Handout — The Feedback Loop

The Two Shapes

Opposing, or adding to

Every feedback loop is a response to a change, and there are only two things a response can do about the change that caused it. It can push against it, or it can add to it. That is the whole distinction, and the names for it are unhelpful enough to be worth saying plainly before anything else.

	Negative feedback	Positive feedback
What the response does	Opposes the change	Adds to the change
Where the level ends up	Back near the set point	Far from where it started
What stops it	Reaching the set point	An event outside the loop
How common in the body	Almost everything	A handful of cases
Does the name mean bad or good?	No. Negative means opposing.	No. Positive means adding to.

Five Loops

The same page, five times over

Each loop below is written the same way, so the shape shows through the different biology: what changed, what noticed, what happened, and where the level ended up.

1 · Blood sugar after a meal — negative: Change: sugar in the blood rises. Detected by: cells in the pancreas. Response: insulin is released, so body cells take sugar in and the liver stores some away. Result: the level falls back toward normal, and the insulin stops. Shape: the response opposed the rise, so this is negative feedback. Go the other way — a long stretch without eating — and glucagon does the opposite job, which is the same loop working from below.
2 · Body temperature on a hot day — negative: Change: core temperature rises. Detected by: nerve endings in the skin and cells in the brain. Response: skin vessels widen, sweat glands release sweat that carries heat away as it evaporates, and you shed a layer or move into shade. Result: the temperature comes back down toward the set point. Shape: negative. On a cold day every part reverses — vessels narrow, muscles shiver — and it is still negative feedback, because the response still opposes the change.
3 · Water in the blood — negative: Change: you sweat all afternoon and drink nothing, so the blood holds less water. Detected by: the brain. Response: a hormone tells the kidneys to take back more water in step two of the filtering, and thirst sends you looking for a drink. Result: less water leaves in the urine, which turns dark and scant, and the level is defended. Shape: negative — and dark urine is the loop working, not failing.
4 · A cut that clots — positive: Change: a vessel is torn and platelets meet the damaged surface. Response: each platelet that sticks releases a chemical that makes nearby platelets sticky, so more arrive and stick, releasing more of it. Result: the plug builds faster as it goes, and the bleeding stops. Shape: positive — each step made the next one larger. What ended it: the tear was sealed, so the surface that started the loop was gone. Nothing returned to a set point.
5 · Labour in childbirth — positive: Change: the baby's head presses on the lower part of the womb. Response: a hormone is released that makes the contractions stronger, which presses the head down harder, which prompts more of the hormone. Result: contractions build in strength rather than settling. Shape: positive. What ended it: the birth, which removes the pressure. Again, an event outside the loop.

Two ways to get it wrong. The first is reading the words as judgements — deciding that clotting must be negative feedback because it is good for you, or that a fever must be positive feedback because it feels bad. The second is sorting by the outcome instead of the mechanism. Bleeding does stop, and stopping sounds like a return to normal, but nothing in the clotting loop was pushing back against the change while it ran. Ask what each step did to the next one.

Try It

Part A · Which shape is it?

Write N for negative or P for positive beside each one. On screen, the answers are behind the button below Part B; on paper they print on a sheet of their own, so you can set it aside until you have finished.

1. ______: Blood pressure rises, sensors in the vessels report it, and the heart slows.
2. ______: A nerve cell begins to fire, which opens channels that make it fire harder, which opens more of them.
3. ______: Calcium in the blood falls, a hormone is released, and the skeleton releases stored calcium.
4. ______: Carbon dioxide builds up in the blood, and breathing becomes faster and deeper.
5. ______: A patch of ice melts, leaving darker ground that absorbs more sunlight, which warms the area and melts more ice.
6. ______: A room gets cold, the furnace runs, and the room warms until the furnace shuts off.
7. ______: The set point is raised during an illness, and the body shivers until its temperature reaches the new target.
8. ______: Blood sugar falls between meals, glucagon is released, and the liver puts sugar back into the blood.

Part B · Finish the loop

Two loops with a step missing. Write the missing step in your own words.

9. A worker drinks a large bottle of water at lunch, so there is more water in the blood than the body needs.

Detected by: the brain. Response: less of the water-saving hormone is sent to the kidneys, so the kidneys take back __________________________.

Result: __________________________________________ . Shape: ______

10. Oxygen in the blood falls at high altitude.

Detected by: sensors in the blood vessels. Response: breathing rate rises and, over weeks, the body makes more __________________________.

Result: __________________________________________ . Shape: ______

Part A 1 · N — the response lowered a level that had risen. 2 · P — each step made the next larger; this is how a nerve signal gets going, as Quiz 16 described. 3 · N — a fall was answered by a release that raises it, and the skeleton is the store, from Quiz 17. 4 · N — more carbon dioxide brings breathing that removes more of it. 5 · P — melting produces the conditions for more melting; this one is not in the body at all, and it returns in Part IX. 6 · N — the house, and the same shape as your own temperature. 7 · N — the target moved, but the loop still opposes the gap between the level and the target, so it is negative feedback aiming at a new number. 8 · N — the mirror image of the insulin loop. Number 7 is the hardest of the eight, and it is worth the time. Changing a set point is not the same as changing the shape of the loop. The body is still comparing, still opposing the difference, still settling at a target — the target is simply higher for a while.

Part B, question 9 The kidneys take back less water, so more of it leaves. The result is a large amount of pale urine and a water level in the blood that returns to normal. The shape is negative: the level rose, and the response brought it down. If you wrote that the kidneys take back more water, check the direction of the original change. There was too much water, so keeping more would push the level further from where it belongs — which would be positive feedback, and would be a serious problem.

Part B, question 10 The body makes more red blood cells, which carry the oxygen, as Quiz 15 described. The result is that more oxygen is delivered despite the thinner air, so the level in the blood is brought back up. Negative feedback again, working over weeks rather than seconds. Anything meaning "more oxygen-carrying capacity" is right here. The point of the item is the direction of the response, not the vocabulary.

Where this comes back

Feedback is not only a body topic, which is why this page is worth keeping past Part III. In Part VI a population grows until food or space pushes back, which is a negative loop on a scale far larger than a person. In Part IX the ice-and-sunlight loop from Part A above is one of the things that makes the earth's climate hard to predict, because a positive loop can carry a change further than whatever started it.

The two questions are the same everywhere. What changed, and what did the response do to it?