inner diameter)A piece of PVC pipe (0.5-in. inner diameter)One T-junction (0.5-in. inner diameter)A shower head with a replaceable cover. All of the holes in the shower head were plugged with Blu Tack on the inner surface of the cover, except for 15 holes. On some shower heads, the holes were drilled through to make the diameter of each hole wider.A piece of garden hoseBonding solutionTwo 2-liter beakers for measurementThe different components of the model were connected to each other using small pieces of PVC pipe. The shower head (representing the glomerular capillary where filtration happens) was connected to the vertical arm of the T-junction. The shower head was kept over a 2-liter beaker (beaker 1 representing Bowman's capsule) to collect the filtered water. The horizontal arms of the T-junction were connected to the two ball valves (representing the afferent and efferent arterioles with variable resistance). The first valve (valve 1) was connected via a hose to a tap (sink faucet). Water input to the model through the tap represented the renal plasma flow coming to the glomerular capillary. The second valve (valve 2) was connected to a hose to collect the unfiltered water. The unfiltered water (representing renal plasma flow in peritubular capillaries) was also collected in a 2-liter beaker (beaker 2). The connections between the pipes and valves and that between the pipes and T-junction were made stronger with the use of bonding solution. All of the water that was collected for measurement was stored in a tub for other uses to prevent a waste of water.Presentation of the Model and Experimental ProceduresThe model was presented to students studying in their first year of medical training in groups of 8–10 students each. This was done during a laboratory session. The students had already been exposed to a comprehensive 18-h module on renal physiology. The objective of using this model in a practical setting was to revise and clarify concepts that had been covered in the theory module. The presentation of the model and the experiments performed took ∼1 h.To begin with, the components of the model were described to the students. A handout explaining the different experiments to be performed was given to them (Supplemental Data S1; supplemental material are available in the data supplement online at the Advances web site). An initial demonstration on how to use the model was conducted by the instructor. Thereafter, the students performed the experiments in different groups simultaneously. Immediately after the session, an anonymous written feedback was obtained to assess the usefulness of this model in understanding the factors affecting the GFR.A total of five different experimental scenarios were tested by altering the following parameters: filtration area, hydraulic conductivity, the inflow, and the input and output valve resistances. Each of these scenarios corresponded to various possible physiological and pathological variations in the factors affecting the GFR.Baseline measurements.The tap was kept at midposition, with both input (valve 1) and output valves (valve 2) open to the midposition (45°). The shower head had 15 holes open, of normal diameter. All other holes were plugged with Blu Tack.Scenario 1: the effect of an increase in filtration area.The shower head was replaced with another having 30 holes of normal hole diameter, with all other parameters the same as in baseline measurements. This increase in filtration area is similar to what is seen with relaxation of the mesangial cells.Scenario 2: the effect of an increase in hydraulic conductivity.The shower head was replaced with a different shower head that had 15 holes, but of larger hole diameter, with all other parameters the same as in baseline measurements. This scenario demonstrates the effect of filtration pore size on the GFR.Scenario 3: the effect of a change in renal plasma flow.Measurements were taken with the tap completely open (increased flow) and open less than the midposition (decreased flow), with all other parameters the same as in baseline measurements. The change in flow in this scenario is similar to what is seen in conditions such as changes in cardiac output or blood pressure (assuming that the renal compensatory mechanisms that regulate GFR have not taken place).Scenario 4: change in afferent arteriolar resistance.Measurements were taken with the input valve (valve 1) more open (afferent arteriolar dilation) and less open (afferent arteriolar constriction) from the midposition, with all other parameters the same as in baseline measurements. The constriction in this scenario is similar to the afferent arteriolar constriction seen with sympathetic stimulation.Scenario 5: change in efferent arteriolar resistance.Measurements were taken with the output valve (valve 2) more open (efferent arteriolar dilation) and less open (efferent arteriolar constriction) from the midposition, with all other parameters the same as in baseline measurements. The constriction in this scenario is similar to a moderate constriction of the efferent arteriole caused by angiotensin II.In each scenario, students measured the volume of filtered water, which represented the glomerular filtrate, and the volume of water exiting the output valve (valve 2), which represented the peritubular capillary flow. All volumes were measured with the tap open for 5 s. Adding both volumes provided the total flow, which represented the renal plasma flow. Filtration fraction was obtained by dividing the filtered volume by the total volume. Students entered the collected values in a tabular column provided in the handout.Leaks in the system can affect the volumes measured. It is important to ensure that there no major leaks. It is also important to ensure that there are no fluctuations of water output through the tap.ResultsBaseline measurements were made with the shower head having 15 open holes of normal diameter.Use of a shower head with more holes or larger diameter holes resulted in an increase in filtration fraction with minimal change in total flow. This was expected because of an increase in the area of filtration and hydraulic conductivity. These are factors that affect the filtration coefficient of the filtration membrane.An increase or decrease in the flow from the tap resulted in a corresponding increase or decrease in filtered volume and total flow, with minimal change in filtration fraction. This corresponds to an effect of uncompensated change in renal plasma flow. In the body, however, myogenic mechanisms and tubuloglomerular feedback mechanisms compensate for the changes in flow to maintain glomerular filtration.An increase in input valve (valve 1) resistance (open less than midposition) resulted in a decrease in filtered volume and total flow, with minimal change in filtration fraction. This represented the effect of afferent arteriolar constriction. A decrease in input valve resistance had the opposite effect on filtered volume and total flow, with minimal change in filtration fraction.An increase in output valve (valve 2) resistance (open less than midposition) resulted in an increased in filtered volume, a decrease in total flow, and an increase in filtration fraction. This represented the effect of efferent arteriolar constriction. A decrease in output valve resistance had the opposite effect.FeedbackAnonymous, voluntary feedback was obtained from the students after the activity to assess the usefulness of the model. There were 61 of 100 students who provided this feedback. The feedback contained three questions. The first question assessed how easy it was to understand the model. The second question assessed how useful the model was in improving the understanding of factors affecting GFR. The third question was whether the students would recommend this model for future batches of students. The results of the feedback are shown in Fig. 2. Questions 1 and 3 used a five-point Likert scale. For question 2, students were permitted to choose more than one option. The feedback form is included as supplemental information (Supplemental Data S2). Permission was obtained from the Institutional Review Board, Christian Medical College, Vellore, to analyze and publish the student feedback.Fig. 2.Student feedback obtained after the laboratory session. A: student response on how easy the model was to understand. B: student response on the usefulness of the model. C: students recommendations on using the model for future batches of students. GFR, glomerular filtration rate.Download figureDownload PowerPointThe model was found to be easy to understand by 77% of students (mean score 3.93). There were 66% of students who recommended the model for future students (mean score 3.9). Most students reported the model as useful, for reviewing existing concepts. The model was also found useful in adding to existing knowledge and in clarifying concepts that they did not understand. Terms such as "creative," "innovative and entertaining," and "very useful" were some of the comments given by the students about the model and the experiment.LimitationsIn the presentation of this model, the size of each group was ∼8–10 due to infrastructure constraints. A group size of three to four would have been ideal.A constant flow through the tap was assumed. The changes in flow were modeled by opening the tap halfway and completely. In reality, there may have been variations in the flow during the period of collection. Ideally the tap outflow pressure would have been measured using a pressure gauge, and this reading could have been used to accurately adjust flow. However, for the sake of simplicity, the knob of the tap was used for this purpose.The different components of the model were not to scale, compared with the different parts of the nephron that they represented.ConclusionThe model described is easy to construct and is inexpensive. Students also reported it useful as a tool to revise and clarify concepts. It can, therefore, be used as an additional teaching tool to complement lecture in renal physiology or as part of a laboratory exercise.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the authors.AUTHOR CONTRIBUTIONSA.B. and V.O. conceived and designed research; A.B. and V.O. performed experiments; A.B. and V.O. analyzed data; A.B. and V.O. interpreted results of experiments; A.B. and V.O. prepared figures; A.B. and V.O. drafted manuscript; A.B. and V.O. edited and revised manuscript; A.B. and V.O. approved final version of manuscript.ACKNOWLEDGMENTSPortions of this work were previously presented at the SIMEDUCON 2018 conference held at Christian Medical College, Vellore, India, on March 3, 2018.REFERENCES1. Barrett KE, Barman SM, Boitano S, Brooks H. Renal function and Micturition. In: Ganong's Review of Medical Physiology (25th ed.). New York: McGraw-Hill, 2016, p. 679.Google Scholar2. Gookin JL, McWhorter D, Vaden S, Posner L. Outcome assessment of a computer-animated model for learning about the regulation of glomerular filtration rate. Adv Physiol Educ 34: 97–105, 2010. doi:10.1152/advan.00012.2010. Link | ISI | Google Scholar3. Hall JE. Urine formation by the kidneys. I. Glomerular filtration, renal blood flow, and their control. In: Guyton and Hall Textbook of Medical Physiology (12th ed.). Philadelphia, PA: Saunders Elsevier, 2011, p. 315.Google ScholarAUTHOR NOTESAddress for reprint requests and other correspondence: V. Oommen, Dept. of Physiology, Christian Medical College, Bagayam Campus, Vellore, Tamil Nadu 632002, India (e-mail: [email protected]ac.in).Supplemental data Data Collection Table - .docx (14 kb) Feedback Form - .docx (15 kb) Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Cited ByEngaging medical students and residents in nephrology education: an updated scoping review5 August 2021 | Journal of Nephrology, Vol. 35, No. 1Demystifying the Nephron: a Call to Action26 September 2021 | Current Pediatrics Reports, Vol. 9, No. 4Appraisal of a novel pedagogical approach to demonstrating neuromuscular transmission to medical studentsSareesh Naduvil Narayanan, Iffath Ahmed, Batul Saherawala, Fatmaelzahraa Foud, and Tarig Hakim Merghani11 August 2021 | Advances in Physiology Education, Vol. 45, No. 3Interactive Metabolism, a simple and robust active learning tool that improves the biochemistry knowledge of undergraduate studentsVitória Costa Pereira Lopes Alves de França and Wellington Ferreira Campos22 April 2021 | Advances in Physiology Education, Vol. 45, No. 2A simple hand mnemonic for teaching the cardiac cycleHui Bian, Yan Bian, Jun Li, Shilian Xu, Xiaoxia Shao, Jiao Li, and Boao Jiang10 December 2019 | Advances in Physiology Education, Vol. 44, No. 1Normal Physiology of Renal System28 June 2020 More from this issue > Volume 42Issue 2June 2018Pages 380-382Supplemental Information Copyright & PermissionsCopyright © 2018 the American Physiological Societyhttps://doi.org/10.1152/advan.00195.2017PubMed29761711History Received 27 December 2017 Accepted 23 March 2018 Published online 15 May 2018 Published in print 1 June 2018 Metrics