Homeostasis, negative feedback and positive feedback, explained with one loop you will use in every chapter of this course. Homeostasis is how your body holds conditions such as temperature and blood pressure inside a narrow range while the world around you changes. Negative feedback is the mechanism that does most of that work. Positive feedback is its opposite: a loop that drives a change further until something outside the loop stops it. Learn the seven parts of the loop here, and every system that follows becomes the same question: what is sensed, where is it compared, and what does the correcting?
Start with a cold morning
You step outside on a winter morning. The air is −5 °C. Your skin cools within seconds, and your fingers go pale. Within a minute or two you start to shiver. Yet if someone took your body temperature an hour later, it would still read close to 37 °C.
Nothing about that is luck. Your skin cooled, and that change was detected. The information was sent to your brain and compared with a target value. Your brain sent out commands, and your blood vessels and muscles carried them out. The result pushed your temperature back toward the target. That sequence is a feedback loop, and it is the pattern this page teaches.
Homeostasis: steady, not still
Homeostasis (homeo- = alike or same, -stasis = standing still) is the maintenance of fairly stable conditions inside your body while conditions outside change. You met the basic idea in the first topic of this course. Here is the precise version.
Homeostasis does not mean that nothing changes. Your body temperature rises and falls a little over the day. It climbs during exercise and dips during sleep. What stays steady is the range: each condition moves up and down within a normal range and is pulled back whenever it drifts. Physiologists call that a dynamic equilibrium: steady on average, because it is constantly being corrected.
Regulated variables and set points
A regulated variable is a condition your body actively holds within a normal range. Examples:
- body temperature
- blood pressure
- blood pH (7.35 to 7.45)
- the concentrations of glucose, oxygen, carbon dioxide, sodium, potassium and calcium in your blood
- the osmolarity of your body fluids
The set point is the target value the body regulates a variable toward: for body temperature, about 37 °C (98.6 °F). The set point sits inside the normal range, and the variable swings around it.
Set points are not fixed forever. Your temperature set point is slightly lower in the early morning and higher in the evening. During an infection it can be moved upward, causing a fever, and your body then works to hold the higher value. The immune system chapter explains how.
The seven parts of a feedback loop
Every homeostatic loop in this course is built from the same seven parts. Figure 1 shows them in order. Learn them in that order, because the feedback loop builder and every system chapter use exactly these names.
1. Stimulus
The stimulus is a change in a regulated variable: your skin temperature falls, your blood pressure drops, the glucose level in your blood rises after a meal. Without a change, the loop has nothing to respond to.
2. Sensory receptor (sensor)
A sensory receptor, or sensor, is a cell or group of cells that detects the change. In your skin, nerve endings respond to cold. In the walls of your large arteries, stretch-sensitive nerve endings respond to changes in pressure. Some sensors are gland cells that detect a chemical directly in the blood.
Always use the full name "sensory receptor" for this part of the loop. A receptor protein is a single molecule that binds a chemical messenger, as you learned in the chemical signaling topic. A sensory receptor is a whole cell or structure that detects a change. The two are easy to mix up.
3. Afferent pathway
The afferent pathway (af- = toward, fer = to carry) carries information from the sensory receptor toward the control center. Usually it is a nerve carrying action potentials. When the sensor and the control center sit in the same gland, the afferent pathway can be very short.
4. Control center
The control center, also called the integrating center, receives the input, compares it with the set point, and decides on an output. For body temperature, the control center is a region of your brain.
5. Efferent pathway
The efferent pathway (ef- = away from, fer = to carry) carries commands away from the control center to the effector. It can be nerves carrying action potentials, or a hormone traveling in the blood. A memory trick: afferent arrives at the control center, efferent exits it.
6. Effector
An effector is the cell, tissue or organ that carries out the command. Effectors are almost always muscles (skeletal, smooth or cardiac) or glands. The smooth muscle in the walls of your skin's blood vessels is an effector. So is a skeletal muscle.
7. Response
The response is what the effector does and the change it makes to the regulated variable. The response feeds back on the variable that started the loop. That is why this is called feedback. Which way the response pushes the variable decides what kind of loop it is.
Negative feedback: the response opposes the change
In negative feedback, the response moves the regulated variable in the opposite direction to the stimulus. A rise causes a response that lowers it. A fall causes a response that raises it. Either way, the variable is pushed back toward the set point. "Negative" means "opposing", not "harmful" and not "lowering".
Almost every homeostatic loop in your body is a negative feedback loop. Here is your body temperature on that cold morning, part by part:
| Loop part | Body temperature falls | Body temperature rises |
|---|---|---|
| Stimulus | Cold air cools the skin and the blood | Exercise or hot weather warms the body |
| Sensory receptor (sensor) | Cold-sensitive nerve endings in the skin; temperature-sensitive neurons in the brain | Warm-sensitive nerve endings in the skin; temperature-sensitive neurons in the brain |
| Afferent pathway | Sensory nerves to the brain | Sensory nerves to the brain |
| Control center | Temperature control center in the brain | Temperature control center in the brain |
| Efferent pathway | Nerves to skin blood vessels and skeletal muscles | Nerves to skin blood vessels and sweat glands |
| Effector | Smooth muscle of skin blood vessels; skeletal muscles | Smooth muscle of skin blood vessels; sweat glands |
| Response | Vasoconstriction keeps warm blood away from the skin, so less heat is lost; skeletal muscles contract in rapid small bursts (shivering), which releases heat | Vasodilation brings warm blood to the skin, so more heat is lost; sweat spreads on the skin and, as it evaporates, carries heat away |
Notice that the response in each column opposes its stimulus. Notice too that the loop switches itself off. As your temperature returns toward the set point, the stimulus shrinks, the sensors signal less, and the effectors ease off. Negative feedback stops itself.
Why the variable wobbles
A negative feedback loop never holds a variable perfectly flat. It takes time to detect a change, send signals and get effectors working. By the time the response takes hold, the variable has already moved. The correction then carries it a little past the set point, and the loop corrects the other way. The result is an oscillation: small swings around the set point, inside the normal range, as Figure 2 shows.
Positive feedback: the response strengthens the change
In positive feedback, the response pushes the regulated variable further in the same direction as the stimulus. The bigger the change, the stronger the response, and the stronger the response, the bigger the change. The loop does not stop itself. Something outside the loop has to end it.
That sounds dangerous, and out of control it is. But your body uses positive feedback for a few events that need to finish quickly and completely.
Childbirth
Late in pregnancy, the baby's head presses on the lower opening of the uterus and stretches it. Stretch-sensitive sensory receptors there send nerve signals to the brain. In response, the pituitary gland releases a hormone into the blood that makes the smooth muscle of the uterus contract harder. Harder contractions push the baby's head down further, which stretches the opening more, which triggers more hormone release, and stronger contractions still (Figure 3). The loop ends with an event outside it: the baby is delivered, the stretch stops, and the signals stop.
Blood clotting
When a blood vessel breaks, platelets stick to the damaged wall. Once stuck, they release chemicals that make more passing platelets stick, and those release more of the same chemicals. The pile of platelets grows faster and faster. The loop ends when the break is sealed and no damaged wall is left exposed. This pile of platelets is only the first stage of clotting. The next stage, in which a mesh of protein fibers forms around the platelets, also runs on positive feedback. The blood chapter follows both in detail.
| Negative feedback | Positive feedback | |
|---|---|---|
| Direction of the response | Opposes the stimulus | Strengthens the stimulus |
| Effect on the variable | Returns it toward the set point | Drives it further from where it started |
| How the loop ends | Switches itself off as the stimulus shrinks | Needs an outside event to stop it |
| How common | Most homeostatic loops | A few events that must finish quickly |
| Examples | Body temperature, blood pressure, blood pH | Childbirth contractions, blood clotting |
Standard feedback examples
These loops come back again and again in this course. Learn which kind each one is now, and fill in the details as you reach each system.
- Body temperature (negative): covered above; the skin chapter adds detail.
- Blood pressure (negative): stretch-sensitive sensory receptors in the walls of large arteries detect a fall in pressure. A control center in the brain then speeds up the heart and triggers vasoconstriction of small arteries, raising pressure back toward the set point.
- Glucose in the blood (negative): after a meal, gland cells in the pancreas detect the rise and release a hormone that lowers the level. Between meals, other pancreas cells release a hormone that raises it.
- Red blood cells and oxygen (negative): when the blood carries too little oxygen, the kidneys detect it and release a hormone. The hormone makes your bone marrow produce more red blood cells, which carry more oxygen, and the signal fades.
- Childbirth (positive): covered above.
- Blood clotting (positive): covered above.
Notice two things. First, a sensor and a control center can be the same cell: the pancreas's gland cells detect glucose and decide how much hormone to release. Second, an efferent pathway can be a hormone rather than a nerve, as in the glucose and red blood cell loops.
Mass balance: input must equal output
Feedback loops explain how your body corrects a change. Mass balance explains where the change comes from in the first place. The amount of any substance in your body stays constant only if the amount coming in equals the amount going out.
- Inputs: what you absorb from food and drink or breathe in, plus what your cells make by metabolism.
- Outputs: what leaves in urine, feces, sweat and breath, plus what your cells use up by metabolism.
If input equals output, the amount in the body stays the same. If input exceeds output, it builds up. If output exceeds input, it falls. Carbon dioxide is a good example: your cells make it constantly by aerobic respiration, and your lungs breathe it out. As long as breathing removes it as fast as metabolism makes it, the amount in your blood stays steady.
Worked example: sodium in and sodium out over three days
Maria usually eats 150 mmol of sodium a day and excretes 150 mmol a day. For three days she eats salty food and takes in 210 mmol a day. For those days her kidneys still excrete only 150 mmol a day. How does her total body sodium change?
- Write the rule: change in the body = input − output.
- Daily change: 210 mmol − 150 mmol = +60 mmol per day.
- Over three days: 60 mmol/day × 3 days = +180 mmol.
- Interpret: her body gains 180 mmol of sodium. The body will not stay out of balance: the gain is itself a stimulus, and negative feedback loops raise sodium output by the kidneys until output again equals input.
Answer: total body sodium rises by 180 mmol over the three days, until the kidneys' output catches up with her intake.
Mass balance and feedback work together. Mass balance tells you what disturbs a variable. Feedback tells you how the body adjusts inputs or outputs to bring it back.
Homeostatic imbalance: disease as failed regulation
A homeostatic imbalance is a state in which a regulated variable stays outside its normal range because regulation has failed. Many diseases are exactly that. Use the loop to find where the failure is:
- The sensor fails. The change is never detected, so no correction starts.
- The control center fails. The input arrives, but no correct command goes out.
- A pathway fails. Damaged nerves, or a gland that stops making its hormone, break the chain. In one common disease, the pancreas stops making the hormone that lowers glucose in the blood, so the glucose level stays high after meals.
- The effector fails or is overwhelmed. The loop works, but the disturbance is bigger than the effectors can oppose. In a long heat wave, the heat load can be larger than sweating and skin blood flow can remove. An older person, whose skin vessels dilate less and who sweats less, overheats first even though the sensors, pathways and control center all work.
When negative feedback fails, positive feedback can take over. For example, if body temperature climbs high enough, the heat speeds up the chemical reactions in every cell. Faster reactions release more heat, which raises temperature further. That is a harmful positive feedback loop, and it is why very high body temperature is a medical emergency.
Much of medical treatment works by standing in for a failed loop part: a warming blanket acts as an outside effector, and a replacement hormone restores a broken efferent pathway.
How to use the loop from now on
For every loop you meet in later chapters, fill in the same seven slots: stimulus, sensory receptor (sensor), afferent pathway, control center, efferent pathway, effector, response. Then ask one question: does the response oppose the stimulus or strengthen it? That tells you whether it is negative or positive feedback. The skin chapter, which comes next, applies it straight away to body temperature.