4.7 — Homeostasis: the Control Loops That Keep You Alive

Your blood glucose right now is somewhere near 5 millimoles per litre. If it falls below about 3 you become confused; below 2 you lose consciousness; below 1 you will have a seizure and can die. If it rises above about 25 you become dangerously dehydrated and acidotic. You have about 4 grams of glucose in your entire bloodstream — roughly one teaspoon — and your brain alone consumes about 120 grams a day.

So the total quantity present is replaced roughly every twenty minutes, indefinitely, whether you eat or not, for your entire life, and the concentration barely moves. That is homeostasis, and once you see the shape of it, most of the diseases in this volume become recognisable as one of a small number of ways a control loop can fail.

The anatomy of a control loop

A negative feedback loop. A change is detected, compared against a set point, and a response is produced that reverses the change — which reduces the original signal and therefore the response. The loop switches itself off as it succeeds. Image: Wikimedia Commons.

Every control loop in your body has the same four components.

1. The variable — what is being controlled. Temperature, glucose, blood pressure, pH, calcium, sodium, oxygen.

2. The sensor (receptor) — something that measures it. Temperature-sensitive neurons in the hypothalamus. Stretch receptors in artery walls that measure blood pressure. Chemoreceptors measuring carbon dioxide. Beta cells in the pancreas measuring glucose directly.

3. The control centre — compares the measurement against a set point and decides on a response. The hypothalamus is the master control centre for most of these, which is why a structure the size of an almond has such an outsized role.

4. The effector — carries out the response. Muscles, glands, the heart, the kidneys.

The loop is closed because the effector's action changes the variable, which changes what the sensor measures. That is the feedback.

Negative feedback: the workhorse

Negative feedback reverses the change. The response opposes the original disturbance, so the variable is pushed back toward the set point, and as it approaches, the response fades.

Almost every control system in your body is negative feedback, and the reason is stability: a loop that opposes deviation settles, and a loop that amplifies deviation runs away.

Worked example: getting too hot

You walk into a hot room. Skin and core temperature rise.

Sensors: thermoreceptors in the skin report the environment; neurons in the hypothalamus measure the temperature of the blood passing through, which is core temperature.

Control centre: the hypothalamus compares this with a set point of about 37 °C and finds core temperature rising.

Effectors, in order of energy cost:

• Blood vessels in the skin dilate. Blood is diverted to the surface, so heat is lost by radiation and convection. This is why you go red, and it is free — it costs no water and no additional energy.
• Sweat glands activate. Evaporating one litre of sweat removes about 2,400 kilojoules (Chapter 1.2). Expensive in water and salt.
• Behaviour — you move to shade, remove clothing, drink. Behavioural regulation is the most powerful effector of all and is usually left out of textbook diagrams, which is a mistake, because in real life it does most of the work.

Result: temperature falls, the sensors report it, the response is dialled back.

Worked example: blood glucose

Glucose rises after a meal. Beta cells in the pancreatic islets sense it directly — glucose enters them, is metabolised, and the resulting ATP closes a potassium channel, which depolarises the cell and triggers insulin release. The sensor and the effector are the same cell, which makes this one of the most direct loops in the body (Chapter 12.5).

Insulin tells muscle and fat cells to insert GLUT4 transporters into their membranes and take glucose up (Chapter 1.4), tells the liver to store glucose as glycogen, and switches off glucose production. Glucose falls, insulin secretion falls.

Glucose falls between meals. Alpha cells in the same islets release glucagon, which tells the liver to break glycogen down and release glucose, and later to manufacture glucose from amino acids and glycerol. Glucose rises, glucagon falls.

Note the design: two opposing hormones rather than one that goes up and down. This gives much finer control, because the balance between two active signals can be adjusted quickly in either direction — the same principle as having both an accelerator and a brake rather than only an accelerator that can be released. Antagonistic pairs appear everywhere in physiology: insulin and glucagon, calcitonin and parathyroid hormone, the sympathetic and parasympathetic nervous systems.

And now the diseases are obvious. Type 1 diabetes destroys the sensor and effector cell (no insulin). Type 2 diabetes leaves insulin present but the target cells no longer respond (Chapter 18.7). Both are failures of the same loop at different points.

Positive feedback: rare, and always with an end point

Positive feedback amplifies the change. The response increases the original stimulus, so the loop accelerates.

This is inherently unstable, so the body uses it only where an explosive, all-or-nothing, one-way process is wanted — and every physiological example has a built-in termination.

Childbirth. The fetal head presses on the cervix. Stretch receptors signal the pituitary to release oxytocin. Oxytocin makes the uterus contract harder. Harder contraction pushes the head down more firmly, stretching the cervix further, releasing more oxytocin. The loop escalates until the baby is delivered, which removes the stimulus and ends it. Chapter 15.7.

Blood clotting. Activated platelets release chemicals that activate more platelets; each activated clotting factor activates many of the next. The cascade amplifies from a tiny initiating event to a clot within seconds. It terminates when the clot seals the vessel and natural anticoagulants confine it to the damaged area — and when that confinement fails, the result is disseminated intravascular coagulation, which is Chapter 7.1.

The nerve impulse. Sodium channels opening let sodium in, which depolarises the membrane further, which opens more sodium channels. This is why an action potential is all-or-nothing. It terminates because the channels automatically inactivate after about a millisecond (Chapter 11.1).

And this is exactly why runaway physiological states are so dangerous. Circulatory shock has a positive feedback component: low blood pressure reduces perfusion of the heart itself, which weakens the heart, which lowers blood pressure further. Once that loop is established it does not self-correct, and it is why shock must be interrupted from outside, urgently (Chapter 23.1).

Set points move on purpose

A set point is not a constant, and treating it as one leads directly to bad clinical decisions.

Fever is the clearest case. During infection, immune signals act on the hypothalamus and raise the set point to, say, 39 °C. The body is not out of control — it is doing exactly what it is told, and it now behaves as though 37 °C is too cold. That is why you shiver and feel cold at the start of a fever while your temperature is objectively rising: your body is trying to reach a higher target. When the infection resolves the set point drops, you are now above it, and you sweat profusely as the body dumps the excess. Chapter 13.4.

Fever is a defence, and moderate fever should not automatically be suppressed — it slows bacterial and viral replication and enhances immune function. Above about 40 °C the balance shifts, and above 42 °C protein denaturation becomes the immediate threat (Chapter 1.3).

Circadian variation shifts several set points daily. Body temperature is lowest around 4 to 6 a.m. and highest in the early evening — about a 0.5 °C swing. Cortisol peaks just before waking. Blood pressure dips at night, and the absence of that nocturnal dip is itself a risk factor for cardiovascular events, which is why 24-hour blood pressure monitoring gives information a clinic reading cannot.

And chronic conditions reset set points, sometimes unhelpfully. In long-standing hypertension the baroreceptors adapt to the higher pressure and start defending it as normal — which is one reason blood pressure must be lowered gradually, because dropping it fast in someone whose brain has adapted to high pressure can cause the brain to be underperfused.

The major loops, in one place

Variable	Normal range	Sensor	Main effectors
Core temperature	36.5–37.5 °C	Hypothalamus, skin	Skin vessels, sweat, shivering, behaviour
Blood glucose	4–6 mmol/L fasting	Pancreatic islets	Insulin, glucagon, liver, muscle
Blood pressure	~120/80 mmHg	Baroreceptors, kidney	Heart, vessels, kidney, hormones
Blood pH	7.35–7.45	Chemoreceptors, kidney	Lungs (fast), kidneys (slow)
Blood oxygen and CO₂	pO₂ ~13 kPa	Chemoreceptors	Breathing rate and depth
Blood calcium	2.2–2.6 mmol/L	Parathyroid glands	Bone, kidney, gut
Blood sodium	135–145 mmol/L	Hypothalamus	Kidney via ADH, thirst
Blood volume	—	Kidney, atria	Kidney, aldosterone, ADH

The narrowness of some of those ranges is the point. Blood pH is held between 7.35 and 7.45 — a range of 0.1 pH unit, which is a 26 percent variation in hydrogen ion concentration. Outside 6.8 to 7.8 is generally incompatible with life. Chapter 10.4 covers how.

Fast and slow, and why both exist

Control systems operate on different timescales, and every variable that matters has both.

Nervous control is fast — milliseconds to seconds, precisely targeted, brief. Your blood pressure is adjusted beat to beat this way. Stand up and the baroreflex increases heart rate within one or two beats to stop you fainting.

Hormonal control is slow — seconds to days, body-wide, sustained. Blood volume and sodium balance are managed this way over hours.

And they cover for each other. In blood pressure control, the nervous baroreflex handles the second-to-second adjustments and has no long-term power at all, because it resets to whatever pressure it experiences. Long-term blood pressure is set almost entirely by the kidney, through how much salt and water it retains. This is why every effective long-term blood pressure treatment acts on the kidney or on the hormone systems that control it, and why the fast reflexes cannot be treated to fix hypertension. Chapter 7.6.

Why almost every disease in this book is a control loop failure

Once you see this, the rest of the volume organises itself. There are only a few ways a loop can break.

The sensor fails. Hypoglycaemia unawareness in long-standing diabetes — the person no longer feels their blood sugar falling, which is dangerous precisely because the warning is what prompts them to eat.

The control centre fails. Heat stroke, in which hypothalamic regulation collapses and the person stops sweating despite a dangerously high temperature. Skin that is hot and dry rather than sweating is the ominous sign (Chapter 23.10).

The effector fails. Type 1 diabetes: the signal cannot be produced. Kidney failure: sodium and water balance cannot be executed.

The target stops responding. Type 2 diabetes, and every drug tolerance in Chapter 1.8.

The set point is wrong. Fever when it is excessive; thyroid disease, where the whole metabolic set point shifts up or down.

The loop's gain is wrong. Too little response and the variable drifts; too much and it oscillates. Cheyne–Stokes breathing — a cycle of deepening then fading breaths seen in heart failure and in some brain injury — is an oscillating control loop, caused by a delay between the lungs changing the blood gases and the brain sensing it. It is a genuine engineering phenomenon appearing in a patient, and it is the same instability that makes a badly tuned thermostat cycle a room between too hot and too cold.

What Part 5 does next

You now have the vocabulary, the tissues, the developmental history and the control principle. From here the volume takes the body one system at a time for eleven Parts, and each follows the same structure: where it is, what it looks like, how big it is, what it is made of, how it formed, what it does, the chemistry behind that, its blood and nerve supply, what breaks, and what happens if it is removed.

Part 5 starts with the skeleton — not as a list of bones to memorise, but as living tissue that rebuilds itself continuously, stores your calcium, manufactures your blood, and tells you a great deal about how someone lived and how they died.