Human Physiology Essentials: 250 Cards on Regulation

250 cards on how each system is regulated, not where it sits.

250 cardsLast updated 2026-08-22

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Physiology is not a longer list of structures. It is a set of control loops, and almost every one has the same shape: something is sensed, compared against a set point, and corrected by an effector that cannot quite finish the job. Once you can see that shape, the baroreceptor reflex, tubuloglomerular feedback, thyroid feedback and temperature regulation stop being four topics and become one idea in four places. So these 250 cards are built on mechanism rather than on naming. All 250 open with a definition and 242 add why it matters, which here almost always means the consequence that makes the mechanism worth knowing: that the ascending limb removes solute without water and is therefore the engine of the concentrating gradient, that surfactant equalizes rather than simply reduces surface tension, that a catalyst-like permissive hormone changes what another hormone can do without doing it itself. The weighting is 42 cards on the cardiovascular system, 38 on the kidney and fluid balance, 32 on respiration, 30 each on cell and nerve function and on the endocrine system, 26 on the gut, 20 on muscle, 18 on blood and haemostasis, and 14 on whole-body regulation. There are no reference ranges in it, and that is deliberate: values differ by source, by method and by the person measured, and a number quoted without its conditions invites a comparison it cannot support. Structure lives in the Human Anatomy Essentials deck and drug effects in the Pharmacology Drug Classes deck, so the three sit alongside one another. Cards are tagged by system and by sub-topic, so you can drill just the cardiac cycle, just tubular transport, or just the endocrine axes.

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What's inside

Showing 100 representative cards from the full 250-card deck.

Front	Back
Equilibrium potential	Definition: The membrane voltage at which the electrical force on an ion exactly balances its concentration gradient, so there is no net movement. Why it matters: Each ion has its own value, so a membrane's voltage sits between them, pulled toward whichever ion it is most permeable to.
Nernst relationship	Definition: The calculation giving an ion's equilibrium potential from the ratio of its concentrations across the membrane. Why it matters: It answers what one ion would do alone, which is why it cannot give the resting potential of a membrane permeable to several ions.
Why potassium dominates the resting potential	Definition: At rest the membrane is far more permeable to potassium than to sodium, so the voltage settles close to the potassium equilibrium value. Why it matters: It sits slightly away from that value because a small sodium leak pulls it, which is the gap the sodium-potassium pump must keep paying for.
Electrochemical driving force	Definition: The difference between the actual membrane voltage and an ion's equilibrium potential. Why it matters: Current flows only when a channel opens and a driving force exists, so an open channel at its equilibrium potential carries no net current.
Threshold potential	Definition: The voltage at which enough voltage-gated sodium channels open for their inward current to exceed the outward potassium current. Why it matters: Beyond that point the process becomes self-reinforcing, which is what makes the response all-or-none rather than graded.
States of the voltage-gated sodium channel	Definition: Closed and available, open, and inactivated, with recovery from inactivation requiring the membrane to repolarize first. Why it matters: Inactivation is a separate state from being closed, which is what makes the refractory periods possible.
Repolarization	Definition: The return toward the resting voltage as sodium channels inactivate and slower potassium channels open. Why it matters: The two events overlap rather than following one another, which is why the falling phase is steeper than sodium inactivation alone would give.
Graded potential	Definition: A local voltage change whose size varies with stimulus strength and which decays with distance. Often confused with: The all-or-none response it may trigger. Graded potentials can add together, which an all-or-none response cannot.
Time constant	Definition: The time taken for membrane voltage to reach a defined fraction of its final value after a step change in current. Why it matters: A longer constant means potentials last longer and overlap more readily, which favours summation over time.
Spatial summation	Definition: Addition of inputs arriving at different locations at the same time. Why it matters: It lets a cell weigh many simultaneous sources, so its output reports a combination rather than any single input.
Ionotropic and metabotropic receptors	Definition: An ionotropic receptor is itself a channel and acts within milliseconds. A metabotropic receptor acts through a second messenger over a longer period. Why it matters: The same transmitter can do both depending on which receptor the target carries, so speed is a property of the receptor rather than the transmitter.
Autonomic tone	Definition: A continuous baseline level of autonomic activity, so a target can be adjusted in either direction from an intermediate state. Why it matters: Without tone a system could only be switched on, so tone is what makes graded bidirectional control possible.
Parasympathetic effects on target organs	Definition: Slowed heart rate, bronchoconstriction, increased gut motility and secretion, and pupillary constriction. Why it matters: Its fibres end on or near the target, so effects are organ-specific rather than body-wide.
Cross-bridge cycle	Definition: Attachment of the myosin head to actin, the power stroke, detachment, and re-cocking of the head. Why it matters: The cycle repeats many times per contraction, so shortening comes from repeated small steps rather than one large movement.
Role of troponin and tropomyosin	Definition: Tropomyosin covers the myosin binding sites on actin, and calcium binding to troponin moves it aside. Why it matters: Regulation is on the thin filament in skeletal and cardiac muscle, which is what makes calcium the switch.
Length-tension relationship	Definition: Active force is greatest at the length giving optimal overlap between thick and thin filaments, and falls when the muscle is shorter or longer. Why it matters: Too much overlap causes interference and too little leaves few cross-bridges, so the curve has a peak rather than a plateau.
Twitch, summation and tetanus	Definition: A single stimulus gives a twitch, repeated stimuli before relaxation add together, and high frequency gives a sustained fused contraction. Why it matters: Summation is possible because the contraction outlasts the electrical event, so calcium is still present when the next arrives.
Size principle	Definition: Smaller motor units are recruited before larger ones as force demand rises. Why it matters: It follows from smaller motor neurons being more easily depolarized, so the ordering is a property of the neuron rather than a control decision.
Smooth muscle contraction mechanism	Definition: Calcium binds calmodulin, which activates a kinase that phosphorylates the myosin light chain, allowing cross-bridge cycling. Often confused with: Skeletal muscle, where regulation is on the thin filament. Here it is on the thick filament and involves an enzyme step.
Muscle energy sources during activity	Definition: Stored ATP first, then creatine phosphate, then anaerobic glycolysis, then oxidative metabolism as activity continues. Why it matters: The order follows how quickly each can supply ATP rather than how much it can supply in total.
Mechanisms of muscle fatigue	Definition: Accumulation of inorganic phosphate and hydrogen ions, impaired calcium release, and depletion of stored glycogen. Why it matters: It is rarely a simple lack of ATP, since concentrations fall far less than force does.
Pacemaker potential	Definition: A slow spontaneous drift toward threshold in nodal cells, driven by an inward current that activates on repolarization together with falling potassium conductance. Why it matters: These cells have no stable resting voltage, which is why the heart beats without any external signal.
Overdrive suppression	Definition: The fastest pacemaker sets the rate and suppresses slower ones by depolarizing them before they reach their own threshold. Why it matters: It is why a single rhythm emerges even though several regions are capable of pacing.
Function of the plateau phase	Definition: Sustained calcium entry maintaining depolarization and supplying calcium for contraction. Why it matters: It prolongs the refractory period, which is why summation and tetanus are impossible in cardiac muscle.
Electrocardiogram waves	Definition: The P wave marks atrial depolarization, the QRS complex ventricular depolarization, and the T wave ventricular repolarization. Why it matters: Atrial repolarization is hidden within the QRS complex, which is why no separate wave appears for it.
Phases of the cardiac cycle	Definition: Isovolumetric contraction, ejection, isovolumetric relaxation, and filling. Why it matters: During the two isovolumetric phases both valves are shut, so pressure changes with no change in volume.
Afterload	Definition: The load the ventricle must overcome to eject, largely set by arterial pressure. Why it matters: Raising it at constant contractility reduces stroke volume, because ejection begins later and ends sooner.
Ejection fraction	Definition: The fraction of end-diastolic volume ejected in one beat. Why it matters: Being a ratio, it reports pump efficiency rather than output, so it can be preserved while output falls.
Ventricular filling	Definition: Rapid passive filling once the atrioventricular valve opens, followed by slower filling and then the atrial contribution. Why it matters: Most filling is passive, so atrial contraction adds a modest amount that becomes important only when filling time is short.
Relationship between flow, pressure and resistance	Definition: Flow equals the pressure difference divided by resistance. Why it matters: It is the pressure difference across a circuit that drives flow, not the absolute pressure at either end.
Series and parallel arrangement of circulations	Definition: The pulmonary and systemic circuits are in series, while most organs are supplied in parallel from the aorta. Why it matters: Parallel supply means each organ receives fully oxygenated blood and its flow can be adjusted without altering the others.
Mean arterial pressure	Definition: The average pressure driving flow through the systemic circuit over the cardiac cycle. Why it matters: It is weighted toward the diastolic value because relaxation occupies more of the cycle than ejection, so it is not the midpoint of the two.
Capillary types	Definition: Continuous, fenestrated and discontinuous capillaries, differing in the size of the gaps in their wall. Why it matters: Permeability follows the type, which is why filtration is minimal in the brain and extensive in the kidney and liver.
Local metabolic control of blood flow	Definition: Accumulating metabolites and falling oxygen dilate local arterioles, raising flow where activity has risen. Why it matters: The tissue adjusts its own supply without any signal from outside, which is why active muscle receives more flow than the nervous system alone would deliver.
Reactive hyperaemia	Definition: A transient increase in flow after a period of occlusion. Why it matters: Metabolites accumulate during the interruption, so the size of the overshoot reflects how long flow was interrupted.
Sympathetic control of vessel calibre	Definition: A continuous sympathetic vasoconstrictor tone that can be increased or reduced. Why it matters: Vasodilation is usually achieved by withdrawing that tone rather than by a dilator nerve supply.
Response to standing up	Definition: Blood pools in the legs, filling falls, and the reflex response raises heart rate and constricts vessels to restore pressure. Why it matters: It shows the reflex correcting a fall it did not cause, which is the clearest everyday demonstration of the loop.
Intrapleural pressure	Definition: The pressure in the thin fluid layer between the lung and chest wall, kept below atmospheric by their opposing recoil. Why it matters: The negative value is what holds the lung expanded against its own elastic recoil, so losing it allows the lung to collapse.
Lung compliance	Definition: The change in lung volume produced by a given change in distending pressure. Why it matters: Low compliance means a stiff lung requiring more effort to inflate, while high compliance means easy inflation with poor recoil on expiry.
How surfactant stabilizes alveoli	Definition: Surfactant molecules crowd closer together as an alveolus shrinks, lowering surface tension more in small alveoli than in large ones. Why it matters: It reverses the instability the Laplace relationship would otherwise create, so it equalizes rather than simply reduces.
Anatomic dead space	Definition: The volume of the conducting airways, where no gas exchange occurs. Why it matters: The first air to reach the alveoli on each breath is dead space air from the previous breath, so it is exhaled unchanged.
Partial pressure as the driving force	Definition: Gases diffuse down their own partial pressure gradients, independently of the other gases present. Why it matters: It is partial pressure rather than concentration that drives movement, which is why the amount dissolved also depends on solubility.
Ventilation-perfusion matching	Definition: Local matching of airflow to blood flow so that neither is wasted. Why it matters: A high ratio behaves like dead space and a low one like a shunt, so both extremes reduce the efficiency of exchange.
Oxygen content versus partial pressure	Definition: Partial pressure describes dissolved oxygen only, while content includes the far larger amount bound to haemoglobin. Why it matters: Content can fall substantially with normal partial pressure if haemoglobin is reduced, so the two are not interchangeable.
Haldane effect	Definition: Deoxygenated haemoglobin carries carbon dioxide more readily than oxygenated haemoglobin. Often confused with: The Bohr effect, which runs the other way. One describes how carbon dioxide affects oxygen binding, the other how oxygen affects carbon dioxide carriage.
Why carbon dioxide is the main respiratory stimulus	Definition: Ventilation responds steeply to small rises in carbon dioxide and only weakly to falling oxygen until it becomes low. Why it matters: The oxygen curve is flat over its usual range, so oxygen contributes little to minute-to-minute control.
Stretch receptor reflex in the lung	Definition: Receptors in the airway walls signalling inflation and inhibiting further inspiration. Why it matters: It operates mainly at large tidal volumes, so it limits over-inflation rather than setting the resting pattern.
Response to altitude	Definition: Low inspired oxygen stimulates peripheral chemoreceptors, raising ventilation, which lowers carbon dioxide and raises blood pH. Why it matters: The resulting alkalinity restrains the very response causing it, until renal excretion of bicarbonate removes that restraint over days.
Voluntary breath holding	Definition: Breathing can be suspended until rising carbon dioxide and falling oxygen make the urge to breathe irresistible. Why it matters: The break point is set mainly by carbon dioxide, which is why prior overbreathing extends it and does so by removing a warning rather than adding oxygen.
Haematocrit	Definition: The fraction of blood volume occupied by red cells. Why it matters: It is a major determinant of blood viscosity, so a rise increases resistance and the work the heart must do.
Erythropoiesis and its regulation	Definition: Red cell production in marrow, stimulated by erythropoietin released from the kidney in response to low oxygen delivery. Why it matters: The kidney senses oxygen delivery rather than oxygen content, so anaemia and low blood flow trigger the same response.
Iron handling	Definition: Iron is absorbed in the duodenum, transported bound to transferrin, and stored as ferritin. Why it matters: There is no regulated route of excretion, so balance is controlled almost entirely at absorption.
Rhesus D antigen	Definition: A protein antigen where antibodies form only after exposure. Why it matters: Sensitization during one pregnancy can affect a later one, which is the basis of the difference from the ABO system.
Vascular response to injury	Definition: Immediate constriction of the damaged vessel, reducing flow through the injured segment. Why it matters: It buys time for the slower steps, so haemostasis begins mechanically before any clotting occurs.
Platelet activation and aggregation	Definition: Adherent platelets change shape, release granule contents including ADP and thromboxane, and recruit further platelets that bind one another through fibrinogen. Why it matters: Released mediators recruit more platelets, so the process amplifies itself and forms a plug quickly.
Fibrin cross-linking	Definition: Conversion of a loose fibrin mesh into a stable one by a factor that forms covalent links. Why it matters: Without cross-linking the plug is soluble and easily dislodged, so stabilization is a distinct step from formation.
Natural anticoagulant mechanisms	Definition: Antithrombin, the protein C and S system, and tissue factor pathway inhibitor, together with intact endothelium. Why it matters: They confine the clot to the injured area, so the boundary of a clot is actively maintained rather than incidental.
The three basic renal processes	Definition: Filtration into the tubule, reabsorption back into blood, and secretion from blood into the tubule. Why it matters: What appears in urine is filtered plus secreted minus reabsorbed, so a substance can be excreted without ever being filtered.
Forces governing filtration	Definition: Glomerular capillary pressure driving fluid out, opposed by pressure within the capsule and by the oncotic pressure of plasma proteins. Why it matters: Oncotic pressure rises along the capillary as protein-free fluid leaves, so the net force falls from the start of the tuft to its end.
Filtration fraction	Definition: The proportion of plasma reaching the glomerulus that is filtered. Why it matters: Raising it concentrates protein in the blood leaving the glomerulus, which increases reabsorption downstream in the proximal tubule.
Tubuloglomerular feedback	Definition: Cells of the macula densa sensing sodium chloride arriving in the distal tubule and signalling the afferent arteriole to adjust its tone. Why it matters: Each nephron regulates its own filtration according to what reached the end of its own loop, so control is local rather than global.
Transport maximum	Definition: The upper limit on how fast a carrier-mediated process can move a substance, set by the number of transporters. Why it matters: Below it essentially all of the substance is recovered, and above it the excess appears in urine, so the appearance is abrupt rather than gradual.
Why sodium reabsorption drives everything else	Definition: The basolateral sodium pump keeps intracellular sodium low, providing the gradient that powers coupled transport of glucose, amino acids, phosphate and hydrogen ions. Why it matters: Blocking the pump stops those processes too, so the pump is the energy source for tubular transport generally.
The ascending limb of the loop	Definition: Actively transports sodium, potassium and chloride out while remaining impermeable to water. Why it matters: Removing solute without water is what dilutes the fluid and concentrates the interstitium, so this segment is the engine of the gradient.
Urea recycling	Definition: Urea reabsorbed from the inner medullary collecting duct adds to medullary osmolarity and re-enters the loop. Why it matters: Urea contributes a substantial share of the concentrating gradient, so the gradient is not built from salt alone.
Osmoreceptors and thirst	Definition: Hypothalamic cells sensing plasma osmolarity, driving both thirst and release of antidiuretic hormone. Why it matters: The same signal drives intake and retention, so the two arms of water balance are triggered together.
Free water clearance	Definition: The volume of solute-free water added to or removed from the urine relative to plasma. Why it matters: A negative value means water is being conserved, so the sign reports the direction of regulation directly.
Renin release and its triggers	Definition: Released from juxtaglomerular cells in response to reduced renal perfusion pressure, reduced sodium chloride at the macula densa, and sympathetic stimulation. Why it matters: The three triggers are different signs of the same underlying problem, which is why they converge on one response.
Natriuretic peptides	Definition: Released from cardiac muscle when the chambers are stretched, promoting sodium and water excretion and vasodilation. Why it matters: They oppose the renin system, so volume regulation is a balance between a retaining and an excreting arm.
Buffering in body fluids	Definition: Weak acid and conjugate base pairs that take up or release hydrogen ions, chiefly the bicarbonate system in extracellular fluid, with proteins and phosphate acting inside cells. Why it matters: Buffering is immediate but conserves nothing, so it limits the change while excretion does the correcting.
Compensation	Definition: The response of the organ not responsible for the primary change, the lungs adjusting ventilation within minutes and the kidney adjusting bicarbonate over days. Why it matters: Compensation reduces the pH change without abolishing it, because the organ doing the correcting is driven by the very error it is correcting.
Generation of new bicarbonate	Definition: Hydrogen ions secreted in the collecting duct are buffered by phosphate and by ammonia, and each one buffered leaves a new bicarbonate behind. Why it matters: Free hydrogen ions can only lower urine pH so far, so buffers are what allow substantial acid excretion.
Hormone transport in blood	Definition: Water-soluble hormones travel dissolved, while lipid-soluble ones travel bound to carrier proteins. Why it matters: Only the unbound fraction is active, so a change in carrier protein alters total measured hormone without altering activity.
Receptor upregulation and downregulation	Definition: Target cells increase receptor numbers during prolonged low exposure and reduce them during prolonged high exposure. Why it matters: Responsiveness is a variable rather than a constant, so a sustained signal produces a smaller effect than the same signal applied briefly.
Levels of endocrine feedback	Definition: A peripheral hormone can inhibit the pituitary, the hypothalamus, or both. Why it matters: The pattern of hormone levels reveals which level is affected, since a failing gland and a failing pituitary move the tropic hormone in opposite directions.
Circadian variation in hormone release	Definition: Predictable daily variation in several hormones, driven by a central clock entrained by light. Why it matters: A single measurement means little without the time it was taken, since the same value can be high or low depending on the hour.
Steps of thyroid hormone synthesis	Definition: Iodide trapping, oxidation and attachment to tyrosine residues on thyroglobulin, coupling of those residues, then storage and release. Why it matters: The hormone is stored extracellularly within the follicle, which is why the gland holds a supply lasting weeks.
Feedback in the thyroid axis	Definition: Circulating thyroid hormone inhibits both the hypothalamic releasing hormone and the pituitary stimulating hormone. Why it matters: The pituitary hormone is the most sensitive indicator of the axis, because it responds to small changes in feedback.
Zones of the adrenal cortex	Definition: An outer zone making mineralocorticoid, a middle zone making glucocorticoid, and an inner zone making adrenal androgens. Why it matters: The outer zone is controlled by the renin system rather than the pituitary, which is why its output can move independently of the others.
Biphasic insulin release	Definition: A rapid first phase from stored granules followed by a sustained second phase from newly produced hormone. Why it matters: The first phase suppresses hepatic glucose output early, so its loss raises the peak after eating even when total output is preserved.
Growth hormone actions	Definition: Direct effects mobilizing fat and opposing insulin, and indirect growth-promoting effects mediated by insulin-like growth factor from the liver. Why it matters: The direct and indirect actions pull glucose handling in opposite directions, which is why its metabolic effect is not simply anabolic.
Parathyroid hormone actions	Definition: Increases bone resorption, increases renal calcium reabsorption, reduces phosphate reabsorption, and activates vitamin D. Why it matters: It raises calcium through three organs at once, and lowering phosphate prevents the two from precipitating together.
Bone remodelling	Definition: Continuous resorption by osteoclasts balanced by formation by osteoblasts, coupled through signalling between them. Why it matters: Bone serves both as structure and as a calcium reservoir, so mineral regulation and structural maintenance compete.
Switch from negative to positive feedback before ovulation	Definition: Sustained high oestrogen changes its effect on the pituitary from inhibition to stimulation, producing a surge. Why it matters: It is one of the few positive feedback loops in the body, and it is self-limiting because ovulation removes the source.
Layers of the gut wall	Definition: Mucosa, submucosa, muscularis with circular and longitudinal layers, and an outer serosa. Why it matters: The two muscle layers act at right angles, which is what allows both propulsion and mixing.
Peristaltic reflex	Definition: A propulsive reflex with contraction above and relaxation below the bolus, moving contents along. Why it matters: It is coordinated locally by the enteric system, so it continues along an isolated segment of gut.
Phases of swallowing	Definition: A voluntary oral phase, then involuntary pharyngeal and oesophageal phases. Why it matters: Once the pharyngeal phase begins it cannot be stopped, so the transition from voluntary to reflex is a one-way step.
Phases of gastric secretion	Definition: A cephalic phase triggered by the sight, smell and thought of food, a gastric phase triggered by distension and protein, and an intestinal phase that mostly inhibits. Why it matters: Secretion begins before food arrives, so anticipation is part of the mechanism rather than an oddity.
Gastric mucosal protection	Definition: A mucus and bicarbonate layer, rapid epithelial renewal, and mucosal blood flow, maintained by locally produced prostaglandins. Why it matters: The stomach is protected by an active process, so anything reducing prostaglandin production removes the defence rather than adding acid.
Intrinsic factor	Definition: A protein from parietal cells binding vitamin B12 and enabling its absorption in the terminal ileum. Why it matters: The absorption site is far from where the factor is made, so loss of gastric function impairs an ileal process.
Bile composition and function	Definition: Bile acids, phospholipids, cholesterol and pigments, emulsifying fat into small droplets. Why it matters: Emulsification multiplies the surface available to lipase, so bile assists digestion without being an enzyme.
Carbohydrate digestion and absorption	Definition: Starch broken to oligosaccharides by amylase, then to monosaccharides by brush border enzymes, absorbed by sodium-coupled and facilitated carriers. Why it matters: Only monosaccharides can be absorbed, so a missing brush border enzyme leaves an otherwise digestible sugar unabsorbed.
Colonic function	Definition: Absorption of water and electrolytes, fermentation of residue by resident bacteria, and storage before elimination. Why it matters: Fermentation produces short-chain fatty acids that the colonic cells themselves use as fuel, so the bacteria feed the tissue that houses them.
Splanchnic circulation	Definition: Blood flow to the gut, rising markedly after eating and reduced during exercise by sympathetic constriction. Why it matters: The two demands compete, which is why digestion and heavy exertion are poorly compatible.
Components of a control system	Definition: A sensor, an integrating centre with a set point, and an effector, connected in a loop. Why it matters: Losing any one component breaks regulation, so identifying which is missing explains the pattern of failure.
Resetting of a control system	Definition: A shift in the set point, so the system defends a different value. Why it matters: Fever is a raised temperature set point rather than a failure of regulation, which is why shivering occurs while temperature is already rising.
Countercurrent heat exchange in limbs	Definition: Warm arterial blood transferring heat to adjacent returning venous blood. Why it matters: It conserves core temperature at the cost of cooling the limb, so the extremity is sacrificed to protect the centre.
Regulation of appetite	Definition: Short-term signals of distension and gut hormones acting on hypothalamic centres, with longer-term signals reporting stored fat. Why it matters: Two timescales are involved, so a single meal and long-term balance are regulated by different signals.
Physiological changes with ageing	Definition: Reduced maximum cardiac output, reduced lung elastic recoil, reduced filtration rate, and reduced maximum responses generally. Why it matters: Resting function is often well preserved while reserve is not, so the change appears under demand rather than at rest.
Why physiological values are given as ranges	Definition: Measured values vary with age, sex, posture, activity, time of day and measurement method, and differ between reference sources. Why it matters: A single figure quoted without its conditions is not comparable to one measured differently, so the conditions matter as much as the number.

Frequently asked

How does it differ from the Human Anatomy Essentials deck?

Anatomy is where a structure is and what it borders. This deck is what the structure does and how that activity is controlled. They share no cards, and they answer different questions about the same body: anatomy tells you the loop of Henle has a descending and an ascending limb, this deck tells you why one is permeable to water and the other is not.

Why are there no reference ranges or normal values?

Because they vary by source, by measurement method, by age and posture, and by the individual, and a value read without those conditions invites a comparison it cannot support. Every card here teaches the relationship instead: what raises a variable, what lowers it, and what the body does about it. The final card says this explicitly.

What is on each card?

A concept on the front. On the back, a definition on all 250 and on 242 the consequence that makes it worth knowing, most often the reason the mechanism is built the way it is. Each line is labeled, so the back reads as a short structured answer rather than a paragraph.

Which systems get the most cards?

Cardiovascular 42, renal and fluid balance 38, respiratory 32, cell and nerve function 30, endocrine 30, gastrointestinal 26, muscle 20, blood and haemostasis 18, and whole-body regulation 14. The weighting follows where the control loops are densest rather than where the anatomy is largest.

How is it organized for studying one system at a time?

Every card carries a system tag and a finer sub-topic tag, across cell and nerve, muscle, cardiovascular, respiratory, blood, renal, endocrine, gastrointestinal and integrated regulation. Filter by a tag to drill one system, or by the finer tag for a single block such as cardiac electrical activity, tubular transport or haemostasis.

Can I import the whole deck on the free plan?

Yes. Importing a saved deck runs no new AI generation and spends no AI credits, so the free plan imports every card. You can study, edit and delete them afterwards.

Will importing it twice create duplicates?

No. Cards you already have are skipped and only cards added in a revision come through. Including re-imports after deleting it, one official deck can be imported three times per account.

Can I edit the cards after importing?

Yes. Once imported they are your cards: edit either side, delete the ones you already know cold, retag them, or move them into another deck.