E. Homeostasis · Homeostasis

Homeostasis and Feedback Loops: How the Body Stays in Balance

E3.1 E2.1 E2.2 A1.12 A1.13About 65 minutes

Learning goals

I can define homeostasis and dynamic equilibrium and give normal ranges for key body variables.
I can identify the stimulus, receptor, control centre, effector and response in a feedback loop.
I can tell negative feedback from positive feedback and give examples of each.
I can compare nervous and endocrine control of homeostasis.
I can build and test a model of a feedback system.

Words to know

homeostasis
The process of keeping the body's internal environment (temperature, blood glucose, pH, water and ion levels) within a narrow range, even when conditions outside or inside change.

dynamic equilibrium
A balance that is kept by constant small adjustments, not by standing still. Body variables wobble slightly around a set point.

set point
The target value a control system tries to maintain, such as a core temperature of about 37 °C.

receptor (sensor)
A structure that detects a change in a variable, such as temperature receptors in the skin or glucose-sensing cells in the pancreas.

control centre (integrator)
The part that compares the receptor's information with the set point and sends out instructions; often the hypothalamus or an endocrine gland.

effector
A muscle or gland that carries out the response.

negative feedback
A control system in which the response reduces or reverses the original change, bringing the variable back toward the set point.

positive feedback
A control system in which the response increases the original change, until some outside event ends the loop.

Minus twenty outside, thirty-seven inside

Picture a January morning in Thunder Bay. It is −20 °C at the bus stop. Six months later, on a humid July afternoon in Windsor, it is +33 °C. In both places, the temperature deep inside your body stays close to 37 °C, usually changing by less than one degree.

The same is true of many other things in your blood. After a stack of pancakes with maple syrup, your blood glucose rises for a while and then comes back down. After a long run, your blood is a little more acidic for a few minutes, then it isn't. Your body is constantly being pushed out of balance and constantly pushing itself back.

This ability to keep a stable internal environment is called homeostasis. It is the big idea of this whole unit. Every system you will study, from the kidneys to the hormones to the neurons, is part of the machinery of homeostasis.

Remember from Grades 10 and 11

In Grade 10 (Tissues, Organs, and Systems), you learned that body systems work together to keep the body in balance, and you met negative feedback with the example of insulin lowering blood sugar. In Grade 11 Biology (Animals: Structure and Function), you saw that chemoreceptors detect rising CO₂ during exercise and the medulla speeds up breathing: another negative feedback loop. This unit explains how these loops work in detail: the receptors, the signals, the cells and the molecules.

What homeostasis means

Your cells live in extracellular fluid: the blood plasma and the fluid between cells. Cells only work properly when that fluid has the right temperature, the right amount of water, the right concentration of ions such as Na⁺ and K⁺, enough glucose and oxygen, and the right pH. Enzymes are especially sensitive. As you saw in the Biochemistry unit, an enzyme's shape (and so its activity) changes with temperature and pH.

Homeostasis does not mean the body is perfectly constant. Each variable wobbles slightly around a set point, staying inside a normal range. Because the balance is kept by constant small corrections, it is called a dynamic equilibrium.

Here are some of the variables your body controls, with typical values for a healthy adult at rest. (Different labs and references give slightly different ranges.)

Variable	Typical normal range	Main organs that control it	Lesson in this unit
Core body temperature	about 36.5 to 37.5 °C	hypothalamus, skin, muscles	Thermoregulation
Blood glucose (fasting)	about 4 to 6 mmol/L	pancreas, liver	Blood Glucose and Diabetes
Blood pH	7.35 to 7.45	lungs, kidneys, blood buffers	Acid–Base Balance
Plasma sodium (Na⁺)	about 135 to 145 mmol/L	kidneys, adrenal glands	Water and Ion Balance
Blood water level (osmolarity)	kept nearly constant	hypothalamus, pituitary, kidneys	Water and Ion Balance

Some variables controlled by homeostasis. mmol/L means millimoles per litre of blood.

The parts of a feedback loop

Every homeostatic control system has the same basic parts:

Stimulus: a change that moves a variable away from its set point (for example, body temperature rises).
Receptor (sensor): detects the change (temperature receptors in the skin and in the brain).
Control centre (integrator): compares the information with the set point and decides what to do. In many loops this is the hypothalamus in the brain.
Effector: a muscle or gland that acts on the instructions (sweat glands, blood vessel muscles).
Response: the action that changes the variable (sweating cools the skin).

The control centre sends its instructions in one of two ways: by nerve impulses (electrical signals along neurons) or by hormones (chemical messengers carried in the blood). Often it uses both.

The parts of a negative feedback loop. Follow the arrows: a change is detected, compared with the set point, and corrected. The dashed arrow shows that the correction removes the stimulus, so the loop quiets down.

Worked example

Example 1: A furnace thermostat as a model

Your home in Sudbury has a furnace controlled by a thermostat set to 20 °C. It is a cold night in February. Use the vocabulary of feedback loops to describe how the thermostat keeps the house warm. Then say what kind of feedback this is.

Set point: 20 °C, the temperature you chose on the thermostat.
Stimulus: heat leaks out through the windows and walls, so the room temperature drops below 20 °C.
Receptor: the temperature sensor inside the thermostat detects the drop.
Control centre: the thermostat's circuit compares the reading (say 19 °C) with the set point (20 °C). It is too cold, so it sends an "on" signal.
Effector: the furnace turns on. Response: warm air raises the room temperature.
When the sensor reads 20 °C again, the control centre switches the furnace off. The response (heating) removed the stimulus (cold), so this is negative feedback.

The thermostat is a negative feedback system: sensor → control circuit → furnace → warmer room, which switches the furnace off. Your hypothalamus works the same way, but with sweat glands, blood vessels and muscles as effectors.

Negative feedback and positive feedback

Negative feedback is by far the most common kind of control in the body. Negative does not mean bad. It means the response acts opposite to the change: if a variable goes up, the response brings it down; if it goes down, the response brings it up. Negative feedback keeps things stable.

Because signals take time, negative feedback is never instant. A variable often overshoots a little and wobbles before it settles, just as a room gets slightly warmer than 20 °C before the furnace's heat fades.

Positive feedback works the other way: the response increases the original change. That would be dangerous for something like body temperature, so positive feedback is used only for processes that need to finish quickly and completely. Then an outside event stops the loop. Examples:

Childbirth: pressure of the baby's head on the cervix sends nerve signals to the brain, which causes release of the hormone oxytocin. Oxytocin makes the uterus contract harder, which pushes the head down harder, which releases more oxytocin. The loop ends when the baby is born.
Blood clotting: platelets that stick to a damaged vessel release chemicals that attract more platelets, until the clot seals the cut.
Ovulation: a rising level of estrogen triggers a sudden surge of the hormone LH. You will study this in the lesson on reproductive hormones.

Left: with negative feedback a disturbance is corrected, overshoots a little and settles back inside the normal range (shaded). Right: with positive feedback the change keeps growing until an outside event ends it.

Feature	Nervous control	Endocrine (hormone) control
Messenger	nerve impulses and neurotransmitters	hormones in the blood
Speed	very fast (milliseconds)	slower (seconds to hours)
Duration of effect	short	often long-lasting
Targets	specific muscles, glands or neurons	any cell with the right receptor, often many organs
Example	pulling your hand off a hot pan	insulin lowering blood glucose after a meal

The two communication systems of homeostasis. In most loops they work together; the hypothalamus links them.

Worked example

Example 2: Analysing two feedback loops

(a) After lunch, blood glucose rises from 5 to 8 mmol/L. Beta cells in the pancreas detect this and release insulin. Insulin causes liver and muscle cells to take in glucose, so blood glucose falls back toward 5 mmol/L. Identify each part of the loop and the type of feedback.

(b) Explain why the oxytocin loop in childbirth is called positive feedback.

(a) Stimulus: blood glucose rises above the set point.
Receptor and control centre: beta cells of the pancreas do both jobs: they sense the glucose level and respond by releasing insulin. (Many endocrine glands act as both receptor and control centre.)
Messenger: the hormone insulin, carried in the blood. Effectors: liver and muscle cells. Response: they take up glucose and store it as glycogen.
Blood glucose falls, which removes the stimulus, so insulin release slows. This is negative feedback.
(b) In childbirth, the response (stronger contractions) pushes the baby harder against the cervix, which increases the stimulus and causes even more oxytocin to be released. Each round makes the change bigger, which is the definition of positive feedback. The loop stops only when birth removes the pressure.

(a) Negative feedback: rising glucose → pancreas (beta cells) → insulin → liver and muscle store glucose → glucose falls. (b) The response strengthens the stimulus until birth ends it, so it is positive feedback.

Hands-on activity

Be the hypothalamus: a model of a feedback system

Grown-up note: use warm tap water only (no kettle or stove). An adult should check that the warm water is comfortable to touch.

This activity builds a working model of negative feedback (curriculum expectation E2.2). You need: a large bowl of tap water at room temperature, a kitchen or aquarium thermometer, a jug of warm tap water, a jug of cold water with a few ice cubes, a measuring cup (about 50 mL), a timer, a partner, paper and a pencil.

Choose a set point about 5 °C above the starting water temperature (for example, 25 °C if the water starts at 20 °C). Write it down.
Decide the roles. One person is the receptor (reads the thermometer every 30 seconds and calls out the number). The other is the control centre and effector: if the reading is below the set point, add one 50 mL cup of warm water; if it is above, add one cup of cold water; if it is within 0.5 °C, do nothing. Stir after each addition.
Run the model for 8 minutes. Record time and temperature in a table.
Graph temperature (y-axis) against time (x-axis). Draw a dashed line at the set point.
Now repeat, but this time the receptor reads the thermometer only every 90 seconds.

Analyse: Does your graph look like the left graph in the diagram above? Which run overshot the set point more? Explain why a delay in sensing makes a feedback system wobble more. Draw a flow chart of your model and label the stimulus, receptor, control centre, effector and response.

What to look for: the temperature should rise toward the set point and then wobble around it. The 90-second run usually overshoots more, because the effector keeps acting on old information. Real body loops have delays too, which is why variables fluctuate inside a normal range.

Just for fun

The word homeostasis comes from Greek words meaning "similar" and "standing still". The American physiologist Walter Cannon made it popular in his 1932 book The Wisdom of the Body. It is a slightly funny name, because your body keeps things steady by never standing still for a second.

Your turn

Define homeostasis in one sentence.

Why is homeostasis described as a dynamic equilibrium?

Name the five parts of a feedback loop in order.

Give the normal range of blood pH.

You start shivering on a cold day. What is the effector, and what is the response?

Explain why positive feedback is not used to control body temperature.

Give one way nervous control differs from endocrine control.

More practice: warm-up

What is the approximate set point for human core body temperature?

Which part of a feedback loop detects a change?

Is the furnace in a thermostat system a receptor, control centre or effector?

Name one example of positive feedback in the human body.

What two kinds of messages can a control centre use to reach effectors?

More practice: on your own

When you are dehydrated, the hypothalamus detects that the blood is too concentrated and the pituitary releases a hormone that makes the kidneys save water. Identify the stimulus, control centre and effector.

A student says "negative feedback is bad because it's negative." Correct the student.

Why does a variable controlled by negative feedback usually overshoot the set point a little?

Blood loss from a small cut triggers platelets to release chemicals that attract more platelets. Explain whether this is positive or negative feedback.

In Example 2, the beta cells act as both receptor and control centre. Explain what that means.

Challenge

A healthy person drinks 1 L of water quickly. Predict how the body's water-balance loop responds over the next two hours, and sketch (in words) the shape of a graph of urine output against time.

Some engineers design "smart" thermostats that start heating before the house gets cold, based on the weather forecast. Is there anything similar in the body? Explain.

Explain why a loss of homeostasis in one variable can affect many others. Use temperature and enzymes as your example.

Check your understanding

Quiz

Which statement best describes homeostasis?

In a feedback loop, which part compares incoming information with the set point?

Sweat glands releasing sweat on a hot day are acting as:

Which is an example of positive feedback?

Compared with nervous control, endocrine control is usually:

In the thermostat model, why does the room temperature overshoot 20 °C slightly?

For parents and teachers

Big idea: homeostasis and feedback loops are the framework for the whole unit. Every later lesson (temperature, kidneys, hormones, neurons) fits the stimulus → receptor → control centre → effector → response pattern.

How to help: Ask your teen to explain a household feedback system to you: the furnace, the fridge, a toilet tank float, cruise control in a car. Then ask them to name each part. If they can map a machine, they can map the body.

Common difficulties:

Thinking "negative" means harmful. It means the response goes the opposite way to the change.
Mixing up receptor and effector. Receptors sense; effectors act.
Calling every hormone loop positive feedback. Most are negative; positive feedback is the exception (birth, clotting, ovulation).

Safety: the activity uses only warm tap water; check the temperature before your teen puts a hand in.