Essential analogies in human anatomy & physiology.
Article Type: Report
Subject: Human anatomy (Study and teaching)
Analogy (Methods)
Classroom management (Management)
Authors: Sundrud, R. Bruce
Hueftle, Kris
Pub Date: 11/01/2009
Publication: Name: The American Biology Teacher Publisher: National Association of Biology Teachers Audience: Academic; Professional Format: Magazine/Journal Subject: Biological sciences; Education Copyright: COPYRIGHT 2009 National Association of Biology Teachers ISSN: 0002-7685
Issue: Date: Nov-Dec, 2009 Source Volume: 71 Source Issue: 9
Topic: Event Code: 200 Management dynamics Computer Subject: Company business management
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 246348918

Teachers know that learning proceeds from the known to the unknown. Analogies help a student learn abstract material by relating it to his or her known experiences. Gentner (1998, p.107) defines analogies as partial similarities between different situations that support further inferences.

Some analogies are so useful that we call them essential analogies. Here is a topical listing of analogies we have used, some of which we consider essential. This list of analogies will be maintained and updated at You are invited to contribute additional analogies at or

* Homeostasis

Maintaining constant internal conditions is a balancing act for all life. Humans are like the baby bear in Goldilocks in our needs: not too hot, not too cold, not too much water, not too little water.

Positive feedback increases the activity: "Don't laugh at his jokes, you'll only encourage him." Negative feedback decreases the activity, and you can cite American Idol examples. A teeter-totter has positive feedback and is difficult to balance evenly. A rocking chair has negative feedback, and returns to a middle state. Pathology is when the body rocks too far and can't correct itself.

* Chemistry

Chemistry can be very intimidating, especially to returning students. To help explain the unequal sharing of electrons, we use comparisons like these: When the author (Hueftle) shared the TV with his little sister, he watched the game and only during the commercials did she get to watch cartoons. When the author (Sundrud) shared an ice cream cone with his big sister, the ice cream cone spent most of the time in her hand, not his.

An enzyme and its substrate are traditionally compared to a lock and key, so protein/enzyme denaturation is like bending the key: It stops working.

An enzyme can also be compared to a matchmaker (Fiddler on the Roof) in that it is not used up by each reaction catalyzed (or marriage arranged). Heating a solution to increase reaction rates is like giving the matchmaker roller skates so she can work faster.

Emulsification of fats works like crunching a hard candy, thus increasing the surface area and letting it dissolve quickly.

To illustrate monomers and polymers, hydrolysis and synthesis, use large toy pop-together beads as monomers. Folding the chain so that the blocks touch would show hydrogen bonds.

The Hindenburg Disaster of 1937 is an example, not an analogy, but useful in teaching about the periodic table, as the next question is: Why was the Hindenburg flammable, but not the Goodyear blimp? Hydrogen has a vacancy in its outer shell; in Helium, the "No Vacancy" sign is blinking.

* Cells

Most students believe in cells, but don't understand the details. The limitations of the light microscope can be explained by citing television or the comics--putting your nose to the screen doesn't show any additional detail. Using the light microscope to see mitochondria is like wearing boxing gloves while feeling for a dropped pin.

The cell is not like a water balloon with marbles in it. It's more like a baggie of Jell-O fruit salad. The fruits are the organelles, the Jell-O is the cytoplasm, and the baggie is the membrane. Put the baggie in a box to visualize a cell wall.

The cell membrane can also be compared to raisin bread, with the bread being the two layers of phospholipids, and the raisins representing proteins. Big raisins (integral) penetrate the bread and small ones (peripheral) are only on the surface. Wrap the bread around a ball (cytoplasm) to give the three-dimensional aspect.

The Golgi complex looks like a squashed stack of incompletely-cooked pancakes. The lysosome, with its powerful digestive enzymes, can be compared to a "suicide pill," and cytoskeleton microtubules compared to reinforcing rods of concrete.

To illustrate diffusion, think of red and blue smoke bombs on opposite sides of a room. What is the eventual uniform color? A front row student with powerful aftershave is another example.

Facilitated versus active transport can be illustrated by specialized gates for goats, chickens, etc. In facilitated transport, the animals go through on their own, but in active transport, the animals are pulled in or out.

Osmosis is a concept students desperately need to know and some never grasp. The author (Sundrud) has taken to scrawling on the side of the whiteboard in large red letters, OSMOSIS SUCKS! The author (Hueftle) explains tonicity by mentioning a sweet tea common in the south. Plain tea is hypotonic, but the tea at the barbecue joints is hypertonic (you almost see sugar crystals forming). Tea with just the right amount of sugar would be isotonic tea. Hypertonic solutions shrivel cells like raisins; hypotonic solutions rupture cells like over-inflated balloons.

In protein synthesis, the ribosome reads the mRNA like a team roster, and lines up the players.

* Cell Division

A discussion of protein synthesis naturally brings us to DNA, a topic daunting to students. In DNA replication, the cell opens up its DNA (unzipping the zipper) like opening up a book, lays it on the copier, and pushes the copy and staple (centromere) buttons.

In prophase, the chromosomes coil, just like strings of holiday lights should be rolled and stored separately in order to move them without tangling.

In human meiosis, the polar body is like a garbage bag in that it is a repository for unwanted halves of homologous pairs.

* Tissues

No, not facial tissue, we patiently explain, but clusters of similar cells. Cartilage can be compared to Swiss cheese, plain (hyaline) or stringy (fibrous and elastic).

Squamous cells are like fried eggs, cuboidal cells are like marshmallows.

The epidermis of our skin has dead outer layers that are being shed, similar to the dead layers found around an onion.

* Integumentary System

The skin, the largest organ in the body, is a complex structure whose functions are often misunderstood: Witness the painted dead lady in the film Goldfinger. Melanocytes spread out melanin like opening up an umbrella. Sebaceous glands act like oil cans, and mammary glands make condensed milk (colostrum) before making whole milk.

* Skeletal System

Most skeletal analogies are obvious enough that we won't list them here.

* Muscular System

Terminology is the challenge in the muscle system. To help explain muscle structure, have a bunch of pencils and plastic wrap to make the endomysium, perimysium, and epimysium. Using a Tootsie Roll, the candy is the muscle, the paper is epimysium, and ends of the paper are tendons.

Muscle cells use the root "sarco" to make sarcolemma, sarcoplasm, sarcoplasmic reticulum, and the sarcomere, just like Batman has the batcave, the batmobile, and the batcopter.

To explain skeletal muscle cell structure, hold a couple of bottle brushes together representing myosin, with the bristles being the crossbridges. Also, if we think of actin as bacon wraps, the actin would be the hot dog, the bacon wrapped around it is the tropomyosin, and the troponin is the toothpick. A myofibril is like a hot dog surrounded by a thin, holey skin (SR). These are chilidogs because between the hot dogs are chili beans (mitochondria). The sarcolemma would be the bun.

Muscle stimulus thresholds are like trying to get someone's attention. Subliminal stimuli gets no response. Refractory periods are like toilets, with tanks that must refill before flushing again.

* Nervous System

It's easy to confuse students about the nervous system--nerves are not wires, and they don't carry a current. Compare the nerve impulse to a row of dominoes; the signal has constant velocity and constant strength no matter how long the row. Glue down a domino to represent local anesthetic (sodium blocker). This domino analogy is the epitome of an essential analogy.

The myelin sheath wraps around the axon like paper towels around the cardboard center.

The neurofibril nodes (of Ranvier) speed the nerve impulse just as email is faster than physically moving a letter with a stamp.

A light switch can represent a synaptic threshold: Tapping the light switch does nothing until the switch flips, then it flips completely.

The dura mater is like the peel of a tangerine and can be easily removed, whereas the pia mater clings like the skin of an apple and can't be removed without damaging the cerebral cortex.

The problem with boxing is that the brain floats in the cranial cavity like pickles in a pickle jar. The cranium rotates from a good punch and the brain wants to stay put.

The pinhole camera needs no lens because the tiny opening causes a large depth of field, which helps understand the contraction of the iris. The smaller the pupil, the greater the depth of field, which is why bright lights make everything seem clear and sharp. Miniature movie monsters can look fake because of the narrow depth of field.

The stomach is the garbage can of the face, as the lacrimal canals carry dust from the eyes to the nasal passages, which drain into the stomach.

Rhodopsin allows light perception like a soldier in a foxhole raising a helmet on a stick to see if the enemy is still there.

* Endocrine System

The endocrine system involves learning a lot of functions and malfunctions. The control of some hormones is like a chain of command: boss/foreman/worker. The author (Sundrud) enjoyed pointing to the hypothalamus and then the hypophysis (pituitary or "master gland") while saying "Hillary--Bill." That reference is out of date, but comparing the hypophysis to a puppet is apt.

The adrenal gland might be compared to a jelly donut, with the dough being the cortex and the jelly being the medulla. The dough isn't completely cooked, so there are three layers--the different zones, with the zona glomerulosa being the outermost.

A lack of insulin would be like having money in the bank (high blood sugar) and forgetting your PIN.

* Cardiovascular System


This is a wonderful system to teach, with much student interest. Push your fist into a soft latex balloon; the layer around your fist is the visceral pericardium, the outer layer is the parietal.

The heart is like a four-room house: The vinyl siding is epicardium, the walls are myocardium, and the glossy interior paint is endocardium.

The valves of the heart open and close like a screen door in the wind. Valves make no sound as they open. You can slam a door but you can't make a loud noise opening it.

The atrioventricular valves are like saloon doors with bungee cords attached on the inner side (chordae tendineae). The chordae tendineae are like parachute cords that prevent the inversion of the AV valves. Imagine tying cords from the handle of an umbrella to the spokes. You might fly like Mary Poppins, but the umbrella won't turn inside out.

Semilunar valves are thicker, so they don't need strings. They're like the Three Stooges trying to go through a door at the same time and just get stuck, so they go back and try again.

To illustrate Starling's law of the heart, think of a rubber band. The more it is stretched, the stronger it contracts. This analogy is flawed--it's not just elastic rebound, but a stronger muscle contraction.

Within normal limits, cardiac output control is like stepping on the gas to speed up. But Marey's Law is like seeing the speedometer needle rising and backing off the gas (negative feedback). In other words, faster heart rate (within normal limits) will raise blood pressure, but rising blood pressure will slow the heart.

In fact, think of controlling the heart rate like driving a car that has a fast idle (sinoatrial node's regular rate is 100 beats/minute). Your heart rate is usually slower, so most of the time you're using the brake (parasympathetic nerve) and seldom the gas pedal (sympathetic nerve).

The AV node and conduction myofibers allow the lower portion of the ventricles to contract first rather than contracting from the top to the bottom, like squeezing a toothpaste tube from the bottom up.

Different sizes of toothpaste tubes would illustrate stroke volume. Preload would be squeezing two different size tubes, so more comes out the bigger tube. Contractility would be squeezing two tubes with different strength. Afterload would be squeezing the tube with the cap on slightly.

A puzzle: Two people are scooping water out of a tub, both moving the same amount of water, but one person is scooping slower. How can this be? Because that person has a bigger scoop, which is similar to having a larger heart capacity due to exercise hypertrophy, which is harmless.

Fibrillation is like a "cramp" in the heart, and can be compared to a human wave going around a stadium. We could electrify the stadium seats and make everyone jump up at once (defibrillation). Then the wave dies out.


Arteries and veins are like hoses of different thicknesses. Capillaries are like a soaker hose, always leaking. The analogy breaks down, though, because albumin in the blood pulls the fluid back in.

A capillary bed is like the delta at the end of a river. All the branching slows down the flow.

The tunica media in arteries and veins is like meat (muscle) and cheese (elastic connective tissue) in a sandwich. Arteries near the heart have more cheese (conducting) while arteries in the appendages have more meat (distributing).

Elastic tissue helps to moderate or average changes in blood pressure just as a line to a popular park ride will fill and empty, but the ride (capillary bed) runs at a constant rate.

The capillaries are more like a washing machine than a buffet, as Starling's law of the capillaries shows active fluid exchange.

Blood viscosity affects blood pressure--think of pouring cheap maple syrup and thick maple syrup through a funnel. Vasodilation increases the size of the funnel opening.

Varicose veins and hemorrhoids are like a balloon that has been overinflated; they don't go back to their original shape.

Cutting out salt can lower blood pressure by reducing blood volume, like letting air out of a tire. Death due to blood loss is actually pressure loss, not blood loss, as 2/3 of the blood is still in the body. But like a tire that is only flat on the bottom, it ceases to function!

The absolute refractory period is like a mandated coffee break, and the sphygmomanometer is an inflatable tourniquet.

Atherosclerosis starts from injury to the arterial wall, just like scratching your Teflon frying pan. Material will stick, which we call the plaque.


Red blood ceils have a biconcave shape; you'd have a slim waist too if you took out all your guts. Cut a pickled egg (common in Pennsylvania) in half and you'll see that the red pickling spice has not penetrated all the way to the yolk. The center of a spherical blood cell might not saturate with oxygen.

Oxygenated blood is bright red because oxygen + iron has a red color, like rust. In other words, the blood "rusts" as it passes through the lungs.

A blood clot is platelets and fibrin-like bricks and cement. One without the other isn't going to hold together well.

DIC (disseminated intravascular coagulation) is a disorder where the clot is destroyed by plasmin as soon as it's made. Sort of like Moe and Larry making a brick wall while Curly sledgehammers it down.

* Respiratory System

In goes the good air, out goes the bad air. The trachea is like a vacuum cleaner hose, being flexible but not collapsible. And the voicebox is pronounced lar-rinks, not lar-nix. Phar-rinks, not pharnix. For shame!

The mucus of the respiratory tract is like flypaper to trap dust, or perhaps like Roach Motel.

The stomach is also the garbage can of the respiratory tract, because dust is carried up the trachea by cilia, and then swallowed.

The branches of the bronchial system can be illustrated by describing a squirrel running out on a limb until it gets to the leaves (alveoli). Alveoli are a bit like a water balloon. If you hold a water balloon by its neck, as you turn your hand the balloon still hangs down. This is not anatomically inaccurate, but explains why you can't drain fluid from the lungs by hanging upside-down.

Surface tension is demonstrated in wet t-shirt contests that show off men's muscles. This gets a chuckle because, of course, this chauvinistic practice usually involves females. This remark will not get you fired. Trust us. No, we don't give job references.

The "bends" (decompression sickness) is similar to the formation of bubbles when the top is removed from a bottle of soda.

Oxygen affinity is like a delivery person who must pick up the package (high affinity) as well as deliver it (low affinity).

* Digestive System

In this humble system, peristalsis is similar to the movement of an earthworm.

Inactive pepsinogen (note the same ending as fibrinogen and trypsinogen) is like a grenade with the pin still in; safe until needed.

In the small intestine, the permanent circular folds (plicae circulares) are like a shag carpet that hasn't been properly stretched. Each shag (thread) is a villus, and the fuzziness would be the microvilli.

A liver lobule is like a spoked wheel with the spokes arranged in groups of three. One spoke has fluid (bile) moving toward the outside, while the other two have fluid (blood) moving toward the hub.

Bilirubin has the word root "ruby" in it, which means red, yet through light skin it appears yellow. Yellow food dye appears red in the container, but turns yellow when diluted in frosting.

Organs such as the liver and pancreas take the shape of the space allotted. One might as well inject latex into the area and see what shape they take. Unlike the heart, the gross anatomy of these organs is not related to function.

* Nutrition

Littered with fads and misconceptions, the topic of nutrition has lots of opportunities to compare honest research with scams from hucksters.

The Krebs cycle is like a water wheel. Acetyl-coA is the water that comes in. The buckets on the wheel are the enzymes that perform the various reactions. The water coming out represents the carbon dioxides. The Krebs cycle can also be compared to a Veg-O-Matic that grinds up the acetyl-coA and spews out the good stuff (hydrogens and their electrons). Or, acetyl-coA can be the farm girl (acetyl-) in the prom dress (-coA) dancing with Freddie Kruger (oxaloacetic acid).

NAD and FAD are like hot pads to carry the hydrogens and their electrons.

One could compare the electron transport system to a Slinky[R] toy going downstairs, carefully unleashing the energy step by step. Another analogy is an exchange bank where one form of high energy compound (NADH and FADH) has to be converted into a useable form (ATP).

A coupled reaction is like a teeter-totter where one side gives up height to raise the other end. As glucose is catabolized, ATP is synthesized, transferring energy from glucose to ATE

ATP is like a wheelbarrow; it doesn't originate the soil (energy), it merely transfers it. Making ATP is like setting a mousetrap. The mousetrap analogy is a good one because there is no matter added to the mousetrap nor removed as the mousetrap is set (high energy state) and sprung (low energy state).

Vitamins are like tire irons. Without a tire iron, you can't change a tire. A box of tire irons won't help you change it faster. This is a good way to explain that excessive vitamins don't help, and may do harm.

* Urinary System

If the student doesn't understand osmosis, he/she will never understand the kidney.

Think of the glomerulus as a ball of string stuffed inside a cup (capsule). The convoluted tubules coil like spaghetti, so that the distal convoluted tubule can have its cells against the afferent arteriole of the glomerulus, allowing for the juxtaglomerular apparatus to form.

Hydrostatic pressure pushes, osmotic pressure pulls. In filtration, the capsular fluid pressure that resists filtration pressure is like everyone trying to leave a classroom when there are already students in the hallway who haven't left yet. Blood colloid osmotic pressure (due to protein that cannot filter out) pulls fluid back into the glomerulus and is like students returning to the classroom to collect forgotten keys and cell phones.

Sodium in the countercurrent multiplier of the loop of the nephron (Henle) is like a leaf caught in a whirlpool in a stream. The water flows by, the leaf remains.

The kidney cleans the blood like cleaning a dresser drawer; it dumps everything out and returns what is wanted. Kidney stones occur when the kidneys concentrate urine, like rock candy forms from an evaporating saturated sugar solution.

The renal pelvis is like a hollow hand. The palm is the pelvis, the fingers are the calyces. The word pelvis means "basin" and it collects the flow from the calyces. The collecting ducts take filtrate from many nephrons, just as a sewer takes sewage from many houses. The ureters attach to the lower side of the bladder, so the ladder is like a hot air balloon where everything goes in and out at the bottom.

* Reproductive System

Some students think they already know about this system, but just because they can drive a Chevy doesn't mean they understand how it works.

The ovary is like an egg carton, a big one that holds lots of eggs. Each egg pocket is a follicle. When an egg is released from a pocket, the empty pocket becomes the corpus luteum. If you bring in an egg carton with eggs, you can pull an egg out and put in some cotton to represent the corpus luteum, making the hormones progesterone and estrogen.

To represent the uterine (Fallopian) tube--use your arm. Put your hand near a round object to represent an ovary. Your fingers are the fimbriae, your palm the infundibulum. Bend your wrist to simulate the ampulla. The rest of your arm is the isthmus.

The Fallopian tube is like a DustBuster. Eggs can't move, so the ciliated uterine tube draws fluid and the egg into the uterine tube.

Because of the amount of mitochondria compared to the size of the cell, the large spiral mitochondria in the body of the sperm are analogous to a sports car with a huge engine.

Sperm pushing their way through the cervical mucus and zona pellucida are like the old WWI war movies, where some soldiers fall on the barbed wire so that the others can get through.

The zona pellucida around the egg is a fertilization barrier, like a moat around the castle, and the corona radiata is like asking the knight to cut through the thorns around a castle before getting to the princess (aka Sleeping Beauty).

In embryonic development, the morula is like a baseball (solid); the blastocyst is like a volleyball (hollow).

The chorion of the embryo is not analogous to a filter. Noxious chemicals, drugs, caffeine, and alcohol can cross easily into the embryo or fetus.

Chromosomal mutations are like a misdeal with a card game. The cards can stick and someone can get too many (chromosomal addition) or not enough (chromosomal deletion).

* Conclusion

We create new analogies all the time, but we must remember that as we age, our analogies can become out of date. We don't want to start sounding like a broken record. Remember those vinyl disks with grooves? No? Neither do our students. The younger students don't know about Lizzie Bordon, Marilyn Monroe, the Vietnam War, the TMI accident, the Arab oil embargo, Rube Goldberg apparatus, and "I'm sorry, Dave. I can't do that." Stay modern, send us your newest analogies, and hang loose. Word up. Wait a minute. Do we still say that?


Gentner, D. (1998). Analogy. In W. Bechtel & G. Graham (Eds.), A Companion to Cognitive Science, (pp. 107-113). Oxford: Blackwell.


R. BRUCE SUNDRUD is Professor of Biology, Harrisburg Area Community College, Harrisburg, PA 17110-2999; e-mail: KRIS HUEFTLE is Instructor of Health and Sciences. Florence-Darlington Technical College, Florence, SC 29501-0548; e-mail:
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