Quiz 19 — Homeostasis

The Guide

Holding a level while the world moves

Homeostasis is the body's way of keeping conditions inside it, such as temperature, water, and blood sugar, within a narrow range while conditions outside it change. The word comes from Greek for “staying about the same.” You have already seen it at work in four earlier quizzes. In Quiz 14 the kidneys took back as much water as the body needed and no more. In Quiz 16 the pancreas released insulin when blood sugar rose and glucagon when it fell. In Quiz 17 the skin sweated and its blood vessels widened when you got hot. In Quiz 18 a fever raised your temperature on purpose and then let it fall again.

Four different systems and four different chemicals, all doing the same thing. The word “about” in that definition matters: nothing is held perfectly still. Levels drift a little and get pushed back.

Here is what is being held, and how narrow the ranges are.

Temperature, around 98.6 degrees Fahrenheit, or 37 Celsius. A small rise, as in an ordinary fever, is the body at work. A temperature near 104 degrees or higher, or near 95 degrees or lower, is a medical emergency.
Blood sugar, enough to supply every cell without damaging vessels over time.
Water and salt, which decide whether cells swell or shrink.
Acidity of the blood, held in a very tight range.
Oxygen and carbon dioxide, and calcium in the blood.

This is not tidiness. Enzymes, from Quiz 14, only work within narrow limits of temperature and acidity, and they are what runs every chemical step in the body. Cells, from Quiz 10, swell or shrivel when the water outside them is wrong. Holding these levels steady is the condition for the chemistry working at all, which is why so much of the body's machinery is spent on it.

What every control system is made of

Any system that holds a level steady needs three parts, and they are easiest to see in something you can put your hand on: the heating in a house.

The set point is the level the control center is aiming at. In the house it is the number you chose on the dial. In the body it is set by the brain, and — as fever shows — it can be changed.

Negative feedback: the response opposes the change

In a house, cold air comes in, the thermometer reads a drop, the thermostat compares that reading with the dial, and the furnace comes on. The room warms. The thermometer reads the new temperature, the thermostat sees the set point has been reached, and the furnace shuts off. The response ended the condition that called for it.

That is negative feedback, and here is the word that trips people. Negative does not mean bad or harmful. It means opposing: the response pushes against the change. Nearly everything holding you alive works this way.

Notice that some of the responses are automatic and some are things you do. Shivering is not your decision; putting on a coat is. Both are effectors, and the body uses whichever is available. Behavior is part of the system.

Fever, explained by the set point. Quiz 18 said a fever is the body raising its temperature on purpose. Now the mechanism is sayable: the brain moves the set point up, to 101 degrees, say. Your temperature is still at 98.6, which is now below the target — so the body treats you as too cold. You shiver, your skin vessels narrow, you pile on blankets, and you feel freezing while running a fever. When the set point drops back to normal, your temperature is suddenly too high, and you sweat it off. The chills and the sweats are the same loop working, on either side of a target that moved.

Positive feedback: the response adds to the change

The other shape is rarer in the body and it works the opposite way. In positive feedback, the response makes the original change larger, which produces more response, which makes the change larger still. Nothing returns to a set point. The loop builds on itself until something outside it ends the situation.

Two examples in the body, and both are ordinary. When a vessel is cut, the first platelets to stick release a substance that makes more platelets stick, which release more of it, and the plug builds quickly. It stops because the hole is sealed — not because a level came back to normal. And in childbirth, the baby's head presses on the cervix, which prompts a hormone that makes the contractions stronger, which presses harder, which prompts more. It ends when the baby is born and the pressure is gone.

In both cases the loop needs an outside event to stop it, because nothing in the loop itself is opposing the change. That is the useful test question when you are asked which shape something is: does the response settle the situation down, or build it up?

When homeostasis is overwhelmed. These systems have limits. Sweating cools you only while there is water to spare and only if the sweat can evaporate, so a hot, humid day with no water to drink can raise your core temperature past what the loop can pull back — that is heat stroke, and it is a medical emergency rather than a matter of toughing it out. Cold does the reverse: shivering makes heat until the body has spent what it has. Knowing the loop tells you what helps. Water, shade and a break restore the body's means of cooling. Anything that keeps a person working in the heat with nothing to drink removes them.

The short version

Homeostasis is holding conditions inside the body within a narrow range while the outside changes, because the chemistry of life only works within those ranges. Every control system has a sensor, a control center holding a set point, and an effector. In negative feedback the response opposes the change and the level returns; in positive feedback the response adds to the change and something outside the loop has to end it.

A handout goes with this quiz. The Feedback Loop is one printable page: both shapes side by side, five worked examples, and a sorting exercise. Feedback comes back in Part VI with ecosystems and in Part IX with the earth's climate, so it is worth keeping.

Worked Examples

Two questions, worked through

Example 1. Blood sugar rises after a meal, insulin is released, and the level comes back down. Which shape is this, and which part is the effector?

Take the shape first. The level went up and the response brought it down, so the response opposed the change. That is negative feedback, whatever you think of blood sugar being high.

Now the parts. Something has to detect the rise — cells in the pancreas do, so the pancreas is the sensor and also the control center here, since it both reads the level and decides to release insulin. The effector is whatever actually changes the level: the body's cells taking sugar in and the liver storing it away.

Negative feedback, with the cells and liver as effectors. Sort the shape by asking which direction the response pushed, then name the parts by what each one does — detect, decide, act — rather than by which organ sounds most important.

Example 2. A person with a fever shivers and asks for a blanket, though a thermometer reads 101 degrees. Why would a body that is already too warm act cold?

The question assumes the set point stayed at 98.6, and that is what has changed. During a fever the brain raises the target, to 101 in this case.

Now run the loop from the body's side. It reads its own temperature, compares it with the new target, and finds itself below it. Below the set point calls for the warming responses: shiver, narrow the skin vessels, seek covers. The person feels cold because by the standard the body is currently using, they are cold.

Because the set point moved, not the loop. A question that seems to show the body doing the opposite of what it should is usually a question about the target rather than the mechanism.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram — questions 1 to 4

Use the two diagrams from the Guide above: the three parts of a control system compared with a house's heating, and the temperature diagram with the set point in the middle.

1.In the house, the furnace is the effector. What is the effector in the body when it gets too hot?

AThe nerve endings in the skin
BThe sweat glands and the blood vessels of the skin
CThe part of the brain holding the set point
DThe thermometer

2.What does the word negative describe in negative feedback?

AThat the effect on the body is harmful
BThat the level always falls rather than rises
CThat the system has failed
DThat the response opposes the change

3.According to the temperature diagram, what does shivering accomplish?

AMuscle activity makes heat, raising the temperature toward the set point.
BIt lowers the set point so the body needs less heat.
CIt widens the blood vessels of the skin.
DIt stops the sensors from reporting the cold.

4.A person's set point is raised to 101 degrees while their temperature is 98.6. What would the body do?

ASweat, because 98.6 is its normal temperature.
BNothing, since 98.6 is a healthy reading.
CShiver and narrow the skin vessels, because it is now below the target.
DLower the set point again immediately.

Table — questions 5 to 7

Readings taken from one worker during a summer shift outdoors. Sweat rate is in liters per hour.

Time	Air temperature	Core temperature	Sweat rate
8 a.m.	72 °F	98.6 °F	0.1
10 a.m.	86 °F	98.9 °F	0.7
12 noon	95 °F	99.2 °F	1.1
2 p.m.	95 °F	100.9 °F	0.4

5.Between 8 a.m. and noon, the air temperature rose 23 degrees. How much did the worker's core temperature rise?

AIt rose by the same amount.
BBy about half a degree
CBy about five degrees
DIt did not rise at all.

6.What do the morning readings show about the loop?

AIt had stopped working by 10 a.m.
BCore temperature is unaffected by the air around a person.
CSweating rose as the air warmed, and the core was held nearly steady.
DSweating raised the core temperature.

7.At 2 p.m. the sweat rate has fallen while the core temperature has climbed. What does that combination most likely indicate?

AThe worker has cooled down and needs to sweat less.
BThe set point has been lowered.
CThe air has become cooler than the readings show.
DThe cooling response is failing, most likely for want of water.

Passage — questions 8 to 10

A worker cuts his forearm on a sheet of metal flashing, the thin metal used to seal roofs. Within seconds, platelets in the blood begin sticking to the torn edge of the vessel. Each platelet that sticks releases a chemical that makes nearby platelets sticky as well, so more of them arrive and stick, releasing more of the chemical.

The plug builds quickly and the bleeding slows and stops. The process ends when the tear is sealed and no more damaged surface is exposed.

8.Which shape of feedback does the passage describe?

APositive, because each step makes the next step larger
BNegative, because the bleeding is brought back to normal
CNegative, because platelets oppose the injury
DNeither; clotting is not a feedback process.

9.What brings the process to an end?

AThe platelet count returns to its set point.
BThe brain sends a signal to stop.
CThe tear is sealed, so the condition that started the loop is gone.
DThe chemical released by platelets runs out on a timer.

10.A student says this shows positive feedback is good for the body and negative feedback is bad. How should that be judged?

ACorrect, since clotting saves lives and fevers are unpleasant.
BWrong, because the words describe the direction of the response, not whether it helps.
CWrong, because both kinds of feedback are harmful.
DCorrect, but only for injuries.

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10 Missed — write the question number and the letter you chose:
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The Key

Answers, and the trap in each one

1. B — the sweat glands and the skin's blood vessels. An effector is whatever acts to change the level. Sweat carries heat away as it evaporates, and widened vessels bring warm blood to the surface where the heat can leave. A names the sensor and C names the control center. Sort the three by their verbs — detect, compare, act — and any question of this kind comes apart cleanly, in a house or in a body.

2. D — the response opposes the change. Negative is being used the way it is in arithmetic, as a direction rather than a judgment. Level goes up, response pushes down; level goes down, response pushes up. A is the everyday meaning of the word and it is wrong here, which is exactly why the test asks. Nearly every loop keeping you alive is a negative one. If negative meant harmful, none of Part III would make sense.

3. A — muscle activity makes heat. Shivering is muscles contracting rapidly without moving anything, and muscle work always releases heat. It is the body's way of running its own furnace. B changes the set point, which is not what an effector does. The set point is what the response is aiming at, so a choice that moves the target instead of the level has confused the two.

4. C — shiver and narrow the vessels. The loop compares the reading with the target, not with what is normal. Below target calls for warming, and 98.6 is below 101. A and B both judge 98.6 by the usual standard rather than the one the body is currently using. This is the fever question, and it explains something everybody has felt: chills while running a temperature.

5. B — about half a degree. 98.6 to 99.2 is six tenths of a degree while the air rose 23. Do the subtraction in both columns and compare the two. D is close enough to feel right if you glance at a core temperature that barely moved, but it did move, and the amount is the point. Nothing is held perfectly still; the loop keeps the drift small.

6. C — sweating rose and the core was held nearly steady. Read the two columns together. The air climbed 23 degrees, sweating went from a tenth of a liter an hour to more than a liter, and the core moved six tenths of a degree. The response grew as the challenge grew, and it worked. D reverses cause and effect: sweating cools rather than heats. And B claims the outside does not affect the core at all, which the 2 p.m. row disproves. Beware of any choice that makes homeostasis sound perfect — it is a system that keeps drift small, not one that prevents it.

7. D — the cooling response is failing, most likely for want of water. Two readings moved the wrong way together. The core is 2.3 degrees above where it started, 1.7 above where it was at noon, and still climbing, and the response that had been holding it down has dropped to well under half of what it was at noon. Sweat is made of water, and a worker who has been losing a liter an hour and not drinking runs out of it to spend. A reads a falling sweat rate as good news, which it would be if the core were falling too — that is why the two columns have to be read together. This is worth taking off the page: rising temperature with less sweating is the danger pattern in the heat, along with headache, confusion, and, in some people, skin that has stopped sweating. It calls for shade, water and help, not for pushing through, and heat stroke can kill.

8. A — positive. The passage gives the shape directly: each platelet that sticks makes more platelets stick. The response adds to the change instead of opposing it, and the plug builds faster as it goes. B is tempting because the bleeding does eventually stop, and a loop that ends in things being normal again feels like a return to a set point. Look at the mechanism rather than the outcome: nothing here was pushing back against the change while it happened.

9. C — the tear is sealed. Positive feedback has no target to reach, so it ends when the circumstances that started it are gone. The passage says exactly that: no more damaged surface is exposed. D invents a timer, which is the kind of stopping mechanism a negative loop might have. It is a fair guess and the passage rules it out — worth remembering that the test rewards taking the given text over the plausible invention.

10. B — the words describe the direction, not the benefit. Negative feedback holds your temperature, your blood sugar and your water balance, and you would die within days without it. Positive feedback seals a wound and delivers a baby, and it also drives some things that harm people. The two words say which way the response pushes and nothing else. A reaches the wrong general rule from one true example, and this pair of words is the most reliable trap in the topic. Say the definitions in plain language once — opposing, and adding to — and every question of this kind is settled.

Your Score

What the number means

8 to 10	Solid. Check topic 19 on your map, and go on to Quiz 20, which closes this part.
6 or 7	Close. Read the whole key, print the handout, and take this again in a few days before you check the box.
5 or fewer	Worth another pass. Work through the handout first — five loops sorted by shape — and then come back to this.

Misses on 1 to 4 are usually the three parts running together, and the house makes them separate again: thermometer, dial, furnace. Misses on 5 to 7 come from reading one column at a time, when the whole point is what two columns do together. Misses on 8 to 10 are the word trap, and it is worth beating for good, because feedback returns in Part VI when a population grows and in Part IX when ice melts and leaves darker ground behind.

The inside weather

In the 1850s the French physiologist Claude Bernard noticed something that had been in front of everybody. A lizard's temperature follows the air. A dog's does not. Bernard argued that animals like the dog carry an inside environment of their own — the blood and fluid surrounding every cell — and that they hold it steady while the world outside them changes. He wrote that keeping that inner setting constant is what allows an animal to live freely, rather than only where conditions happen to suit it.

The idea sat quietly for sixty years. In the 1920s the American physiologist Walter Cannon put a name to it, from Greek words meaning to stay about the same, and set out the pattern this quiz describes: a level, a target, and responses that push back whenever the level drifts.

What Bernard saw is why you can work a roof in July and sleep outdoors in November and still be running at 98.6 degrees. The weather inside you is your own, and holding it costs a great deal of the food you eat.