Homeostasis and Response

The response cancels itself

Homeostasis is almost always introduced as keeping conditions inside the body constant. That sentence is where the trouble starts, because it is not quite true and it hides the machinery. ⚠️⚠️ NOTHING IS HELD CONSTANT. YOUR TEMPERATURE RISES AND FALLS ALL DAY. YOUR BLOOD GLUCOSE CLIMBS AFTER A MEAL AND DROPS BETWEEN THEM. YOUR WATER LEVEL SWINGS WITH EVERY DRINK. What the body actually does is keep bringing them back. The graph in the textbook is a wavy line crossing a level, not a flat one sitting on it, and that shape is the topic rather than an untidiness in it. ⭐⭐ AND THE REASON IS THE STRANGEST PROPERTY IN THE WHOLE SUBJECT: EVERY ONE OF THESE RESPONSES MAKES ITSELF UNNECESSARY. Sweating cools you, until you are no longer hot enough to sweat. Shivering warms you, until you are no longer cold enough to shiver. Insulin lowers blood glucose, until there is no longer a raised level for insulin to be released about. ⭐ LOOK AT WHAT THAT MEANS. THE ACTION BEING TAKEN IS THE THING THAT REMOVES THE REASON FOR TAKING IT. That is what negative feedback means, and it is where the word negative comes from. The response opposes the change that triggered it, and in opposing it successfully, switches itself off. ⭐⭐ NOW THE PAYOFF, BECAUSE THIS EXPLAINS THE THING THE CONSTANT VERSION CANNOT: WHY THE LEVEL ENDS UP WAVING AROUND A NORM RATHER THAN SITTING ON IT. A response that switched off too early would never finish correcting anything. A response that could not switch off at all would keep going past the norm and out the other side. ⚠️ SO THE WAVE IS NOT A FAILURE OF THE SYSTEM. IT IS WHAT A SELF-CANCELLING CORRECTION LOOKS LIKE WHEN YOU DRAW IT. ⭐ AND IT TELLS YOU WHAT IS MISSING FROM MOST EXAM ANSWERS. STUDENTS DESCRIBE THE RESPONSE AND STOP: YOU GET HOT, SO YOU SWEAT. ⚠️ THAT IS HALF A LOOP. THE MARKS ARE IN THE OTHER HALF: WHAT NOTICED THE CHANGE, AND WHAT SWITCHES THE RESPONSE OFF AGAIN. Carry one question through everything that follows: what switches this off? The answer is always its own success.

Three loops, one shape

The same four questions asked of three systems. Notice that the last row is the one students leave out.

Tap the two that switch off

Tap the TWO statements that describe a response SWITCHING ITSELF OFF rather than just happening.

• Sweating cools the body until it is no longer hot enough for sweating to continue
• Insulin lowers blood glucose until there is no longer a raised level to release it for
• Shivering produces heat in the muscles
• The kidneys adjust how much water is returned to the blood

Why the line is wavy

A graph of body temperature over a day is a wavy line crossing the norm rather than a flat line on it. Why is that the expected shape?

• Because each response only switches off once it has succeeded, so the level is always being corrected back rather than held in place
• Because the measuring instrument is not accurate enough to show a flat line
• Because that person's temperature regulation is not working properly
• Because the responses are too slow to keep up with the changes

The terms the loop is made of

Five terms, each defined by what it is. The middle two are the parts of the loop most answers never name.

Match each change to the loop it starts

Two that follow from self-cancelling

Select the TWO statements that follow from every response switching itself off once it works.

• Levels wave around a norm rather than sitting on it, and that shape is what a working system produces
• An answer that names the response but not what switches it off has described only half the loop
• A perfectly healthy person would show a flat line for body temperature across a day
• The response to being too hot is simply the reverse of the response to being too cold

Writing a loop rather than a response

Almost every question on this topic gives you a change and asks what the body does. The change is the easy part; the marks are distributed across the loop, and most answers spend all their words in one place. ⭐⭐ WRITE THE LOOP IN FOUR MOVES, AND NAME ALL FOUR EVEN WHEN THE QUESTION ONLY SEEMS TO ASK FOR ONE. SAY WHAT CHANGED, AND IN WHICH DIRECTION. Above or below the norm. ⚠️ AN ANSWER THAT IS VAGUE ABOUT THE DIRECTION USUALLY GOES ON TO DESCRIBE THE WRONG RESPONSE. ⭐ SAY WHAT DETECTED IT. This is the first commonly missing piece. Something has to notice before anything can happen, and naming it is often worth a mark on its own. SAY WHAT CARRIED OUT THE RESPONSE, AND HOW IT OPPOSES THE CHANGE. ⚠️ THE WORD OPPOSES IS DOING WORK: A RESPONSE THAT DID NOT PUSH BACK AGAINST THE CHANGE WOULD NOT BE PART OF THIS TOPIC AT ALL. ⭐⭐ AND SAY WHAT SWITCHES IT OFF. THIS IS THE SECOND MISSING PIECE AND THE MORE VALUABLE ONE, BECAUSE IT IS WHAT SHOWS YOU UNDERSTAND WHY THE SYSTEM IS STABLE RATHER THAN JUST WHAT IT DOES. ⚠️ THREE HABITS THAT COST MARKS HERE. ⚠️ THE FIRST IS STOPPING AT THE RESPONSE. You get hot, so you sweat, full stop. It is true and it is half a loop, and the half left out is the half the question was really about. ⚠️ THE SECOND IS SAYING THE BODY KEEPS THE LEVEL CONSTANT. ⭐ IT DOES NOT, AND THE BETTER SENTENCE IS THAT IT KEEPS BRINGING THE LEVEL BACK. THAT WORDING COSTS NOTHING AND SHOWS THE MECHANISM. ⚠️ AND THE THIRD IS REACHING FOR NUMBERS. This topic can be answered entirely in rises, falls, more and less, and a half-remembered value for a normal level is a risk with no reward attached. One last thing. When a question describes something going wrong, ask which part of the loop is broken: the detecting, the responding, or the switching off. That single question turns a confusing stem into an ordinary one.

One day of blood glucose

Follow the level through an ordinary day. No numbers appear here, because every point can be made in rises and falls, and a half-remembered value would add risk rather than accuracy. You eat. Glucose is absorbed from the gut into the blood, and the level climbs. The pancreas notices the climb and releases insulin into the blood. ⭐ INSULIN TRAVELS EVERYWHERE THE BLOOD GOES, AND ITS EFFECT IS TO MAKE CELLS TAKE GLUCOSE IN. The liver takes up a good deal and stores it. So the level comes down. ⭐⭐ AND NOW THE MOMENT THIS WHOLE MODULE IS ABOUT. AS THE LEVEL COMES DOWN, THE PANCREAS HAS LESS AND LESS REASON TO RELEASE INSULIN, SO IT RELEASES LESS. THE THING THAT BROUGHT THE LEVEL DOWN WAS SWITCHED OFF BY THE LEVEL COMING DOWN. ⚠️ NOBODY DECIDED TO STOP. THE STOPPING IS BUILT INTO THE ARRANGEMENT. Hours pass without eating. Cells go on using glucose, so the level drifts downwards, below where it should be. The pancreas notices that too, and releases a different hormone, glucagon. Its effect is roughly the reverse: the liver releases some of its stored glucose back into the blood, and the level rises. ⭐ AND AGAIN, THE RISE REMOVES THE REASON FOR THE RELEASE. ⚠️⚠️ NOTE CAREFULLY WHAT THE TWO HORMONES ARE NOT. THEY ARE NOT AN ON SWITCH AND AN OFF SWITCH FOR THE SAME PROCESS. They are two separate responses, released by the same organ in opposite circumstances, doing opposite jobs, each ending itself. ⭐ WHICH IS WHY THE LEVEL SPENDS THE WHOLE DAY CROSSING AND RE-CROSSING THE MIDDLE RATHER THAN SETTLING ON IT. ⚠️ DIABETES IS A CONDITION IN WHICH THIS ARRANGEMENT DOES NOT WORK IN THE USUAL WAY, SO THE LEVEL IS HARDER TO BRING BACK. ⚠️ HOW IT IS MANAGED IS A MEDICAL MATTER AND NOT SOMETHING THIS MODULE ADVISES ON. WHAT THE COURSE ASKS IS THAT YOU CAN SAY WHICH PART OF THE ARRANGEMENT IS NOT WORKING AS USUAL.

Find where the blood is cleaned

This diagram shows the organs that adjust the water content of the blood. Tap the organ where blood arriving from the body has its water content adjusted and its waste removed.

Order one turn of the loop

Put these six stages into the order they happen in any homeostatic response.

• A condition moves away from the norm
• A receptor detects the difference
• A signal is sent, by nerves or in the blood
• An effector carries out a response that opposes the change
• The condition moves back towards the norm
• The receptor no longer detects a difference, so the response stops

Assemble the sentence that closes the loop

This is the sentence most answers are missing. Assemble it.

The loop run

Five questions on detecting, opposing and switching off. Three lives.

Complete the regulation facts

Keeping conditions inside the body within narrow limits, by correcting changes as they happen, is _____. A response that opposes the change which triggered it, and so switches itself off once it has worked, is _____. A cell or structure that detects a change in conditions is a _____. A muscle or gland that carries out the response is an _____.

Three answers missing a half

Three students answer questions on this topic. Nothing they write is false; what is absent is the problem.

• Asked what happens when body temperature rises, a student writes that you sweat and the sweat evaporates, cooling you. How would you complete it?
• An answer says homeostasis keeps conditions in the body constant. Is that acceptable?
• A question describes a condition in which blood glucose stays high after eating. A student writes that the person should change what they eat. What is wrong with that answer?

Explain one loop, all the way round

A friend says that homeostasis means the body keeps everything constant, and cannot see why a temperature graph is wavy. Explain what actually happens, using one complete loop as your example.

• Correct the wording: say what the body does instead of keeping conditions constant
• Choose one condition and say what makes it move away from the norm
• Say what detects the change, before anything responds
• Say what carries out the response, and how it opposes the change
• Explain what switches the response off, and why that is its own success
• Use that to explain why the graph is a wave crossing a level rather than a flat line
• Keep to rises, falls, more and less, and give no values you are not certain of