Near the middle of John Keats’s Lamia, published in 1820, the story stops for a moment of open complaint. A young man of Corinth, Lycius, is celebrating his marriage to a woman who is really an enchantress, and at the wedding feast his old teacher, the philosopher Apollonius, fixes his gaze on the bride. Under that stare she will vanish, and Lycius will die. Before it happens, the narrator turns to the reader:
Do not all charms fly At the mere touch of cold philosophy? There was an awful rainbow once in heaven: We know her woof, her texture; she is given In the dull catalogue of common things.
Philosophy, Keats goes on, meaning natural philosophy, the science of his day, will “clip an Angel’s wings, / Conquer all mysteries by rule and line, / Empty the haunted air, and gnomed mine— / Unweave a rainbow.” The lines had a history. At a dinner given by the painter Benjamin Haydon on 28 December 1817, Keats had agreed with Charles Lamb that Newton had destroyed all the poetry of the rainbow by reducing it to the prismatic colours. Keats knew the science he was quarrelling with better than most poets. He had trained in medicine at Guy’s Hospital before he gave it up for verse.
The complaint is an old one and still a live one: that explanation turns wonder into an inventory, that the rainbow, once it is understood, becomes one more item in a list of things known. What interests me about Keats’s lines is that they get the order of events backwards for anyone who has studied the body. For a student of biology, the catalogue comes first. The body arrives as a list: the bones of the skull, the muscles of the forearm, the parts of the cell, the stages of a cycle, each named, labelled and drawn, each to be learned in order and reproduced on demand. The dull catalogue of common things is where the student begins. What rescues the student from it is not the absence of explanation but its arrival: the moment when the parts on the list begin to act on one another, and the inventory turns out to have been a description of a machine that works.
The title of this essay speaks of the physiology textbook, in the singular, and readers may expect a particular book, a particular edition, a remembered page. I will not name one or describe a copy. The experience in question, the first time the body becomes an explanation rather than a list, is common to a great many students of medicine and biology, and it is that category of experience, and the genre of book that produces it, that this essay is about. Its question is simple: what changes when the body becomes an explanation, and what does an explanation still leave out?
Two Ounces and a Thousand Beats #
The best way to see what explanation does is to watch it happen at a turning point, and one of the clearest examples in medicine is a calculation.
By the early seventeenth century, the vessels of the body had been described and named with great care. Anatomists knew the arteries and the veins, the chambers of the heart, the valves in the veins that Harvey’s teacher Fabricius had described. What they had was, in a sense, a superb list. The framework that organised it, inherited from Galen, held that blood was made continuously in the liver from digested food and was consumed by the tissues it nourished; arteries and veins carried different substances for different purposes. Everything was named. Nothing quite added up.
William Harvey’s De Motu Cordis, published at Frankfurt in 1628, made it add up, and the decisive step was arithmetic. In the ninth chapter, in Robert Willis’s nineteenth-century translation, Harvey asks the reader to assume a quantity:
Let us assume either arbitrarily or from experiment, the quantity of blood which the left ventricle of the heart will contain when distended to be, say two ounces, three ounces, one ounce and a half—in the dead body I have found it to hold upwards of two ounces.
Assume, he continues, that only a fraction of that is thrown into the aorta with each contraction, a quarter, a sixth, even an eighth. Now, in half an hour the heart beats more than a thousand times. Multiply the small quantity by the large number and the heart must send into the arteries, in half an hour, “a larger quantity in every case than is contained in the whole body!” He checks the reasoning against animals he has examined: if a sheep’s heart sent only a scruple of blood with each beat, it would pump about three and a half pounds in half an hour, and the body of a sheep, as he had found for himself, does not contain more than four pounds. The liver cannot possibly make blood at that rate from food. The blood that leaves the heart must come back to it. It goes round.
What makes this an explanation rather than another entry in the catalogue is the shape of the argument. Harvey does not need his numbers to be right. He deliberately chooses small ones, “assumed merely as a ground for reasoning,” and says that even if the interval were an hour, or a day, the conclusion would stand. The argument is built to survive error. It also commits itself. A list can be incomplete; it cannot be wrong in the way that an explanation can, because it makes no claim about how its items relate. Harvey’s calculation makes such a claim, and in doing so exposes itself to refutation: if the heart did not eject blood with each beat, if the body held vastly more blood than it does, the argument would fall. That vulnerability is the price of explanation and its value. It is what makes the explanation worth believing when it survives.
Harvey then tested a prediction on a living arm. In the eleventh chapter he describes binding the arm of a lean man with a fillet of the kind used for bloodletting. Tied as tightly as can be borne, the band stops the pulse beyond it; the artery throbs and swells above the band, and the hand stays its natural colour. Loosened to a middling tightness, so that arterial blood can pass in but venous blood cannot get out, the hand and arm become, within ten or fifteen beats, “excessively distended, injected, gorged with blood.” Physicians had long said that ligatures draw blood, and explained the drawing by heat, or pain, or a horror of the vacuum. Harvey replaced the old word with a mechanism. The blood is not drawn. It arrives by the arteries and is prevented from leaving by the veins.
There is one more thing in Harvey’s book that bears on Keats’s complaint. Its dedication to King Charles I opens with a sentence that is pure poetry: “The heart of animals is the foundation of their life, the sovereign of everything within them, the sun of their microcosm, that upon which all growth depends, from which all power proceeds.” The same man who multiplied drachms by heartbeats wrote that. In the founding text of modern physiology, the arithmetic and the image sit in the same volume, and neither seems to have embarrassed the other.
The Genre #
The physiology textbook is the genre that grew from that moment, and its distinguishing feature is grammatical. An anatomy atlas is a book of nouns. A physiology textbook is a book of verbs: the heart pumps, the kidney filters, the receptor fires, the vessel constricts, the hormone rises, the level falls. Its characteristic mark is the arrow.
Two ideas of the nineteenth and early twentieth centuries gave the genre its organising theme. The French physiologist Bernard, working in Paris in the middle of the nineteenth century, argued that the cells of complex animals live not in the outside world but in an internal environment, the milieu intérieur, and that the constancy of that inner environment is what allows the animal to live a free and independent life in a changing world outside. In his Introduction to the Study of Experimental Medicine of 1865 he also insisted that theories are only hypotheses, to be tested by facts and abandoned when the facts go against them. In 1926 the American physiologist Walter Cannon gave the constancy a name, homeostasis, and in 1932 wrote a book about it, The Wisdom of the Body. From then on, the physiology textbook had a plot. The body is a system of regulated variables, and physiology is the study of how they are kept within bounds.
One of the most widely used modern examples of the genre is the Textbook of Medical Physiology first published in 1956 by Arthur Guyton of the University of Mississippi. Guyton had meant to be a cardiovascular surgeon. In 1946, in the final year of his residency, he contracted polio, which left him with paralysis in his right leg, his left arm and both shoulders; during nine months of recovery at Warm Springs, Georgia, he designed a motorised wheelchair steered with a joystick, a hoist for lifting patients and special leg braces. Unable to operate, he turned to research and teaching. The first eight editions of his textbook, through 1991, were written entirely by him, a single voice explaining the whole of human physiology, with a new edition arriving about every five years for thirty-five years. From the ninth edition John E. Hall joined him as co-author, and the book, under Hall’s name and Guyton’s, reached its fifteenth edition in 2025.
A single-author textbook of that kind is a peculiar literary object. It is not an encyclopaedia, though it is nearly as large as one, because it has an argument: that the body can be understood as a set of controlled systems, each with its sensors and effectors, its set points and its time scales. And it has a voice, patient and declarative, the voice of a teacher who has decided what a student needs to understand first and what can wait. For a student who has spent years learning lists, that voice is the sound of the lists beginning to move.
That moment of crossing is, for many students, less a single revelation than a change of question. Anatomy asks what a structure is and where it lies. Physiology asks what happens when something changes: when a person stands, eats, runs, bleeds, sleeps, climbs a mountain or catches a fever. The first question can be answered once and for all. The second has to be answered with a sequence, and a sequence has a direction, a speed and consequences. Once a student has learned to ask the second question, the lists of the earlier years do not disappear. They become the cast of a play whose plot the student can now follow, and in which each named part has something to do.
Following One Variable #
The genre’s power is easiest to see by following one variable through one ordinary event. Take blood pressure, and take the act of standing up.
When a person who has been lying down stands, gravity pulls blood toward the legs. Several hundred millilitres shift out of the chest into the veins of the lower body, and for a few seconds less blood returns to the heart. The heart can only pump out what comes in, so the amount ejected with each beat falls, and arterial pressure dips. If nothing answered, the brain, which sits at the top of the column, would receive less blood, and the person would grow light-headed and might fall.
Something answers. In the walls of the carotid sinuses in the neck and the arch of the aorta in the chest are stretch receptors, baroreceptors, which fire faster as the vessel wall is stretched by pressure and more slowly as it relaxes. Their signals travel along the glossopharyngeal and vagus nerves to a cluster of neurons in the brainstem, the solitary nucleus, which uses the rate of firing as a measure of pressure. When pressure falls, the firing slows, and the brainstem responds by withdrawing the parasympathetic brake on the heart and increasing sympathetic drive. The heart beats faster and more forcefully; small arteries throughout the body narrow; veins stiffen and push blood back toward the chest. Within seconds, before the person has finished straightening, pressure has been largely restored. The reflex can begin to act within less than the span of a single heartbeat.
Stated like that, the event has the structure that every physiology textbook teaches a student to look for: a regulated variable, a sensor, an integrating centre, effectors and a time scale. It also has assumptions that the tidy statement hides. The receptors respond more to changes in pressure than to its absolute level, and over time they reset to whatever level they are exposed to. The response varies with age, with fitness, with hydration and with medicines. And the reflex is only one of several controllers, working on one time scale among several.
What the explanation buys, even in this simplified form, is the power to reason about situations no one has listed. If standing is a challenge because blood pools in the lower body and must be returned, then anything that enlarges the pool, or shrinks the supply, or slows the correction, should make standing harder. Heat, which opens the vessels of the skin, enlarges the pool. Loss of fluid, from vomiting or sweating or too little drinking, shrinks the supply. Medicines that relax blood vessels or slow the heart, and diseases that damage the nerves carrying the reflex, slow the correction. A student who has memorised a list of causes of light-headedness on standing knows the items on the list. A student who has understood the mechanism can generate the list, extend it to cases it does not mention, and see why two causes that seem unrelated, a hot afternoon and a new tablet, might add together in the same person. That is the practical meaning of the difference between a catalogue and an explanation. The catalogue tells you what has been seen. The explanation tells you where to look next.
Guyton put the larger picture in a short paper in Science in 1991. The arterial pressure of an adult, he wrote, rarely deviates from normal by more than ten to fifteen per cent during a day, and that constancy is achieved by a network of controls: neural receptors that respond within seconds, hormonal controllers that follow within minutes, and, within hours or days, a system in the kidneys that adjusts the volume of body fluid, raising it when pressure falls and lowering it when pressure rises. That kidney-fluid system, he argued, is the dominant method of setting pressure over the long term. In one paragraph the variable acquires a hierarchy of guardians, each faster or slower than the last, and the student who has followed it has learned something no list could teach: that the same quantity is defended by different mechanisms on different clocks.
What the Model Predicts #
An explanation earns its keep by predicting, and the most useful predictions are the ones that could fail.
If the baroreflex is what holds blood pressure steady, then cutting it ought to make pressure unsteady. And if the baroreflex is what sets the level of blood pressure over the long term, then cutting it ought also to make pressure high. These are two different predictions, and the second is the more interesting, because a reasonable student, having just learned how elegantly the reflex corrects a fall in pressure, would probably expect it to be true.
In 1973 Allen Cowley, Jean-François Liard and Guyton reported an experiment that tested both. In dogs whose baroreceptor nerves had been surgically cut, they made continuous recordings of arterial pressure through the day. A later review of the field summarised the result: after denervation the mean pressure was only minimally increased, while the fast variability of pressure increased dramatically with changes of posture and physical activity. The first prediction held. The second largely failed. The reflex, it appeared, is a buffer against rapid swings, not the thing that decides where the pressure settles over days. That conclusion fitted Guyton’s argument for the kidneys, and it has been argued over ever since; the same review notes that later studies came to diverse results, some in favour of a long-term role for the baroreflex and some against.
Clinical medicine uses the same logic every day, in a humbler form. Because the explanation says that standing challenges pressure and that the body normally corrects the challenge within moments, it tells a clinician what to measure and when: pressure lying down, and again after a minute and after three minutes of standing. A widely used consensus definition calls it orthostatic hypotension when systolic pressure falls by at least twenty millimetres of mercury, or diastolic by at least ten, within three minutes of standing. The definition is a prediction turned into a test. It says, in effect, here is what the mechanism should achieve; if it does not, look for the reason.
That is the deepest difference between a list and an explanation, and the one that changes how a student thinks. A list can be memorised or forgotten, complete or incomplete. An explanation can be checked. It says what would count as being wrong, and so it invites the experiment, the measurement or the observation that might show it to be wrong. The student who has crossed from the first kind of knowledge to the second has not merely learned more. He has acquired a different relation to what he knows.
The Feeling of Understanding #
That different relation has a hazard built into it, and psychology has measured it. In 2002 Leonid Rozenblit and Frank Keil published a series of studies under a title that names the problem: “The misunderstood limits of folk science: an illusion of explanatory depth.” People, they found, feel that they understand how complex things work with far greater precision, coherence and depth than they actually do, and the feeling shrinks when they are made to explain; another essay on this site, on the library desk, describes the experiment. Two further findings in the same paper bear directly on the physiology textbook. The illusion, they reported, was far stronger for explanatory knowledge than for other kinds of knowledge, such as knowledge of facts, procedures or narratives. And it was most robust where the environment supports explanations with visible mechanisms.
Put plainly, the crossing from list to explanation is also a crossing into a new kind of overconfidence. A student who has memorised a list knows, more or less, how much of it he has memorised; he can recite it and find where he stumbles. A student who has understood a mechanism, or feels that he has, carries a picture that seems whole, and the wholeness of the picture is exactly what makes its gaps invisible. A good diagram, with its clean arrows and its visible parts, is the kind of environment in which the illusion flourishes. It shows the mechanism so clearly that the viewer mistakes seeing it for knowing it.
The remedy Rozenblit and Keil’s studies imply is the same one the textbook’s own method provides: explain it, step by step, out loud or on paper, and see where the chain breaks. Ask what happens if the sensor is cut, if the supply falls, if the clock runs slower. A mechanism that has been explained in that way, and has survived the explaining, is understood. A mechanism that has only been looked at is admired. The difference matters a great deal in a clinic, where the feeling of understanding a patient’s physiology can arrive long before the understanding does.
The Diagram Has Edges #
A textbook diagram looks like the body in miniature. It is not. It is a selection, made by someone, for a purpose, and its clean arrows sometimes cover live disputes.
Guyton’s own work supplies a striking example. Among his best-known contributions were the venous return curves, graphs relating the pressure in the right atrium of the heart to the flow of blood returning to it, which he derived from experiments in the 1950s and which became a staple of physiology teaching. In 2011 Daniel Beard and Eric Feigl published a reanalysis in the American Journal of Physiology arguing that Guyton’s interpretation had interchanged the independent and dependent variables: in his experiments, they wrote, cardiac output set by an artificial pump was the causal variable, and right atrial pressure followed from it, so that the idea of atrial pressure as a back-pressure limiting venous return had confused physiologists and clinicians for decades. They suggested the concept be removed from educational materials. In 2013 The Journal of Physiology staged a formal debate on the question, with one side arguing that Guyton’s curves should be taught and the other, in a paper by Beard and Feigl, that they should not.
The kidneys have had their own argument. In 2016 Theodore Kurtz and colleagues reviewed salt-loading studies and argued that the evidence conflicted with the widely held view that normal people resist salt-induced rises in blood pressure because their kidneys rapidly excrete the extra salt; in properly controlled studies, they wrote, salt-resistant subjects retained substantial amounts of salt too. Whatever the outcome of that argument, its existence is the point. The arrow in the textbook that runs from kidney to volume to pressure represents a body of evidence still being contested by serious people.
A diagram also has edges in a plainer sense. It leaves out what its author judged secondary: the effects of age on the stiffness of arteries, the influence of a full stomach or a hot day, the medicines a person takes, the fear that raises pressure in a clinic and not at home. It shows the average case, and nobody is the average case. None of this is a criticism of diagrams. A map that included everything would be as large as the territory and as useless. It is a reminder that the diagram is an argument about what matters most, and that a student who mistakes it for the body has stopped reading at the moment the book was trying to begin a conversation.
Measurement Enters #
Between the explanation and the patient stands the number, and the number has a history of its own.
The first person to measure blood pressure directly was Stephen Hales, a clergyman who was perpetual curate of Teddington, outside London. In Haemastaticks, published in 1733, he described inserting fine tubes into the arteries of animals, a horse among them, and measuring the height to which the column of blood rose. A historian of the episode has written that Hales combined a mechanistic, quantitative approach with what he saw as a duty to discover and wonder at the wisdom and goodness of God in creation. He is a useful figure to set against Keats. Here was a man unweaving a rainbow, reducing the force of the blood to a column of a certain height in a glass tube, and doing it precisely in order to wonder more.
A century and a half later, measurement came to the bedside in a form that did not require opening an artery. In 1905 the Russian physician Nikolai Korotkov described the sounds that can be heard through a stethoscope below an inflated cuff as it is slowly released, sounds that mark the pressures at which blood begins to spurt through the compressed artery and then flows freely. Almost every blood pressure reading taken by hand since then has depended on them.
What the history teaches is that a measured number is three things that are easily confused. There is the number itself. There is the method that produced it: the position of the body, the size of the cuff, the time of day, the minutes of rest beforehand, the patient’s anxiety, the observer’s ear. And there is the interpretation, which depends on what the number is compared with. Laboratory reference ranges are commonly built from the central ninety-five per cent of values in a healthy reference population, which means that, for any single test, about one healthy person in twenty will fall outside the range by construction. A number outside the range is a question, not a verdict. A single reading is a sample, not a state. The explanation in the textbook tells a clinician what the number means if the method was sound and the person resembles the population behind the range. Neither of those conditions can be read off the number.
Patients, for their part, often arrive with lists of their own: a column of readings from a home machine, a printout of results with some values flagged, a sheet of medicines with doses. In a curious way the patient’s experience of medicine can repeat the student’s. The body comes to them as a catalogue of numbers and names, each one marked high or low or normal, and nothing on the page says how the items relate. Part of what a physician owes such a patient is the same service the textbook once performed for the student: to turn the list, where possible, into an explanation, to say why this number moves with that one, what the reading taken in the car park means compared with the one taken after a quiet morning at home, which flags matter and which are the expected noise of a reference range. An explanation offered in that spirit does not make the patient a physiologist. It gives her a way to hold her own numbers that is less frightening than a list, and more useful.
The Mechanism and the Person #
Here the essay’s own enthusiasm has to be tested. A mechanistic explanation can feel complete when it is not. It can be elegant, internally consistent, beautifully diagrammed, and wrong about what matters most to the person in front of the clinician.
Medicine has a famous lesson on the point. By the 1980s it was well established that people who had survived a heart attack and had frequent extra beats from the ventricles were at higher risk of sudden death. The mechanism seemed obvious: the extra beats were the sparks that set off the fatal rhythm, so drugs that suppressed them should save lives. The Cardiac Arrhythmia Suppression Trial was designed to test exactly that hypothesis. Patients whose extra beats could be suppressed by one of several drugs were randomly assigned to active drug or placebo. The arms using encainide and flecainide were stopped early because of excess deaths. In the published report of those arms, which included 1,498 patients followed for an average of ten months, 43 patients taking an active drug had died of arrhythmia, against 16 taking placebo. The drugs did what the mechanism said they should do to the extra beats. The patients taking them died more often. The authors wrote that the mechanisms underlying the excess mortality remained unknown.
The lesson is not that mechanisms are worthless; the trial was designed from one, and physiology is how anyone came to understand why such drugs can provoke the rhythms they are meant to prevent. The lesson is that a plausible mechanism is a reason to test, not a substitute for testing. Harvey’s explanation of the circulation was right, and bloodletting, whose fillet he used to demonstrate it, went on being practised for two centuries afterward. An explanation can be correct and still not tell a clinician what to do, and it can be persuasive and still lead the clinician astray.
There is a second limit, and it is not about evidence. A person who feels faint on standing is not a baroreflex. She is someone whose stairs have become frightening, who has begun to avoid the bus, who worries about falling at night on the way to the bathroom, who takes several medicines from several doctors, and who may care more about keeping her independence than about any number. The mechanism makes some of her experience intelligible; it can suggest where to look and what might be tried. It does not tell anyone what she values, what she fears, or what trade she would accept between a symptom and a side effect. Those things have to be asked.
Keats saw this danger, though he described it as a danger to poetry. In Lamia, the philosopher’s stare does not merely dispel an illusion. It kills the young man who loved her. Apollonius is right about Lamia, and his rightness is pitiless; he looks at the bride and sees only what she is made of. That is the true risk of explanation in medicine. Not that it drains the world of wonder, but that it can stand in for attention, so that the clinician who understands the mechanism stops looking at the person in whom it is happening.
The Book Remains Open #
Keats’s complaint has had many answers, and the most persuasive have been lives rather than arguments. Stephen Hales, measuring the force of the blood so that he could wonder at it more exactly, is one. Another essay on this site, on a teacher who made chemistry make sense, considers a second, a Punjabi poet who spent years as a forest chemist. The physiology textbook, at its best, makes the same reply in prose. It replaces the list with an explanation, and the explanation does not put out the student’s wonder. It gives the wonder a new object: no longer the strangeness of so many named parts, but the fact that parts so various manage to work together at all. But the textbook’s deepest lesson is in its own history, written on its title page. Guyton’s book is in its fifteenth edition. Each edition is a public admission that the previous one was incomplete, and one of its best-known diagrams has been the subject of a debate in a major journal about whether it should be taught at all. Bernard said that theories are only hypotheses. A textbook that is periodically revised, and argued over between revisions, is a monument to that sentence.
So the right way to honour the book that first turned the body into an explanation is not to treat it as an authority but as a beginning. The explanation is a hypothesis that has survived a great deal of testing and will face more. The diagram is an argument about what matters most. The number is a sample taken by a method. And the person in whom the mechanism runs is larger than the mechanism, as Keats feared and as every patient knows.
Keats’s rainbow is a good place to end, because it has outlived his complaint. Newton explained its colours a century and a half before Lamia, and it still appears after rain, still stops people in the street, still looks, to anyone who knows its optics, like light doing something astonishing. Unweaving it did not take it out of the sky. It only made it possible to understand why it is there, and to go on looking.
Evidence and Editorial Note #
Documented findings. The essay relies on the author’s public biography: pre-medical sciences at Khalsa College, Amritsar, 1994–1996, and medical training and practice since. John Keats, Lamia (1820), Part II, quoted from Project Gutenberg eBook 2490; the dinner at Benjamin Haydon’s on 28 December 1817 and Keats’s agreement with Lamb about Newton and the rainbow: Wikipedia, “Lamia (poem)”, citing Denise Gigante, PMLA 117(3), 2002. William Harvey, An Anatomical Disquisition on the Motion of the Heart and Blood in Animals (1628), translated by Robert Willis, quoted from Project Gutenberg eBook 67065 (dedication to Charles I; chapter IX, the quantitative argument; chapter XI, the ligature experiment). Galen’s account of blood made in the liver and Fabricius’s description of venous valves: standard reference knowledge. Bernard’s milieu intérieur and his 1865 Introduction à l’étude de la médecine expérimentale (theories as hypotheses): Wikipedia’s biographical article on Bernard; the paraphrase of his view that constancy of the internal environment is the condition of a free life is standard reference knowledge of his 1878 lectures. Walter Cannon’s coinage of homeostasis (1926) and The Wisdom of the Body (1932): Wikipedia, “Homeostasis”. Arthur Guyton’s life, polio in 1946, inventions at Warm Springs, the textbook’s first edition (1956), single authorship of the first eight editions through 1991, John E. Hall’s co-authorship from the ninth and the fifteenth edition (2025): Wikipedia, “Arthur Guyton”, citing his obituary in The Physiologist (2003). Baroreceptor locations, nerves, the solitary nucleus and the speed of the reflex: Wikipedia, “Baroreflex”, citing Ganong’s Review of Medical Physiology. The shift of blood on standing and the consensus definition of orthostatic hypotension (systolic fall of at least 20 mm Hg or diastolic of at least 10 mm Hg within three minutes): the 1996 consensus statement of the American Autonomic Society and the American Academy of Neurology and its 2011 revision (Freeman and colleagues, Clinical Autonomic Research), as summarised in Wikipedia, “Orthostatic hypotension”. A. C. Guyton, “Blood pressure control—special role of the kidneys and body fluids,” Science 252, 1991, 1813–1816 (PubMed 2063193, abstract). A. W. Cowley Jr, J. F. Liard and A. C. Guyton, “Role of baroreceptor reflex in daily control of arterial blood pressure and other variables in dogs,” Circulation Research 32, 1973, 564–576 (PubMed 4713198), with its findings as summarised in M. Di Rienzo, G. Parati, A. Radaelli and P. Castiglioni, “Baroreflex contribution to blood pressure and heart rate oscillations,” Philosophical Transactions of the Royal Society A 367, 2009, 1301–1318 (PMC2635500). D. A. Beard and E. O. Feigl, “Understanding Guyton’s venous return curves,” American Journal of Physiology: Heart and Circulatory Physiology 301, 2011, H629–H633 (PubMed 21666119), and “CrossTalk opposing view: Guyton’s venous return curves should not be taught,” Journal of Physiology 591, 2013, 5795–5797 (PubMed 24293526). T. W. Kurtz and colleagues, “An alternative hypothesis to the widely held view that renal excretion of sodium accounts for resistance to salt-induced hypertension,” Kidney International 90, 2016, 965–973 (PubMed 27546606). Stephen Hales, Haemastaticks (1733), measurement in animals including a horse: Wikipedia, “Stephen Hales”; his combination of quantitative method and religious wonder: O. Lewis, “Stephen Hales and the measurement of blood pressure,” Journal of Human Hypertension 8, 1994, 865–871 (PubMed 7884783, abstract). Korotkov’s sounds (1905): Wikipedia, “Korotkoff sounds”. Reference intervals as the central 95 per cent of a reference population: standard reference knowledge of laboratory practice. L. Rozenblit and F. Keil, “The misunderstood limits of folk science: an illusion of explanatory depth,” Cognitive Science 26(5), 2002, 521–562 (PubMed 21442007; PMC3062901; abstract, including the findings that the illusion is stronger for explanatory knowledge than for facts, procedures or narratives and most robust where mechanisms are visible; the procedure is described in the linked essay on this site). D. S. Echt and colleagues, “Mortality and morbidity in patients receiving encainide, flecainide, or placebo: the Cardiac Arrhythmia Suppression Trial,” New England Journal of Medicine 324, 1991, 781–788 (PubMed 1900101; abstract figures quoted). The continuation of bloodletting long after Harvey: standard reference knowledge. Vendor representations. None.
Recollection and tradition. That the author studied the pre-medical sciences and later medicine is biography. The essay names no textbook he used, describes no copy, annotation, teacher, lecture or moment of understanding from his own studies, and asserts no remembered scene. The person who feels faint on standing is a general description of a common clinical situation, not a patient.
Analysis and forecast. The reading of Keats and Harvey, the contrast between a list and an explanation, and the account of what models, diagrams and measurements can and cannot establish are the author’s interpretation as of November 6, 2026. Nothing in the essay is a recommendation about the diagnosis or treatment of blood pressure or any other condition. Independence. This essay is not sponsored by any institution, company, health system, publisher or technology developer. It is personal reflection, not medical advice; readers seeking care should contact their own physician’s office.
About the Author #
Kanwar Partap Singh Gill, MD, traces his family roots to Khadur Sahib in Punjab’s Majha region and grew up in Amritsar. He attended Spring Dale School and completed pre-medical studies at Khalsa College in 1994–1996. He earned his MBBS from Sri Guru Ram Das Institute of Medical Sciences and Research in Amritsar in 2002.
He completed PGY-1 at the University of Arkansas for Medical Sciences in Little Rock in 2007–2008, and PGY-2 and PGY-3 with UCSF Fresno at Community Regional Medical Center from 2008 through 2010. He has practiced medicine in California since 2010.
His career has crossed four distinct information eras: predominantly paper-based medicine, early and fragmented electronic records, the integrated-EHR era, and the emerging age of generative and agentic artificial intelligence.