How the body holds itself steady, and what it looks like when it stops
Anatomy tells you where things are. Physiology tells you what they are doing, and it is the subject that turns a set of observations into a picture of a patient. Every number a nurse records is a physiological measurement, and every one of them is the body either succeeding or struggling to hold something steady. A rising pulse with a falling blood pressure is not two facts; it is one story about a body compensating. Students who learn physiology properly stop seeing a chart as numbers to enter and start seeing it as a patient getting better or worse, often hours before anyone has said so.
Pulse, blood pressure, breathing rate, temperature, oxygen saturation, urine output and level of consciousness are the standard observations, and every one of them is a physiological variable. They are recorded together because they move together. A nurse who understands why they move together reads a chart as a single picture instead of seven separate boxes, and that is the whole practical purpose of this subject.
The body does not fail all at once. It compensates, and compensation is visible. When circulating volume falls, the heart speeds up and vessels narrow so that blood pressure is held steady — which means the pulse rises long before the pressure drops. A blood pressure that has fallen is therefore a late sign, not an early one, and a rising pulse in an otherwise quiet patient is worth more attention than most students give it.
Published ranges describe most healthy people, not every person. An athlete may sit far below the usual pulse range; an older person may live at a blood pressure a textbook would flag. What matters is this patient's own baseline and the direction of travel. A reading inside the range that has moved substantially from where that patient usually sits is more concerning than a stable reading slightly outside it.
Physiology is taught with diagrams and feedback loops and can feel far from the ward. It is not. Every loop in this manual is something you will watch happening in a real person: sweating when feverish, breathing faster when acidotic, passing less urine when dry. The loop explains what you are looking at, and looking at patients with the loop in mind is how the subject becomes permanent.
Homeostasis is the maintenance of a stable internal state despite changes outside. Temperature, water, salts, acidity, glucose and oxygen are all held within narrow limits, and being held there is what allows cells to work at all. Almost every illness is, at some level, a failure of one of these controls, and almost every treatment is an attempt to support the control rather than to replace it.
Most control in the body works by negative feedback: a change is detected, and the response opposes it. Temperature rises, sweating begins, temperature falls. The pattern needs three parts — something that senses, something that decides, something that acts — and when a system fails it is usually worth asking which of the three has failed. A patient who cannot sweat and a patient who cannot sense heat both overheat, for different reasons.
Positive feedback amplifies rather than opposes, and the body uses it only where a process needs to run to completion: clotting, labour contractions, the nerve impulse itself. It is rare because it is dangerous. Several of the worst clinical spirals are positive feedback loops that should not be running, where falling output worsens the very thing causing it.
Healthy organs have far more capacity than daily life requires, which is why serious disease can be silent for a long time. A person can lose a great deal of kidney function before any test looks abnormal. The practical consequence is that the first visible sign of failure often appears under stress — illness, surgery, dehydration — when the reserve is finally called on and is not there.
Every cell is wrapped in a membrane that lets some substances through and not others. This selectivity is the basis of nearly everything else in physiology: nerve impulses, muscle contraction, kidney function and drug action all depend on what can cross a membrane and what cannot. A membrane that loses its selectivity is a cell that is dying, which is why cell injury produces such consistent patterns.
Diffusion is movement from where a substance is concentrated to where it is not, and it needs no energy. Osmosis is the same idea applied to water: water moves towards the side with more dissolved particles. These two explain why oxygen enters blood at the lungs, why a wound weeps, why cells swell in some fluids and shrink in others, and why the composition of an intravenous fluid is not a detail.
Some substances must be moved against their gradient, and that requires energy. The pump that moves sodium out of cells and potassium in is running constantly in every cell and consumes a substantial share of the body's energy. When cells run short of energy — in shock, in severe illness — these pumps fail, sodium and water enter, and cells swell. That is a large part of what tissue damage physically is.
Potassium sits mostly inside cells and sodium mostly outside, and the difference across the membrane is what makes nerves and muscle able to fire. Small changes in the potassium outside cells therefore have large effects on the heart, which is muscle. This is why potassium is measured so often, why it is replaced carefully rather than quickly, and why both high and low values are treated as urgent.
Most of the body is water, and it sits in two main places: inside cells, and outside them. The fluid outside cells divides again into the fluid between cells and the plasma inside blood vessels. Fluid moves between these compartments constantly, and almost every fluid problem in nursing is a problem about which compartment has too much or too little rather than about the body as a whole.
Two forces act at the capillary: the pressure of blood pushing fluid out, and the pull of proteins in the plasma drawing it back. Normally these nearly balance. Raise the pressure — as in heart failure — and fluid is pushed out into the tissues. Lower the protein — as in malnutrition or liver disease — and less is pulled back. Both produce swelling, by opposite routes, and the distinction matters.
Fluid is lost in urine, in sweat, in breath, in stool and through wounds, and different losses carry different amounts of salt. This is why replacing large losses with plain water alone causes problems, and why oral rehydration for diarrhoeal illness contains salts and sugar rather than water alone. The principle behind one of the most effective treatments in global health is straightforward physiology.
Much of fluid assessment is nursing observation: intake and output, daily weight, skin and mouth, pulse, blood pressure lying and standing, capillary refill, and whether the patient is thirsty or confused. Daily weight is the most underused and most reliable of these, because a rapid change in weight is a change in water rather than tissue. A litre gained or lost is roughly a kilogram.
Blood is plasma, which is mostly water carrying proteins, salts, nutrients and waste, together with three kinds of cell: red cells carrying oxygen, white cells defending against infection, and platelets involved in clotting. Almost every blood test a nurse sees is a measurement of one of those components, and knowing what each does makes the result interpretable rather than merely abnormal.
Red cells carry oxygen bound to haemoglobin, and the amount of oxygen the blood can carry depends far more on how much haemoglobin there is than on anything else. A severely anaemic patient can have a perfectly normal oxygen saturation while their tissues are short of oxygen, because saturation measures the proportion of haemoglobin carrying oxygen, not the amount of oxygen present. This single distinction is examined constantly and misunderstood constantly.
White cells are the immune system in circulation, of several types with different jobs. A rising count commonly accompanies infection, but not always, and a falling count in a patient who should be mounting a response is a more worrying sign than a high one. In the very young, the very old and the immunosuppressed, the count may not rise at all, which is why the absence of a fever or a raised count never rules infection out.
Clotting is a chain of steps that ends in a mesh trapping cells to seal a vessel, and it is deliberately hard to start and hard to stop once started. Too little clotting gives bleeding; too much gives clots where they are not wanted, which is why immobility is treated as a real risk rather than an inconvenience. Both ends of this balance are managed constantly in hospital patients.
The heart generates its own electrical signal from a region in the right atrium and conducts it through a defined path to the ventricles. It does not need a nerve from the brain to beat, which is why a transplanted heart still beats. Nerves and hormones modify the rate rather than create it, and this distinction explains why the heart continues in patients whose nervous system is profoundly impaired.
The amount of blood the heart delivers each minute is the rate multiplied by the amount ejected each beat. Either can change to compensate for the other, but only within limits. A very fast heart fills less between beats, so beyond a certain rate output falls rather than rises. This is why an extremely fast pulse is dangerous in itself, not merely a sign of something else.
Three things determine how much the heart ejects: how full it is before contracting, how hard it must push to eject, and how strongly the muscle contracts. Most cardiac treatment adjusts one of the three, and most cardiac failure is a problem with one of the three. Learning them as three questions about a patient makes an otherwise dense topic manageable.
A rising pulse is one of the earliest signs that something is wrong, because it is the body's first and cheapest compensation for almost anything — blood loss, dehydration, fever, pain, fear, low oxygen. It is non-specific, which is exactly why it is sensitive. A pulse that has risen steadily across a shift, in a patient nobody is worried about, is one of the most useful observations in nursing.
The upper number is the pressure while the heart contracts; the lower is the pressure between beats, while the heart fills. The lower number reflects how tight the vessels are, and the gap between the two says something about how much volume is being ejected. A narrowing gap, with the two numbers converging, can indicate falling volume before either number alone looks alarming.
Pressure sensors in the large arteries report continuously, and the response adjusts heart rate and the tightness of vessels within seconds. Slower control comes from the kidneys, which adjust the volume of fluid in the system over hours and days. Fast control by nerves, slow control by kidneys — and treatments aimed at blood pressure are generally aimed at one or the other.
On standing, blood pools in the legs and the pressure at the head briefly falls; the reflex response restores it within a few beats. When that reflex is impaired by age, dehydration, prolonged bed rest or certain medicines, the person feels faint or falls. Measuring pressure lying and then standing is a simple bedside test of a physiological reflex, and it explains a large share of falls in hospital.
Because compensation works, a seriously unwell patient can maintain a normal blood pressure for a long time by increasing heart rate and narrowing vessels. The cost is that when it finally falls, a great deal of reserve has already gone. This is the single most important practical consequence of this chapter, and the reason experienced nurses watch the pulse, the breathing and the urine output rather than waiting for the pressure.
Breathing is driven mainly by carbon dioxide rather than by oxygen. A rise in carbon dioxide, sensed as a change in acidity, increases the depth and rate of breathing. Oxygen becomes the main driver only when it falls to low levels. This is why a patient can be breathing hard with a near-normal oxygen reading, and why the breathing rate is a more sensitive early warning than the saturation.
Of all the standard observations, breathing rate is the one most often estimated rather than counted, and it is the one that changes earliest in deterioration. It rises in response to low oxygen, to acid in the blood, to fever, to pain and to anxiety. Counting it properly for a full period, without telling the patient you are doing it, costs a minute and is repeatedly shown to be worth more than that minute.
At the alveolus, oxygen moves into blood and carbon dioxide moves out, both by diffusion across a very thin barrier. Anything that thickens the barrier, floods it with fluid, or reduces the surface available impairs exchange. Carbon dioxide crosses far more readily than oxygen, which is why oxygen levels fall first in most lung disease and carbon dioxide rises only later, when the problem is severe or the patient is tiring.
Breathing costs energy, and a patient struggling to breathe is spending a great deal of it. Visible effort — using neck and shoulder muscles, nostril flaring, inability to complete a sentence, sitting forward — is a physiological measurement that requires no equipment. A patient who has been working hard and suddenly appears calmer with no treatment may be tiring rather than improving, and that is one of the most dangerous misreadings in nursing.
A saturation reading tells you what proportion of the available haemoglobin is carrying oxygen. It does not tell you how much haemoglobin there is, how much blood is flowing, or whether tissues are receiving what they need. A patient who has lost a great deal of blood may show a perfectly reassuring saturation, and that reassurance is false.
Oxygen delivered to tissues depends on how much is in the blood, how much blood is flowing, and whether it reaches the tissue. Anaemia affects the first, a failing heart or blood loss the second, and poor perfusion from shock or cold the third. Asking which of the three is failing is a far more useful question at a bedside than asking whether the saturation is normal.
A saturation probe measures light passing through tissue, so it depends on blood actually pulsing through the finger. Cold hands, poor circulation, movement, nail varnish and certain pigments can all produce a wrong or absent reading. A reading that does not fit the patient in front of you should be questioned rather than recorded, and the patient should be looked at rather than the number.
When carbon dioxide rises substantially it causes headache, flushing, tremor and progressive drowsiness. A patient with severe lung disease who becomes sleepy and settled after a period of distress may be accumulating carbon dioxide rather than resting. Recognising drowsiness as a possible sign of worsening rather than of improvement is one of the clearest examples of why nurses need physiology.
Digestion breaks food into pieces small enough to cross the gut wall; absorption takes them in. Carbohydrates, proteins and fats each require their own enzymes and their own conditions. Most absorption happens in the small intestine, whose lining is folded and covered in projections that make its surface area enormous — which is why disease that flattens that lining causes malnutrition even when the person is eating.
Muscle in the gut wall moves contents along in coordinated waves. This movement can be slowed by immobility, by dehydration, by certain medicines and by pain, and speeded by infection and inflammation. Bowel sounds, abdominal distension and the passage of flatus are direct observations of this physiology, which is why they are asked about so persistently after surgery.
Everything absorbed from the gut travels first to the liver before reaching the rest of the body. The liver processes nutrients, stores some, converts others, makes plasma proteins and clotting factors, and deals with substances the body needs to remove. This explains why liver disease produces bleeding, swelling, jaundice and confusion together — several unrelated-looking problems from one organ's workload.
A patient who is not eating is not merely uncomfortable. Wound healing, immune function, muscle strength and the ability to breathe deeply all depend on nutritional state. Nursing records of intake, of weight and of whether a patient can actually reach and manage their food are physiological observations, and they are frequently the only record of a decline that nobody has yet named.
The kidney filters a very large volume of fluid from the blood and then reclaims almost all of it, adjusting precisely what is kept and what is lost. Working this way — filter everything, take back what is needed — allows fine control over water, salts and acid. It also means kidney function depends completely on blood flow, which is why low blood pressure damages kidneys quickly.
Urine output reflects how well the kidneys are being perfused, and falling output is one of the earliest signs that circulation is inadequate. It is also one of the few continuous measurements nursing controls entirely. A patient whose output has fallen steadily over several hours is telling you something before any blood test will, provided somebody is adding the figures up rather than only recording them.
When the body is short of water, a hormone signals the kidney to reclaim more, and urine becomes concentrated and scant. When there is too much, the hormone falls and dilute urine is passed freely. Concentrated, dark, small-volume urine in a patient who is not otherwise unwell is usually a statement about their fluid intake, and it is a statement a nurse can act on directly.
When kidneys fail, waste accumulates, fluid is retained, potassium rises, acid builds up, blood pressure climbs and, over time, anaemia and bone disease follow. The scattered nature of this list is the point: it is one organ with many jobs. Recognising the pattern rather than the individual abnormalities is what makes kidney failure identifiable at the bedside.
The acidity of blood is held within very narrow limits, because enzymes stop working properly outside them. Three systems defend it: chemical buffers act instantly, the lungs adjust within minutes by changing how much carbon dioxide is blown off, and the kidneys adjust over hours and days. Fast lungs, slow kidneys — that pairing explains most of what this topic contains.
When acid accumulates, as in severe infection, in uncontrolled diabetes or in poor tissue perfusion, the lungs compensate by breathing deeply and rapidly to remove carbon dioxide. A patient breathing hard with clear lungs and a normal saturation may therefore be compensating for an acid problem elsewhere in the body. This is one of the most valuable inferences in this whole manual.
Compensation buys time; it does not fix the cause. A patient compensating well may look stable while the underlying problem worsens, and when the compensation fails the deterioration is abrupt. This is why the effort a patient is making is recorded, not only the result they are achieving, and why a tiring patient is an emergency even if the numbers still look acceptable.
Large losses from the gut disturb both salts and acid balance, and the direction depends on where the loss is from. Prolonged vomiting loses acid from the stomach; severe diarrhoea loses alkaline fluid from the intestine. This is why the nursing record of what was lost, how much and for how long is genuinely clinical information rather than administration.
The body produces heat continuously and loses it through skin, breath and excreta. Control sits in the brain and works by adjusting blood flow to the skin, sweating, shivering and behaviour. When we feel cold we seek warmth, and that behavioural part is as physiological as the rest — which matters for patients who cannot move, cannot speak or cannot reach a blanket.
In fever the body's target temperature is raised, and everything else follows: the person feels cold, shivers and puts on blankets while their temperature climbs towards the new setting. This explains the otherwise confusing picture of a hot patient complaining of cold. When the setting returns to normal, the patient feels hot and sweats as the excess is lost.
A temperature below normal is a serious sign in the very young, the very old and the severely unwell, and it is easy to overlook because attention is trained on fever. Some patients with serious infection become cold rather than hot. A low reading should never be dismissed as a faulty thermometer without checking the patient.
Illness, immobility, exposure during procedures, cool rooms, wet linen and being unable to ask for a blanket all cool patients, and the very people least able to complain are the least able to compensate. Keeping a patient warm is not comfort care separate from clinical care; being cold impairs clotting, wound healing and drug handling, and increases oxygen demand through shivering.
Body temperature, hormone release, alertness and appetite all follow a daily rhythm set largely by light. Hospitals disrupt every input that rhythm depends on: lights at night, noise, observations at fixed hours, no daylight, meals at unfamiliar times. The result is not only tiredness — disrupted sleep worsens confusion in older patients, raises pain, impairs glucose control and slows recovery. Protecting sleep is a physiological intervention.
A nerve carries information as a brief electrical change travelling along it, produced by movement of sodium and potassium across the membrane. Because the impulse depends on those ions, disturbances of salts in the blood produce weakness, abnormal sensation, confusion and abnormal heart rhythm. The link between a blood result and a symptom at the bedside is exactly this mechanism.
A nerve does not touch the muscle it controls; it releases a chemical across a tiny gap which triggers contraction. This junction is where several important disorders and several classes of drug act, and it explains why weakness can result from a problem in the nerve, in the junction or in the muscle itself, with different patterns of fatigue and recovery.
A reflex is a response produced without the brain deciding: the signal goes to the cord and back. Reflexes protect — withdrawing from heat, coughing, blinking, gagging. Losing a protective reflex is what makes reduced consciousness dangerous, because the airway is no longer defended. A great deal of positioning and airway care exists because a reflex is absent.
Consciousness depends on a region in the brainstem keeping the rest of the brain aroused, and on the brain having enough oxygen, glucose and normal chemistry. This is why altered consciousness has so many causes — low oxygen, low glucose, salt disturbance, infection, injury, drugs — and why a change in a patient's alertness is treated as urgent until explained. It is often the first thing a family notices and the last thing recorded.
Hormones travel in the blood and act on any cell able to receive them, so their effects are slower to begin and longer lasting than nerve signals. This is why hormone disorders develop over weeks and months and are frequently attributed to something else — tiredness, mood, weight, appetite — before anyone measures anything.
Blood glucose is held steady by opposing hormones: one lowers it by moving glucose into cells, others raise it by releasing stores. Illness, injury and stress raise glucose even in people without diabetes, because the body is mobilising fuel. This is why glucose is monitored in acutely unwell patients generally, and not only in those with a known diagnosis.
Injury, illness, surgery, pain and fear all trigger a coordinated response: heart rate and blood pressure rise, glucose is released, fluid is retained, and the body prepares to withstand insult. It is protective and it is also costly, and much of what is seen in a deteriorating patient is this response rather than the disease itself.
Hormone disorders declare themselves in the things nurses record daily: weight, appetite, thirst, urine volume, temperature, pulse, mood and sleep. A pattern assembled across days by the person doing the observations is frequently what prompts investigation, and that pattern exists only if the observations were recorded properly rather than approximated.
The first defence is physical: intact skin, the lining of the airway with its mucus and moving hairs, stomach acid, the flushing action of urine and tears. Nursing spends much of its time maintaining these barriers, and most invasive devices deliberately breach one. Every cannula, catheter and tube is a hole in a defence, which is why the question is always whether it is still needed.
When tissue is injured or invaded, local vessels widen and leak, and defensive cells arrive. The result is redness, heat, swelling, pain and loss of function — the classic signs, and all of them consequences of the same process. Recognising that these signs are the response rather than the damage explains why they appear in sterile injury as well as in infection.
Beyond the general response, the body builds specific defences against particular organisms and remembers them, which is the basis of both recovery and vaccination. Memory is why a second encounter is usually milder and faster, and why protection can be produced deliberately without the illness. It is also why immunity can be lost when the cells that carry it are suppressed.
Patients whose immunity is suppressed by disease, by treatment or by age may not mount the usual signs — no fever, no raised white count, little inflammation — so infection presents as confusion, falling appetite or simply not being right. At the other extreme, an overwhelming response to infection damages the patient's own tissues and circulation. Both extremes are recognised at the bedside before they are confirmed in a laboratory.
Deterioration rarely announces itself in one number. It appears as a pattern: breathing rate creeping up, pulse rising, blood pressure still normal, urine output falling, the patient a little less engaged than yesterday. Each observation alone is unremarkable. Together they describe a body compensating, and this is precisely what early warning scores are built to catch.
Breathing responds to low oxygen, to acid, to fever, to pain and to anxiety, which makes it the most sensitive single observation and the earliest to change. It is also the one most often estimated rather than counted. Counting it accurately is probably the highest-value minute in routine observation, and it costs nothing but attention.
A patient's own statement that something is wrong, or a family member's statement that this is not how they usually are, carries real predictive weight. It is information about a baseline that nobody else has. Recording it, and escalating on it rather than waiting for a number to cross a threshold, is supported by the physiology in this manual: by the time the number moves, compensation has already been spent.
A patient who was distressed and is now quiet may be improving or may be exhausted, hypoxic or accumulating carbon dioxide. Distinguishing the two requires looking at the whole picture — effort, colour, alertness, the trend of the observations — rather than at the relief of no longer hearing distress. This is the single most repeated warning in acute nursing, and it rests entirely on physiology.
With age, the maximum heart rate falls, vessels stiffen, the reflex response to standing weakens, kidneys lose reserve, the thirst sensation blunts and temperature regulation becomes less reliable. The consequence is that an older patient has less capacity to compensate and deteriorates with less warning, and may do so without a fever, without a fast pulse and without complaining. A smaller change in their observations means more.
Specialised nerve endings respond to damage or to the threat of it, and send signals to the cord and up to the brain. Pain is not a measurement of tissue damage — it is the brain's output after weighing that signal against everything else it knows, including fear, memory, exhaustion and what the person has been told. This is why two people with identical injuries report very different pain, and why neither of them is exaggerating.
Signals from touch and pressure travel on faster fibres than those carrying most pain, and they can reduce how much pain signal passes through the cord. This is the mechanism behind rubbing a knock, behind warmth and cold, behind position changes and behind a hand held. These are not merely kind gestures filling time until something real is given; they act on a described physiological pathway.
Acute pain accompanies injury and settles as healing proceeds; it is useful, because it enforces rest. Persistent pain continues after healing and the nervous system itself becomes more responsive, so less stimulus produces more pain. Treating persistent pain as though it were prolonged acute pain fails, and treating the patient as though their pain should have stopped by now is both wrong physiologically and damaging to trust.
Pain raises heart rate and blood pressure, increases oxygen demand, stops patients breathing deeply, prevents them moving, coughing, eating and sleeping, and therefore contributes directly to chest infection, clots, pressure damage and delayed healing. A patient left in pain is not merely uncomfortable — measurable harm follows, which is why pain is counted among the routine observations rather than alongside them.
There is no instrument that measures pain. The person's own report is the measurement, and when they cannot report it — in dementia, in very young children, after a stroke, in intensive care — it is inferred from behaviour: facial expression, guarding, restlessness, changes in breathing, withdrawal. These patients are consistently under-treated, and the reason is that their measurement is harder to obtain rather than absent.
Entrance papers ask mechanism: what drives breathing, what a hormone does, which ion does what. Registration papers ask interpretation: here are the observations, what is happening and what would you do. Both reward understanding the loop rather than memorising the endpoint, because a loop can be reasoned from and a memorised fact cannot.
Several questions recur across papers because they catch people reliably: that oxygen saturation is a proportion and not a quantity, that a normal blood pressure does not exclude serious blood loss, that carbon dioxide rather than oxygen normally drives breathing, and that a sleepy patient with lung disease may be worsening. Each of those is worth understanding rather than remembering.
The fastest way to find out whether you understand a physiological mechanism is to explain it aloud to another student without notes. Gaps become obvious immediately, in a way they do not while reading. This is more effective than re-reading for this subject specifically, because the subject is made of chains of cause and effect and a chain either connects or does not.
Every set of observations you take is a physiology examination with a real answer. Before writing the numbers down, ask what they say together, and whether the trend across the chart is towards better or worse. Students who form this habit early find that the subject stops being revision and becomes the way they see patients.