Evidence-based physiotherapy and rehabilitation resources
Editorial & review policy
Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Physiology · Foundations
Every system in this subject does one job: it keeps the fluid around your cells steady while the world outside keeps changing. Learn the logic of that once, here, and every later chapter becomes a variation on it.
Part 1 · Foundations · Chapter 1 of 47
By the end of this page you should be able to:
A physiotherapist spends the working day disturbing homeostasis on purpose. You sit a patient up after a week in bed. You walk someone with heart failure. You raise the intensity of an exercise session until breathing and heart rate climb.
Each of those is a deliberate challenge to a control system. The patient tolerates it only if the system can answer. When it cannot, you see the result: dizziness on standing, breathlessness out of proportion, a heart rate that will not settle.
Homeostasis is therefore not background theory. It is the thing you are dosing, every session. Understanding it tells you what to monitor, when to stop, and why training makes the next session easier.
In the nineteenth century the French physiologist Claude Bernard pointed out that our cells do not live in the outside world. They live in a fluid of their own, which he called the milieu intérieur, the internal environment [1].
That fluid is the extracellular fluid. It surrounds every cell, brings it oxygen and fuel, and carries away its carbon dioxide and wastes. A cell can survive only while that fluid stays within narrow limits of temperature, acidity, salt content and supply.
In the 1920s the American physiologist Walter Cannon gave the process of keeping it steady a name: homeostasis [1, 3]. The word means “staying similar”, not “staying the same”. That difference matters, as you will see.
Figure 1 · The internal environment
Notice that only four organs actually exchange material with the outside. The lungs take in oxygen and remove carbon dioxide. The gut takes in water and nutrients. The kidneys remove water, salt, acid and wastes. The skin loses heat and sweat.
Everything else serves those four. The heart and vessels move the fluid between them and the cells. The nervous and endocrine systems coordinate them. This is why physiology can be learned system by system and still make sense as one subject.
Water makes up about 60% of body weight in a lean young man, and somewhat less in women and older adults. It sits in two main compartments separated by the cell membrane.
About two-thirds is inside cells, the intracellular fluid (ICF). About one-third is outside, the extracellular fluid (ECF). The ECF is itself split by the capillary wall into interstitial fluid, which bathes the cells, and plasma, which circulates.
Figure 2 · Where the body’s water is
| Compartment | Share of body weight | Volume in a 70 kg man | Main features |
|---|---|---|---|
| Intracellular fluid | about 40% | about 28 L | High K⁺, Mg²⁺, phosphate and protein; low Na⁺ |
| Interstitial fluid | about 15% | about 10.5 L | Similar ions to plasma, little protein; the fluid cells actually touch |
| Plasma | about 5% | about 3.5 L | High Na⁺ and Cl⁻; rich in protein; moves the whole ECF around the body |
| Total body water | about 60% | about 42 L | Varies with sex, age and body fat |
Two facts from this figure come back again and again. First, the cell membrane keeps the ICF and ECF chemically very different, using constant pumping. Second, the capillary wall lets water and small solutes through but holds back most plasma protein.
The first fact underlies every nerve impulse and muscle contraction in Part 2. The second explains oedema, which you will meet whenever you treat a swollen limb. Chapter 4 returns to both in detail.
Many properties of the ECF are regulated. Each has a normal range, and each has its own control system. The values below are typical adult reference ranges; individual laboratories quote slightly different limits.
| Variable | Typical normal range | Main controllers |
|---|---|---|
| Core temperature | about 36.5–37.5 °C | Hypothalamus; skin vessels, sweat glands, shivering |
| Arterial pH | 7.35–7.45 | Lungs (CO₂), kidneys (acid, bicarbonate), blood buffers |
| Arterial PO₂ | about 80–100 mmHg (10.7–13.3 kPa) | Breathing, via peripheral chemoreceptors |
| Arterial PCO₂ | 35–45 mmHg (4.7–6.0 kPa) | Breathing, via central chemoreceptors |
| Plasma sodium | 135–145 mmol/L | Kidneys; antidiuretic hormone, aldosterone, thirst |
| Plasma potassium | 3.5–5.0 mmol/L | Kidneys; aldosterone, insulin |
| Fasting blood glucose | about 3.9–5.5 mmol/L (70–100 mg/dL) | Pancreas (insulin, glucagon), liver |
| Plasma osmolality | about 275–295 mOsm/kg | Antidiuretic hormone, thirst |
| Mean arterial pressure | about 70–105 mmHg | Baroreceptor reflex short term; kidneys long term |
Look at how narrow some of these ranges are. A potassium level outside its range can stop the heart. A pH below about 7.0 or above about 7.8 is life-threatening. Other variables, such as blood pressure, are allowed to swing widely with activity and posture.
Almost every homeostatic system works by negative feedback. Something changes the variable; the system detects the change; it then acts to push the variable back the other way.
Physiology teachers have found that students learn this best by naming the same parts every time [1]. Learn these five terms and you can describe any system in this subject.
Figure 3 · The negative feedback loop
| Component | What it does | Temperature example | Blood pressure example |
|---|---|---|---|
| Regulated variable | The property being kept steady | Core temperature | Mean arterial pressure |
| Sensor (receptor) | Measures the variable | Thermoreceptors in skin and hypothalamus | Baroreceptors in carotid sinus and aortic arch |
| Control centre (integrator) | Compares the value with the set point and decides on a response | Hypothalamus | Cardiovascular centres in the medulla |
| Effector | Changes the variable | Sweat glands, skin vessels, skeletal muscle (shivering) | Heart, arterioles, veins |
| Response | The effect of the effector on the variable | Heat lost or conserved | Pressure raised or lowered |
The word “negative” does not mean harmful. It means the response has the opposite sign to the disturbance. If temperature rises, the response lowers it. If pressure falls, the response raises it.
Modell and colleagues point out a common source of confusion. Textbooks often blur the variable being regulated with the things the effectors change to regulate it [1]. Heart rate, for example, is not itself held constant. It is changed, freely, to keep blood pressure constant.
The set point is the value the system aims for. The error signal is the difference between the measured value and the set point. The control centre acts on the size of that error.
No real system holds the variable exactly at its set point. It oscillates within a normal range, and recovers after a disturbance with some overshoot. This is why homeostasis is called a dynamic steady state rather than a fixed one [3].
Figure 4 · Homeostasis is dynamic, not static
Gain describes how well a system corrects an error. It is the correction achieved divided by the error that remains. A high-gain system leaves only a small residual error; a low-gain system leaves more.
Classic textbook estimates put the gain of temperature control far above that of the baroreceptor reflex. That fits experience. Core temperature barely moves on a hot day, while blood pressure on standing dips noticeably before it recovers.
Set points are not fixed either. In fever, the hypothalamic set point for temperature is moved upward. The patient shivers and feels cold while their temperature climbs, because the system is defending the new, higher value [3].
Two other control strategies exist, and examiners like to ask about both.
Figure 5 · Three control strategies
In positive feedback the response reinforces the change. During labour, stretch of the cervix releases oxytocin, which strengthens contractions, which stretch the cervix further. Blood clotting and the rising phase of the nerve impulse work the same way.
Positive feedback is useful only because something outside the loop ends it: delivery of the baby, sealing of the vessel, closure of sodium channels. When a positive feedback cycle runs without a stop, it is usually disease, as in some forms of circulatory shock [3].
In feedforward control the system acts before any error appears. Heart rate and breathing start to rise at the very onset of exercise, before blood gases change. Feedforward reduces the error that feedback then has to correct [3].
Control works at three broad levels. Local (intrinsic) control happens within a tissue, such as a working muscle dilating its own arterioles. Neural control is fast and targeted, as in the baroreceptor reflex. Hormonal control is slower but widespread and lasting.
Important variables are usually guarded by several systems at once, often with effectors pulling in opposite directions. Billman describes this redundancy and hierarchy as what gives the body fine and flexible control [3]. It also explains why a patient may cope with one failing system until a second is stressed.
The baroreceptor reflex is the clearest negative feedback loop in clinical practice. You trigger it every time you sit a patient up or bring them to standing.
Figure 6 · Worked example: standing up
Read it as a loop. Standing is the disturbance. Arterial pressure is the regulated variable. The carotid sinus and aortic arch baroreceptors are the sensors. The medulla is the control centre. The heart, arterioles and veins are the effectors.
In a healthy person the dip in pressure lasts only a few heartbeats. When the reflex is weakened, the pressure fall is larger and lasts longer. The brain is under-perfused and the patient feels light-headed, greys out or faints.
Bed rest removes the daily stimulus of gravity and activity. Plasma volume falls, the heart and leg muscles decondition, and the baroreceptor reflex has less to work with. Studies of bed rest and spaceflight describe orthostatic intolerance and reduced exercise capacity as the result [5].
The best-known demonstration is the 1968 Dallas study. Healthy young men spent three weeks in bed, and their maximal oxygen uptake fell substantially. A subsequent training programme restored it and then took it beyond the starting level [4].
For practice, this means early, graded mobilisation is itself a treatment for the control system. It also means the first sit-up after prolonged bed rest is a test of that system, and should be monitored as one.
Exercise raises heat production, oxygen use and carbon dioxide output many times over. Homeostasis during exercise is the coordinated response of breathing, circulation and thermoregulation to that load.
Training improves the response. That is what you are doing when you progress a programme: enlarging the disturbance the patient’s control systems can absorb.
Vital signs are the readings of homeostatic control. Measure them before, during and after a challenge, and interpret them as a loop that is coping or failing.
| Before and during a session | What it tells you about control |
|---|---|
| Lying and standing blood pressure | Whether the baroreceptor reflex keeps pressure up on standing |
| Heart rate response to effort and recovery | How hard the cardiovascular loop is working, and how quickly it settles |
| Oxygen saturation | Whether breathing and gas exchange keep up with demand |
| Breathlessness and perceived exertion | The patient’s experience of the load on their control systems |
| Skin colour, sweating, temperature | Thermoregulatory and autonomic responses |
| Symptoms: light-headedness, nausea, visual greying | Under-perfusion of the brain; stop and lie the patient down |
Practical skill · postural blood pressure
Rest the patient supine for about five minutes and record blood pressure and heart rate. Stand them up, with support if needed, and repeat at one and three minutes. A sustained fall of at least 20 mmHg systolic or 10 mmHg diastolic within three minutes meets the consensus definition of orthostatic hypotension [6]. Ask about symptoms at every reading, and never leave the patient unsupported.
Define homeostasis. Describe the components of a negative feedback system, with one example. Add a note on positive feedback.
Textbooks present homeostasis as a list of regulated variables with a feedback diagram for each. Students often memorise the diagrams without a shared vocabulary, so the same idea looks different in every chapter.
Physiology educators have built and validated a common framework for teaching homeostasis. Faculty from many kinds of institution agreed which ideas a student must hold, and rated how important and how difficult each is [2].
The same group proposed one standard model, with consistent terms for sensor, integrator, effector and the regulated variable, because inconsistent textbook wording was itself a cause of misunderstanding [1]. Billman argues that homeostasis remains the central organising principle of physiology, and that disease is best understood as its disruption [3].
Use one loop and one vocabulary for every system in this subject. When a later chapter introduces a new reflex or hormone, ask the same questions: what is regulated, what senses it, what decides, what acts, and what happens when a link fails.
10 questions on this page. Tap one to see the answer and the reasoning.
Answer: (B) Cells live in, and exchange with, the extracellular fluid. Keeping it constant is what homeostasis means [1, 3].
Answer: (C) The sensor measures and the effector acts. The comparison with the set point happens in the control centre [1].
Answer: (B) The response, evaporative heat loss, opposes the rise in temperature. That is the defining feature of negative feedback [1].
Answer: (C) Contractions release more oxytocin, which strengthens contractions. The loop ends only when delivery removes the stimulus [3].
Answer: (B) The change comes before any error has been measured, so it is anticipatory. Feedback then trims the remaining error [3].
Answer: (B) Homeostasis is dynamic. Variables oscillate within a range and higher centres can move set points, as in fever [3].
Answer: (C) Lower baroreceptor firing reduces vagal and increases sympathetic outflow. Heart rate rises and vessels constrict [6].
Answer: (B) The 2011 consensus statement uses 20 mmHg systolic or 10 mmHg diastolic, within 3 minutes of standing or head-up tilt [6].
Answer: (B) Maximal oxygen uptake fell substantially after bed rest and was restored, then exceeded, by training [4].
Answer: (C) Redundant and opposing effectors give finer, more reliable control. Temperature uses skin vessels, sweat glands and shivering [3].
Key takeaways
| Source | Why it earns its place |
|---|---|
| Hall JE, Hall ME — Guyton and Hall Textbook of Medical Physiology | The classic treatment of control systems, gain and the body fluids |
| Boron WF, Boulpaep EL — Medical Physiology | Deeper cellular and quantitative treatment for advanced readers |
| Silverthorn DU — Human Physiology: An Integrated Approach | The clearest student account of reflex control pathways |
| Modell H et al. — Advances in Physiology Education, 2015 | A short, readable standard model of homeostasis [1] |
| Billman GE — Frontiers in Physiology, 2020 | The history and the case for homeostasis as physiology’s central idea [3] |
Reviewed by the Physiotherapist India Team. · Human Physiology contents