Stable Internal Conditions - Entry to Homeostasis

Homeostasis is holding an internal condition inside the range the body can work in, by detecting changes and acting to reverse them. The rest of this lesson is about the second half of that sentence — the detecting and the acting — because that is the part that can break.

First, though, it is worth asking why there is a range at all. Almost everything your cells do is run by enzymes: proteins folded into a particular shape, where the shape is the whole point. An enzyme works because a molecule fits into it, the way a key fits a lock. Heat is molecular movement, so as a cell gets hotter its enzymes vibrate harder, and past a certain point they shake themselves out of shape permanently. A bent key does not open the lock slowly — it does not open it at all.

That is why there is a range at all, and why going outside it gets worse the further and the longer you go. Human core temperature sits near 37 °C, and enzymes are not all equally fussy — there is no single temperature at which they all fail at once. What happens instead is that the hotter you get and the longer you stay there, the more of them lose their shape, and the more cell processes stop working. A fever of 39 °C for an afternoon is a very different amount of damage from 41 °C for an hour, even though both are "above the range". This is also why the treatment for heatstroke is to cool the person down fast: getting the temperature back inside the range stops the damage accumulating. The number 37 is not a rule the body has chosen to follow — it is roughly where this machinery works best.

So a living thing has a problem. The outside world moves — a Darwin summer, a Thredbo winter, a lunchtime sprint — and the inside cannot be allowed to move with it. Something has to notice the drift and push back against it. That something is a feedback loop, and it has the same five parts every time.

The diagram above is that loop drawn twice, once for each direction. The value in the middle, 37 °C, is the set point: the temperature the loop pulls back towards. Read the top path and you are following what happens when you get too hot — your body widens the blood vessels near the skin so that warm blood runs close to the surface and loses heat, which is called vasodilation, and it starts sweating. Read the bottom path and you are following the opposite correction: the same vessels narrow to keep warm blood away from the surface, which is vasoconstriction, and your muscles shiver to generate heat.

Notice what the two paths have in common. Each one produces a change that undoes the change that started it. Getting too hot triggers cooling. Getting too cold triggers warming. Because the response always opposes the original change, biologists call this negative feedback — negative in the sense of opposing, not in the sense of bad. Nearly every loop in your body works this way, and it is the reason you can be measured at roughly 37 °C in two very different places.

Here is a single case followed from start to finish: you are running around at lunchtime in the sun, and your core temperature starts to climb. Every feedback loop in your body has the same five parts, so trace this one carefully and you have the pattern for all of them.

1. Stimulus — your core temperature rises above 37 °C

   Working muscles release heat as a by-product, and the sun adds more. The stimulus is the change itself, not the discomfort you feel. Nothing has responded yet.

2. Receptor — thermoreceptors detect the rise

   Nerve endings in your skin sense the outside heat, and a set of receptors deep in the brain senses the temperature of the blood flowing past them. This step is easy to skip and it is the one that matters most: a change that nothing detects cannot be corrected. A room with no thermostat still gets cold.

3. Control centre — the hypothalamus compares the reading to the set point

   The hypothalamus is a small region at the base of your brain. It holds the set point and does the one job a thermostat does: it compares what is happening to what should be happening, and works out which direction the correction has to go. Because it compares rather than simply reacts, the same centre can trigger opposite responses — cooling if the reading is high, warming if it is low.

4. Effector — sweat glands and skin blood vessels are told to act

   The effectors are the parts that actually do something: here, the sweat glands and the muscles in the walls of the skin's blood vessels. The control centre decides; the effector acts. Keeping those two apart is what lets you say exactly where a loop has gone wrong.

5. Response — you sweat, your vessels widen, and your temperature falls

   Sweat takes heat with it as it evaporates, and wider vessels bring more warm blood to the surface to lose heat. Your temperature drops back toward 37 °C. And now the important part: that falling temperature is a smaller stimulus. The receptors report less of a rise, the hypothalamus calls for less correction, and the sweating eases off. The loop switches itself off by succeeding.

That last sentence is the whole idea of negative feedback, and it is why the loop does not overshoot into hypothermia. The response feeds back and reduces the very thing that triggered it, so the correction fades as the problem does. A loop that instead made the change bigger would run away from the set point rather than back to it.

One traced example proves the loop exists. A second one proves the loop is a pattern rather than a fact about sweating — so here is water balance, run through the same five slots.

The marathon runner in the last card lost water to protect their temperature, so now their blood is slightly more concentrated than it should be. That concentration is the stimulus. Receptors in the hypothalamus — the same region, doing the same comparing job for a second variable — detect it, and this time there are two effectors rather than one. The pituitary gland releases a hormone that tells the kidneys to return more water to the blood instead of passing it into urine, which is why a dehydrated person produces a small amount of dark urine. At the same time you feel thirsty, and thirst makes you go and find a drink.

Look at what thirst is in this picture. It is not a symptom of anything wrong; it is the response step of a working loop, and it is a slightly unusual one because it runs through your behaviour rather than through an organ. Drinking dilutes the blood, the concentration falls back toward its set point, the receptors report less of a change, and the thirst goes away. Once again the response reduces the stimulus that caused it, and once again the loop turns itself off by working.

The graph above is worth a second look now, because it shows the thing a definition cannot. The line never sits flat on the set point — it wanders above and below it continuously. That wandering is not the loop failing. It is the loop working: a correction can only start after a drift has been detected, so a healthy variable is always a little late and always moving. A perfectly flat line would mean nothing was ever being corrected, because nothing was ever allowed to drift.

Different variable, different organs, different speed — and the same five slots in the same order. That is the point of running a second case: you are not learning two facts, you are learning one shape twice.

Take one more variable: the amount of glucose in your blood. After lunch it rises, and that rise is the stimulus. Cells in the pancreas detect it — they are acting as receptor and control centre together — and release the hormone insulin into the blood. Insulin reaches the effectors, which are your liver and muscle cells, and tells them to take glucose out of the blood and store it. Blood glucose falls back toward its set point, so less insulin is released. Five slots again.

Now break one slot deliberately and see what follows. In type 1 diabetes, a person's own immune system has destroyed the pancreas cells that make insulin. Trace it through: the stimulus still happens, the glucose still rises. But the signal is never sent, so the effectors are never told to act, so the glucose is never stored — and it stays high. Nothing is wrong with the liver or the muscles. They are waiting for an instruction that no longer arrives.

That is one precise way a disease can impair normal functioning, and it is the reason this lesson sits third in a unit about disease. Lesson 1 defined disease as something that impairs normal functioning without saying how; here is one specific mechanism — a control loop that has lost one of its five parts and can no longer correct itself. It also explains why the treatment is what it is. Injected insulin does not cure anything; it replaces the missing message, so the rest of an intact loop can run.

Be careful how far you take this. A broken loop is not what every disease is. Influenza impairs your functioning badly, and no part of any loop has been lost — in fact the fever it causes is your thermostat being deliberately reset upwards, which is a loop working, not a loop failing. Most of the rest of this unit is about diseases with a cause outside you, and the loop model says nothing about them. What it does explain is the group Lesson 15 calls the non-infectious diseases, which is why it is worth learning now.

Use this one as a self-test rather than as a picture to look at. Cover the labels and name the five parts in order, then check yourself. If you can do it for temperature, do it again for water and for blood glucose — the diagram is the same shape all three times, which is the reason it is worth learning as a shape.