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 Table of Contents  
Year : 2018  |  Volume : 3  |  Issue : 2  |  Page : 111-115

“Graph's disease” and students' anxieties in understanding physiology

Department of Physiology, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia

Date of Submission22-Oct-2018
Date of Acceptance03-Dec-2018
Date of Web Publication26-Dec-2018

Correspondence Address:
Prof. Hwee-Ming Cheng
Department of Physiology, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/bjhs.bjhs_38_18

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Graphs are visual summaries that explain the phenomenon in different disciplines. Students who approach physiology in rote-learning mode usually have more difficulty and anxieties to understand the graphical information. Graphs can be used in many ways to conceptualize much of physiology. The relationship between the X-axis and Y-axis variables can indicate a cause and effect scenario as in the hemoglobin-oxygen association/dissociation. Temporal, sequential events are illustrated with the X-axis as the time parameter and several changing parameters can be followed along the Y-axis in a dynamic way as in the Wigger's cardiac cycle diagram. Graphs can also provide a different perspective on the fluctuations between two parameters that is produced by a third physiologic event as seen in the ventricular volume-pressure loop. Hopefully, this article on physiological graphs will help students to relax and enjoy looking and thinking through the different line profiles in graphs they face in their “Physbook.”

Keywords: Correlation, graph, interpretation, physiology

How to cite this article:
Cheng HM, Hoe SZ. “Graph's disease” and students' anxieties in understanding physiology. BLDE Univ J Health Sci 2018;3:111-5

How to cite this URL:
Cheng HM, Hoe SZ. “Graph's disease” and students' anxieties in understanding physiology. BLDE Univ J Health Sci [serial online] 2018 [cited 2021 Sep 21];3:111-5. Available from: https://www.bldeujournalhs.in/text.asp?2018/3/2/111/248557

There are diverse ways to teach and communicate physiology. Information in PowerPoint boxes are the main mode of delivery in classes. When informative graphs are shown that are meant to summarize and highlight physiology events, the teacher's common experience is to notice a general lack of engagement by students. This lack of focus is often associated with anxiety and difficulty in processing the graphical information. Moreover in a lecture setting, the limited time a PowerPoint slide is projected does not give the unhurried time to appreciate the good graph's data. The students' learning indifference or avoidance of graphs is reflected in a weaker performance in test questions that are formulated in relation to graphs. This article is written to help students to include in their learning armory, the arrows represented in the axes of graphs. Hopefully, graphs will become useful for their targeted understanding of key physiology.

  Graphs that Illustrate Cause and Effect Top

Lower the Y-axis and make horizontal both lines

Two lines at right angle to each other are the cause of much students' troubles and anxieties when reading their physiology (this assuming they still bother to read and think outside their power point boxes, to read books, hard, or electronic).

If these two lines are aligned horizontally in sequence, then they become more familiar and less threatening. Let the X-axis be the first horizontal line and the Y-axis be the subsequent horizontal line. The X parameter becomes the cause and the Y parameter is then the effect of any physiological event that was represented as a graph when the Y-axis was perpendicular to the X-axis parameter [Figure 1].
Figure 1: Horizontal alignment of X-and Y-axes

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Some cause and effect are proportionate, and this is represented by a linear line or linear portion of a graph. A linear line in the renal handling graph for glucose will be that of the filtered load (Y-axis) plotted against the plasma glucose concentration (X-axis) [Figure 2].
Figure 2: Renal handling of glucose as a function of plasma glucose concentration. Tm, transport maximum

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A linear portion of the nonlinear graph will be seen in the reabsorption rate line in the renal handling of glucose. The earlier portion of the line shows that at the proximal convoluted tubule, the rate of glucose reabsorbed is proportionate to the plasma glucose concentration. Thus, the line of the filtered glucose runs identically along the same track as the reabsorbed glucose line. The reabsorbed glucose line then separates from the filtered load line and plateaus out. This is the point when the sodium-coupled glucose symporters at the proximal convoluted tubule are fully saturated by the filtered glucose. Maximal transport of glucose has been reached.[1]

  Mood Swings and Shifting Of Graph Top

If the static graph alone worries the students, they become more anxious when discussion dwells on the shifting of the graph with different physiologic or pathophysiologic scenarios. Once the cause/effect of the X-/Y-axis areappreciated, these rightward or leftward movements of the graph are quite easily handled and understood.

Let us illustrate with the Starling's cardiac mechanism of the heart [Figure 3]. The initiating variable along the X-axis is the ventricular filling or end diastolic volume (EDV). Changes in EDV cause or produce proportionate increase in stroke volume (SV) (Y-axis). This represents the intrinsic, inherent property of myocardium, independent of extrinsic sympathetic neural or hormonal stimulation on ventricular contraction. This would be the Starling's cardiac graph of an isolated, denervated heart.
Figure 3: Frank-Starling curves of the heart

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Now, we add on sympathetic neural or adrenaline action on the heart when the adrenal gland secretes the catecholamines. In a normal heart, both the sympathetic and hormonal inputs will increase the myocardial contractility defined by a greater ejection fraction. Most students would remember that ejection fraction means that for a given EDV, a higher SV is ejected (SV/EDV). This simply indicates that for any given EDV on the X-axis, a higher point is achieved along the Y-axis (SV). If an EDV point along the X-axis is now associated with a higher value along the Y-axis in the presence of extrinsic sympathetic/adrenaline action, this will result in a graph that will be shifted left of the intrinsic, Starling's cardiac line.

Pathophysiologically, in cardiac failure, there is pump failure. Ejection fraction is reduced for any given EDV, which leads to cardiac and vascular congestion in the closed loop of the systemic circulation.[2] Each point along the EDV X-axis is associated with a reduced SV value along the Y-axis. The failing heart is thus shown by a graph that is shifted right of the normal, intrinsic Starling's cardiac line.

  Plateaus, More Saturation, Less Flow Fluctuations Top

Let's bring physiologic meaning to the plateaus.

An unchanging plateau line remains a line in a graph to be memorized by unthinking students. When we inject and unpack the meaning embedded in these horizontal lines along the graph, students will be engaged to grasp the essential mechanism behind the specific physiological phenomenon.

Let's take two examples, one on autoregulation of blood flow and the other on the hemoglobin-oxygen (Hb-O2) saturation graph.

Vascular autoregulation

The X-axis is blood pressure that perfuses the tissues, and the Y-axis is the blood flow [Figure 4].
Figure 4: Autoregulation of blood flow in the brain, heart, and kidneys

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Autoregulation is an intrinsic mechanism observed in some organs that demonstrate the ability to maintain relatively constant blood perfusion over a certain defined range of blood pressure (BP) fluctuations. Over the range from about 60–160 mmHg, an initial change in the BP will either acutely decrease or increase the blood flow. Soon the autoregulatory mechanisms kick in, and blood flow is sustained at an optimal normal rate. Thus, the autoregulatory events are represented by the plateau where the flow (Y-axis) is unchanged.

This autoregulation is significantly evident in the cardiac and coronary circulations for obvious metabolic importance to supply uninterrupted, adequate blood flow to the heart and the brain. The renal autoregulation mechanism also regulates renal blood flow but not primarily for metabolic reactions but to maintain an optimal normal glomerular filtration.[2]

Thus, we have a plateau autoregulatory line that is inherent in the three organs, the heart, the brain, and the kidneys. One line to rule them all!

Oxy-hemoglobin dissociation graph

The relationship between oxygen and its unique carrier hemoglobin is not linear. The sigmoid profile of the Hb-O2 saturation graph in [Figure 5] shows a plateau that starts to emerge at 60 mmHg (X-axis). This is quite a significant point as the Hb-O2 saturation has reached 90% (Y-axis) at 60 mm Hg. If the relationship was linear, we might expect only a 60% saturation at 60 mmHg. The plateauing portion of the graph from 60 onwards to 100 mm Hg implies a few physiological characteristics pertaining to lung oxygenation.
Figure 5: Oxy-hemoglobin dissociation curve

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Lung oxygenation involves diffusion of oxygen from alveolar air to the pulmonary capillary. The partial pressure gradient for lung oxygenation decreases quickly along the alveolar capillaries as oxygenation is rapid (alveolar air 103 mmHg and initial deoxygenated mixed venous blood 40 mmHg). Also as seen in the Hb-O2 saturation graph, above 60 mmHg, the saturation changes much less, increases about 10%.

This plateau and the phenomenon of perfusion-limited lung oxygenation indicates that hyperventilation is not the primary reason for increased oxygen supply to the tissues during physical activity.[3] It is actually the increase in right ventricular cardiac output and the pulmonary blood flow that increase the rate of oxygen delivery to the tissue since the oxygen content per unit volume of blood is not markedly increased at the lungs by a higher alveolar PO2. The primary reason for hyperventilation is more toward the removal of metabolic carbon dioxide and preventing hypercapnia and a respiratory acidosis during physical activity.[1]

Hence, the students can now have a physiologic plateau learning experience from this understanding.

Besides a cause and effect use of graphs, they can also be helpfully drawn to show the ongoing sequence of changes in two parameters that are produced by a third causative event. Here, we can look at the example of the ventricular volume/pressure cardiac loop diagram.

  Loop the Loop and Students' Tachycardia Top

The cardiac cycle is also represented graphically with respect to the two changing parameters in the ventricles, namely, pressure and volume. The volume-pressure cardiac loop [Figure 6] can be mind-spinning for students that have graph anxieties. To help students to be less dizzy when looking at the information in the cardiac loop, a few highlights can be useful.
Figure 6: Volume-pressure cardiac loop. ESV = end systolic volume, EDV = end diastolic volume

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  1. Note that, it is ventricular volume (X-axis) and pressure (Y-axis). In this cardiac loop, the cause/effect mentioned above for other graphs does not apply. The ventricular volume does not cause or result in the ventricular pressure changes. The cardiac loop shows instead the associated volume and pressure in the ventricles at various points in the cardiac cycle. It is the ventricular contraction that produces the pressure not the volume per se
  2. The cardiac loop should be ascended and descended in an anticlockwise manner. I believe the ferris wheel also moves in that direction
  3. The two parallel lines ascending on the right and descending on the left would then indicate the sharp increase and drop in ventricular pressure at unchanged ventricular volume. These would be respectively the isovolumetric contraction phase (beginning of systole) and iso-volumetric relaxation phase (beginning of diastole)
  4. The beginning of systole and diastole are both marked by the closure of valves. Thus, the bottom right (before pressure ascending) (point B) would be the closure of the mitral valve (1st heart sound) and the upper left (before pressure descending) (point D) would be the closure of the aortic valve (2nd heart sound)
  5. By taking note that the X-axis is ventricular volume, the minimum volume would be end systolic volume (ESV, points D and A) and the maximum volume would be EDV (points B and C). The horizontal bottom line, arrow moving from left to right will be the filling phase of diastole. The upper line, arrow moving from right to left would be the reduction in ventricular volume during the ejection phase, showing some further increase in intra-ventricular pressure during myocardial contraction.[4]

  Temporal, Sequential Events Top

Graphs are useful to follow changing parameters in physiological cyclical events. More than one parameter can be followed dynamically with the X-axis as Time. For example in the respiratory cycle, changes in the various pressures during normal negative pressure breathing are demonstrated [Figure 7]. Students can take a deep breath and follow the lines representing intra-pleural pressure, intra-alveolar pressure and the trans-pulmonary pressure as air is inspired and expired as tidal volume.[5]
Figure 7: Changes in pressures and volume during a normal breathing cycle

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This brief narrative on graphs in physiology hopefully will take away some anxieties from students that are averse to graphs. For fellow teachers, we would encourage you to devote some unhurried time during small group tutorials to have a good graphing time with your students.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Cheng HM. Physiology Question-Based Learning. Cardio, Respiratory and Renal Systems. London: Springer International Publishing Switzerland; 2015.  Back to cited text no. 1
Cheng HM, Jusof FF. Defining Physiology: Principles, Themes, Concepts. Cardiovascular, Respiratory and Renal Physiology. Singapore: Springer; 2018.  Back to cited text no. 2
Cheng HM, Hoe SZ. Students' responses under “negative pressure” to respiratory questions at the 15th physiology quiz international event: 100 medical school teams from 22 countries. BLDE Univ J Health Sci 2017;2:105-8.  Back to cited text no. 3
  [Full text]  
Cheng HM, Mah KK. Cardiovascular Physiology – Figure-Based Instructions. Singapore: Cengage Learning Asia; 2014.  Back to cited text no. 4
Mah KK, Cheng HM. Learning and Teaching Tools for Basic and Clinical Respiratory Physiology. London: Springer International Publishing Switzerland; 2015.  Back to cited text no. 5


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]


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Graphs that Illu...
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