AMEI’s Current Trends in Diagnosis & Treatment
Volume 7 | Issue 1 | Year 2023

Engaging Physiology with Metaphor and Allegory

Hwee-Ming Cheng1, See-Ziau Hoe2

1,2Department of Physiology, Faculty of Medicine, Universiti Malaya, Kuala Lumpur, Wilayah Persekutuan, Malaysia

Corresponding Author: Hwee-Ming Cheng, Department of Physiology, Faculty of Medicine, Universiti Malaya, Kuala Lumpur, Wilayah Persekutuan, Malaysia, Phone: +603 79674920, e-mail:

How to cite this article: Cheng HM, Hoe SZ. Engaging Physiology with Metaphor and Allegory. AMEI’s Curr Trends Diagn Treat 2023;7(1):21–25.

Source of support: Nil

Conflict of interest: None

Received on: 06 February 2023; Accepted on: 26 February 2023; Published on: 20 July 2023


Use of imagination and images are helpful visuals to communicate ideas and concepts in Physiology. Story telling always engages the audience. The teacher can creatively use analogies, metaphors, and allegories to communicate events in Physiology. These approaches direct attention to the many homeostatic interactions and cross-talk between organs, electrolytes, transporters. The added use of cartoons makes Physiology learning enjoyable. The students themselves can also develop their own imaginative doodles to summarize and tell the Physiology story.

Keywords: Allegory, Concept, Metaphor, Physiology.


Metaphor and Allegory

A metaphor is similar to a simile. A simile makes a comparison using the words “like” or “as.” Example: Body temperature control is like the thermostatic regulation in a room. A metaphor makes the comparison directly. The brain is an ultra-intelligent, integrative machine.

An allegory is a story, poem, or picture that can be interpreted to reveal a hidden meaning, typically a moral or political one. The classic book by John Bunyan, “Pilgrim’s Progress” is an allegory of the spiritual journey.1 The wisdom of the body, a term by Walter Cannon describes the discernment of the homeostatic responses to different changes and stresses.2 Cross-talk, interactions between organs, solutes can be allegorized in inter-organ dialogues and conversations. These illustrated, creative narrations can engage students in learning and making sense of physiology.3

1. Physiological Metaphors for Cardiovascular System

A. The Cardiovascular System is a Constant, Smooth Traffic Highway

The systemic and pulmonary circuits are arranged in series to each other and together make a closed circulatory loop. Blood flow rate is evenly delivered through each part of this cardiovascular circuit, with the driving pressure provided by the two rhythmic ventricular pumps connected in series.4

Traffic jam occurs when the flow of vehicles along the highway becomes uneven. Traffic congestion develops. When the cardiac output from the right and the left ventricles are not equalized, blood traffic congestion or vascular congestion likewise results. In left ventricular failure, the vascular congestion will happen in the pulmonary circuit with the development of pulmonary edema. In right cardiac failure, vascular congestion with peripheral edema is the consequence.

The right and left cardiac outputs are equalized by Starling’s mechanism of the heart that matches the ventricular filling with the ejected stroke volume.

B. The Cardiovascular Circulation is a Bloody River

There are regions of greater resistance to blood flow, or vascular resistance, and most of this is at the smooth muscular arterioles. The basal smooth muscle tone and sympathetic innervations at the arterioles change the total peripheral vascular resistance (TPR) significantly.5

It is important to distinguish the effect of increased arteriolar resistance on the upstream and the downstream blood pressure. Increasing the TPR by arteriolar constriction in selected organs (coronary and cerebral are spared) will increase the upstream arterial blood pressure. At the same time, the downstream capillary pressure will be decreased.

In hypovolemia, the baroreflex sympathetic vasoconstriction produces this reduction in capillary hydrostatic pressure, which in turn favors a transcapillary shift of fluid from interstitial fluid (ISF) as a compensation to maintain vascular volume.

The other organ where this upstream/downstream concept is useful to understand is at the renal glomerular capillary which is sandwiched between the afferent and efferent arterioles. Text-books discuss each renal arteriole separately, where constriction of pre-glomerular afferent arteriole will reduce the downstream glomerular hydrostatic pressure. Conversely, constriction of the post-glomerular efferent arteriole will raise the upstream glomerular hydrostatic pressure. The renal peritubular capillary is in series with the glomerulus, joined by the efferent arteriole or downstream from the efferent arteriole.6

Filtration fraction (FF) at the glomerulus will affect the downstream hydrostatic pressure in the peritubular capillary (Ppt). An increased FF will lower the Ppt.

2. Metaphors for Respiratory System

A. The Lungs and Chest Wall Pull Each Other in a Tug-of-war

In pulmonary physiology, the lung and chest wall are physically associated and move together with inspiration and expiration. The elastic lungs are expanded from birth and thus there is always an inward recoil force for the lungs. The chest wall is a rigid structure and the direction of its recoil force depends on its position. Think of the chicken breast bone and how attempts to compress or stretch the two prongs of the bone produce an opposite recoil force.

In a normal respiratory cycle, at the end of a normal breath (functional residual capacity, FRC), the lung inward recoil is balanced by the outward chest wall recoil. A good analogy is the game of tug-of-war with opposite teams pulling.

If lung elastic tissues are damaged as in emphysema, the lung inward recoil is diminished. With the normal chest wall outward recoil, the balance of the forces will shift to a greater FRC, a clinical feature in emphysematous patients.

Conversely, in pulmonary fibrosis, the inward lung recoil is enhanced, reducing the lung compliance. The FRC volume is decreased.

B. The Lungs are Pulmonary Elastic Balloons

In physiological language, any elastic structure has an elastic recoil and this recoil force opposes stretch or distension. The lung compliance is then inversely related to the lung elasticity. The other recoil force that is also inversely related to lung compliance is the alveolar surface tension force.5

It is harder to expand a balloon from its collapsed state. Once a balloon begins to fill with air, less effort is needed to enlarge it. But as it nears the end of its distension limit, a greater effort is needed again to expand the balloon further.

This balloon characteristic quite well describes the changing lung compliance with lung volume. In the lung compliance curve relating transpulmonary distending pressure and lung volume, the graph is a sigmoid shape. The lung compliance (change in lung volume per change in distending pressure) is best at the steepest part of the curve. At small lung volume and also at large lung volume, the lung compliance is reduced.

C. Lung Oxygenation is Passenger Transport from Fresh Air Transit Lounge

Imagine that the pulmonary blood flow to be shuttle buses that transport airline passengers to their hotels (cells). The rate of lung oxygenation (mL O2/time) has been described as perfusion or flow-limited. This is due to the rapid equilibration between alveolar air partial pressure of oxygen (PO2) and pulmonary blood PO2 during the alveolar capillary transit time. The analogy here with the shuttle buses will be when the seats in each bus are all taken. The only way to transport more passengers to their hotel is to increase the number of buses to pick up more travelers at the airport.

The rate of lung oxygenation is increased during physical activity. This is primarily due to a greater cardiac output from the right ventricle, delivering more deoxygenated blood to be re-oxygenated.

3. Metaphors for Renal Events

A. Renal Tubular Fluid Flow is a Slow Yellow River with Diverse Riverside Activities

There are two flow-determined events in renal physiology which are normally presented to students, namely, reduced tubular fluid flow and urea retention; and increased tubular fluid flow and hyperkaliuria.

Reduced Glomerular Filtration Rate (GFR) and Urea Retention

Urea is passively reabsorbed at the proximal late segment after isosmotic water reabsorption at the early proximal segment. In renal failure, when the GFR and tubular filtrate fluid flow are decreased, more water will be osmotically reabsorbed following more sodium reabsorption. Thus, the urea concentration downstream will be higher. A greater gradient exists for more urea reabsorption and thus urea retention.7

Metaphor: The tubular fluid flow represents kids on a boat, holding sodium yellow flowers and the proximal tubular epithelial cells are pretty girls out to claim those flowers. If the boat flows past slower, more sodium flowers will be grabbed. Iso-osmotic reabsorption of water that follows sodium will be increased.

Increased Tubular Fluid Flow during Diuresis and Urinary Potassium Excretion

Passive events are characteristically affected by flow rates. In the case for potassium secretion at the principal cells of renal collecting ducts, diuresis tends to enhance the luminal membrane passive potassium secretion step via potassium channels. A secondary loss of potassium is observed.

Metaphor: The tubular fluid flow represents empty boats with fixed potassium passenger seats. When the flow rate or more boats are passing the principal cells, more potassium from the tubular epithelial cells can be picked up by the boats.

4. Physiological Allegories

A. Soddy Talks to Potty

A “Phyairy” Tale of Soddy and Potty

Once there were two Cats named Soddy and Potty.

They lived in a Gated community called Cellominium.

When they are resting, Soddy prefers the outdoors while Potty sleeps inside.

And frequently, when Soddy decides to enter the house, Potty will go outside.

Soddy and Potty appear to like to have their own space.

Soddy also often likes to bring friends into the Cellominium.

The creative physiology students will recognize the two Cats (Cations!) as SODium and POTassium. Sod is the signature extracellular fluid (ECF) cation and Pot is the major intracellular fluid (ICF) cation. Do you notice the sentence indicating the Sod/Pot exchange representing the cationic fluxes during an action potential? And the last line is obviously sodium coupled membrane transport! And Gated Community was already in Physiology before the housing developer copied the term!

B. Skeletal Muscle and Smooth Muscle in Conversation

The qualitative differences between skeletal and smooth muscle are featured in this allegory (Fig. 1). The neurotransmitter acetylcholine released at the endplate of the skeletal muscle is always excitatory. Autonomic neural and hormones that act on smooth muscle can be contracting or relaxing. The arteriolar smooth muscle is the major site of vascular resistance and plays a key role in blood pressure regulation. In the skin, cutaneous vasodilation is a homeostatic response to lose heat during thermoregulation. The contractile filaments, myosin and actin are common to both muscle types although their intracellular arrangement is different. A rise in intracellular calcium is the trigger for skeletal, smooth (and cardiac) muscle contractions. For skeletal muscle, all the trigger calcium is from the sarcoplasmic reticulum. Extracellular calcium influx participates in smooth and cardiac muscle functions.

Fig. 1: Skeletal muscle and smooth muscle in conversation

C. Conversation between the Two Contractors, Rectum and Bladder

The smooth muscle physiology of the rectum and urinary bladder is in this allegory (Fig. 2). Brad’s urinary bladder is filled less when body fluid is lost as sweat. Dehydration leads to production of a concentrated smaller urine volume. The micturition reflex involves the autonomic parasympathetic and sympathetic coordinated responses. When urine is evacuated, the parasympathetic activity increases to contract the bladder and reduced sympathetic input relaxes the urinary sphincter. Both the rectum and the bladder have smooth muscle plasticity that enables them to store feces and urine, respectively with no great increase in muscle tension.

Fig. 2: Conversation between the two contractors, rectum and bladder

D. Rina Talks to Brain (Nephrons and Neurons in Conversation)

In this allegory, the kidney Rina declared that her brainy friend Brian always autoregulate his blood flow to ensure his essential neuronal functions (Fig. 3). Brian understands that Rina’s renal autoregulatory function is overridden when there is a need to restore blood volume/blood pressure. Rina’s two autoregulation mechanisms are the myogenic and the macula densa responses. So, in hypovolemia, Rina’s renal blood flow is temporarily reduced by renal vasoconstriction as part of the compensatory homeostatic reflex to increase TPR and decrease filtered sodium load (normal sodium balance determines blood volume). No Kidding, Rina’s momentary sacrifice when renal sympathetic arteriolar vasoconstriction overrides myogenic vascular autoregulatory response preserves the blood flow for her brainy friend Brian.

Fig. 3: Nephrons and neurons in conversation

E. Sodium and Water Countercurrent Relationships

The renal countercurrent mechanism for osmoregulation or water balance involves the subpopulation of juxtamedullary nephrons. The differential permeabilities of the different nephron segments and the collecting ducts to water and sodium contribute to the generation of an osmotic stratification in the renal medulla, reaching a maximum of 1300 milliOsm/L.

Allegory: Imagine a conversation between Walter speaking (water) to Soddy (sodium) at various sites.

Proximal convoluted tubule: “I will follow you.” Isoosmotic reabsorption of water where the generation of a local osmotic gradient drives water reabsorption following sodium/solute reabsorption.

Descending loop of Henle: “You can’t follow me.” Water is osmotically absorbed by the surrounding hypertonic renal medulla. Sodium remains in the tubular fluid and is increasingly concentrated, up to a maximum at the tip of the loop.

Ascending loop of Henle: “I can’t follow you.” The ascending loop of Henle is uniquely impermeable to water. It is expected that there is no expression of aquaporins here on the membranes of the tubular epithelial cells. Sodium, however, is reabsorbed here and this includes the activity of the Na/K/2Cl neutral symporter.

F. Urine My Heart (Conversations between the Heart and the Kidneys; Myo and Reno)

Reno: I know when you are tired.

Myo: No Kidding! How do you know?

Reno: When you are tired, I also feel less stretched out.

Cardiac output (CO) determines the effective circulating volume to peripheral tissues including the kidneys. A lower CO is monitored by the intrarenal stretch/baroreceptors at the pre-glomerular afferent arteriole.

Reno: I make sure you have enough blood.

Myo: Huh? I thought I am the one that supplies you with blood.

Reno: That is true also but I am the one that ensures that the volume of blood we share is always enough.

Circulating blood volume is fixed and the cardiac ventricular filling is affected by changes in blood volume and venous return. The kidneys regulate sodium balance which determines the ECF/blood volume.

Myo: When I am tired, I will signal you for sympathetic help.

Reno: Certainly, I will surely respond to your sympathetic message.

A decrease in arterial blood pressure when the heart weakens will activate a baroreflex increase in sympathetic activity to the kidneys to trigger blood pressure compensatory mechanisms.

These mechanisms include secretion of renin, renal arteriolar constriction, and less urinary excretion of sodium to conserve ECF/blood volume.


Teaching Physiology is a learning journey for educators. Since whole body Physiology is an integrated homeostatic entity, there are different possible approaches to communicate and tell the story of mechanistic events in the body. It is useful to identify the key areas in Physiology and illustrate and elaborate these using creative instructional methods in class. The use of metaphors and allegories presented in this article engages, entertains the students and makes their study less burdensome as they study and sift through a large volume of knowledge in Physiology. Students themselves can imagine and enrich their appreciation of physiology by generating their own metaphors, analogies, and allegories by writing, drawing, and perhaps even dramatizing Physiology with their classmates.

Dependent learning is spoon feeding as in the helpless baby. Self-directed learning as popularly encouraged push students to stretch and enlarge their capacities to select, discern, and retain foundational information for future applications. The ability to use metaphors and allegories reflect a good understanding and also a versatile ability to explain accurate Physiology to others in interesting ways.

Aphorisms can also be used in Physiology teaching. This will be short concise engaging physiology observations that highlight key principles in Physiology. A few physiologic aphorisms are given in Table 1. We hope this article will stimulate Physiology colleagues to explore other innovative ways to deliver their own creative versions of Physiology.

Table 1: Cheng’s “aPhyorism” (a fusion of Physiology and aphorism)
No. Aphorism Explanation
1 Cells always like when hemoglobin (Hb) dislikes O2 When hemoglobin’s affinity for O2 is reduced, it always benefits the cells that can then receive more O2
 This increased O2 unloading from Hb exactly happens when the cells are more active and the cellular environment is then warmer, more acidic and hypercapnic
2 The renal autonomic nerve is always sympathetic to sodium The renal sympathetic nerve (RSN) vasoconstricts both the renal arterioles. This vascular action contributes to the raised TPR to maintain blood pressure when hypovolemia activates via baroreflex an increased sympathetic activity that includes the renal sympathetic action
 In addition, the sodium excretion is always reduced by RSN. This is achieved in three concerted ways; by a reduced filtered sodium load when the renal blood flow/GFR is decreased by arteriolar constriction, direct action to increase proximal tubular sodium reabsorption and release of hormone renin from the juxtaglomerular cell
3 CO2 has a concerted role in oxygenating cells This sounds unfamiliar and strange. If we consider CO2 merely as a metabolic waste product, this statement does not make sense.
 But CO2 is a physiologic multitasker, especially during increased cellular metabolism.
 CO2 is a vasodilator and this increases the perfusion of oxygenated blood to the tissues. CO2 by Bohr’s effect unloads more O2 to the active cells
 And increased arterial partial pressure of CO2 (PCO2) is a more potent and the principal stimulus for increased alveolar ventilation rather than decreased PO2 during physical activity

GFR, glomerular filtration rate


See-Ziau Hoe


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2. Cannon WB. Wisdom of the Body. Rev. and Enl. Ed. Philadelphia:W. W. Norton & Company, 1963.

3. Cheng HM, Hoe SZ. Using portmanteau to create meanings in new physiology words. AMEI’s Curr Trends Diagn Treat 2022;6(1):18–22. DOI: 10.5005/jp-journals-10055-0139.

4. Ming CH. Concept-applied physiology thinking. AMEI’s Curr Trends Diagn Treat 2021;5(1):58–61. DOI: 10.5005/jp-journals-10055-0116.

5. Cheng HM, Jusof F. Defining physiology: Principles, themes, concepts: Cardiovascular, Respiratory and Renal Physiology. Singapore:Springer Nature, 2018.

6. Cheng HM, Hoe SZ. Untangle the convoluted NOTs: learning renal physiology. AMEI’s Curr Trends Diagn Treat 2021;5(2):116–120. DOI: 10.5005/jp-journals-10055-0127.

7. Cheng HM. Physiology Question-Based Learning: Cardio, Respiratory and Renal Systems. Cham:Springer International Publishing Switzerland, 2015.

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