The shape of the lesson
By the end of the lesson, learners can
- A control loop with five parts: Press Too hot (core temperature jumps to 38.5 °C) and watch the light travel round the loop from stimulus to receptor, coordinator, effector and response.
- Nerves or hormones: two coordination systems: Core temperature is controlled mainly through the nervous system; blood glucose and blood water potential are controlled through hormones.
- Homeostasis = keeping the internal environment within narrow limits.
- Receptor detects; coordinator sends instructions; effector (muscle or gland) responds.
- Negative feedback: the response reverses the change.
- Nervous = fast and brief; endocrine = slower and longer-lasting.
One picture every idea returns to
Homeostasis is the maintenance of a constant internal environment: core temperature, blood glucose concentration and blood water potential are all kept within narrow limits around a set point. This matters because enzymes work best within a narrow range of temperature and pH, and cells shrink or swell if the water potential of the tissue fluid changes. A change away from the set point is a stimulus, which may be internal (more heat from respiration during exercise) or external (a cold day). Receptors detect it; a coordination system, either the nervous system (fast, short-lived, impulses along neurones) or the endocrine system (slower, longer-lasting, hormones in the blood), sends instructions; effectors, which are muscles or glands, bring about a response. In negative feedback the response reverses the original change, so the value returns towards the set point.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Keeping the inside steady.. Outside, the temperature swings and meals come and go. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: A control loop with five parts | Press Too hot (core temperature jumps to 38.5 °C) and watch the light travel round the loop from stimulus to receptor, coordinator, effector and response. Project tab 1 of the lab (Homeostasis lab, negative feedback) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Nerves or hormones: two coordination systems | Core temperature is controlled mainly through the nervous system; blood glucose and blood water potential are controlled through hormones. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Nerves or hormones: two coordination systems, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Negative feedback does not mean that something bad happens. | It means the response is in the opposite direction to the change: a rise causes responses that lower the value, a fall causes responses that raise it. |
| Arterioles do not move up and down in the skin. | Vasodilation and vasoconstriction change the diameter of arterioles, which changes how much blood flows through the surface capillaries. |
Principles of homeostasis: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Explain what is meant by homeostasis and name two factors that are controlled by homeostasis in mammals.
Explain why it is important for a mammal to keep the water potential of its blood and tissue fluid constant.
Using the example of a rise in core body temperature during exercise, explain the principle of negative feedback.
Distinguish between an internal stimulus and an external stimulus, giving one example of each that could affect core temperature.
A person's core temperature is recorded every minute. It rises from 37.0 °C to 37.6 °C over 10 minutes of hard exercise, then falls back to 37.1 °C over the next 15 minutes of rest. Describe and explain these results.
Compare the ways in which the nervous system and the endocrine system coordinate homeostatic responses.
Answer key and marking notes
- Homeostasis is the maintenance of a constant internal environment, keeping factors within narrow limits around a set point (despite changes inside or outside the body).
- Any two of: core body temperature, blood glucose concentration, water potential of the blood (also pH, carbon dioxide concentration).
- If the tissue fluid has a lower water potential than the cells, cells lose water by osmosis and shrink / metabolism is disrupted.
- If the tissue fluid has a higher water potential, cells gain water by osmosis and swell (and may burst).
- Changes in water content alter the concentrations of solutes and enzymes in cells, so reactions do not run at a steady, efficient rate.
- The rise above the set point (stimulus) is detected by thermoreceptors in the hypothalamus (and skin).
- The hypothalamus (coordinator) sends nerve impulses to effectors.
- Sweat glands secrete more sweat, which evaporates and removes heat; arterioles in the skin dilate so more blood flows near the surface and more heat is lost.
- Core temperature falls back towards the set point and the response then decreases: the response has reversed the original change.
- Internal: a change that originates inside the body, for example heat produced by respiration in contracting muscles / fever.
- External: a change in the surroundings, for example a fall in air temperature or swimming in cold water.
- Temperature rises by 0.6 °C during exercise because heat is produced by respiration in muscles faster than it is lost.
- Receptors detect the rise and the hypothalamus increases sweating and vasodilation.
- During rest less heat is produced, so heat loss exceeds heat production and the temperature returns towards 37 °C (negative feedback); it does not return exactly, showing fluctuation around the set point.
- Nervous: electrical impulses along neurones (chemical at synapses); endocrine: hormones secreted into and carried by the blood.
- Nervous responses are rapid and short-lived; hormonal responses are slower to start and often last longer.
- Nervous signals reach specific effectors; hormones affect any target cells that have the specific receptors.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement best describes homeostasis?
- Akeeping conditions of the internal environment within narrow limits around a set point
- Bkeeping every factor in the body at exactly the same value at all times
- Cchanging the internal environment to match the external environment
- Dremoving the waste products of metabolism from the body
Which of these is an effector in a homeostatic control system?
- Athe muscle in the walls of arterioles in the skin
- Bthe thermoreceptors in the hypothalamus
- Cthe osmoreceptors in the hypothalamus
- Dthe sensory neurones from the skin
A person sits in a sauna and their core temperature begins to rise. Which response is an example of negative feedback?
- Asweat glands secrete more sweat, so more heat is lost by evaporation
- Bskeletal muscles shiver, so more heat is produced by respiration
- Carterioles in the skin constrict, so less blood flows near the surface
- Dthe metabolic rate rises, so core temperature increases further
Which is a feature of coordination by the endocrine system rather than by the nervous system?
- Athe signal is carried in the blood and can affect any cell with a matching receptor
- Bthe signal travels as an electrical impulse along a neurone
- Cthe response is produced within a few milliseconds of the stimulus
- Dthe response stops almost as soon as the stimulus is removed
Why is it important for a mammal to keep its core temperature close to the optimum?
- Aenzymes work efficiently, so metabolic reactions continue at a suitable rate
- Bcells cannot take in oxygen by diffusion at any other temperature
- Call enzymes are denatured if the temperature falls by more than 1 °C
- Dhaemoglobin only carries oxygen at exactly 37 °C
The graph shows the core temperature of a swimmer who stays in cold water for 20 minutes and then gets out. Which statement is fully supported by the data?
- Acore temperature fell by 0.8 °C in the water and had not quite returned to 37.0 °C after 40 minutes
- Bcore temperature fell by 0.8 °C in the water and was back to exactly 37.0 °C after 30 minutes
- Ccore temperature fell steadily at the same rate throughout the 20 minutes in the water
- Dcore temperature began to rise while the swimmer was still in the cold water
During a fever, the hypothalamus raises the set point for core temperature from 37 °C to 39 °C. At the start of the fever, while core temperature is still 37 °C, which response is expected?
- Ashivering and vasoconstriction in the skin, because the body is below the new set point
- Bsweating and vasodilation in the skin, because the body is above the new set point
- Cno response, because core temperature has not changed
- Dsweating and shivering together, because the receptors receive mixed signals
Insulin release is controlled by negative feedback. Which observation would show that this system is working?
- Aas blood glucose falls back to normal after a meal, insulin secretion also falls
- Binsulin secretion keeps rising for as long as blood glucose is falling
- Cblood glucose keeps rising after insulin is secreted
- Dinsulin is secreted at the same rate whatever the blood glucose concentration
Quiz key
Homeostasis keeps factors such as core temperature, blood glucose and blood water potential within narrow limits, fluctuating slightly around a set point. Nothing is held exactly constant: negative feedback only acts once there is a change, so values oscillate a little. Matching the external environment is the opposite of homeostasis. Removing metabolic waste is excretion, which contributes to homeostasis but is not its definition.
Effectors are muscles or glands. Smooth muscle in arteriole walls contracts or relaxes to change blood flow through the skin, which alters heat loss. Thermoreceptors and osmoreceptors are receptors: they detect stimuli. Sensory neurones carry impulses from receptors to the coordinator; they are part of the coordination system, not effectors.
A rise in temperature must be met by a response that lowers it; increased sweating removes heat as the water evaporates. Shivering and vasoconstriction are responses to a fall in temperature, so here they would make the rise worse. A higher metabolic rate that raises the temperature further would be positive feedback, which amplifies a change instead of reversing it.
Endocrine glands secrete hormones into the blood, which carries them throughout the body; only cells with specific receptors respond. Impulses along neurones, responses in milliseconds and very short-lived effects are all features of nervous coordination. Hormonal responses are slower to start and often last longer, because the hormone remains in the blood until broken down.
Enzymes have an optimum temperature; below it reactions slow, and well above it enzymes are denatured. Keeping the core temperature steady keeps metabolism efficient. Diffusion of oxygen happens at any temperature, just at different rates. A small fall in temperature slows enzymes but does not denature them. Haemoglobin binds oxygen over a wide range of temperatures, although the dissociation curve shifts.
The trace falls from 37.0 °C to 36.2 °C (a fall of 0.8 °C) by 20 minutes and has reached only about 36.9 °C at 40 minutes, so it is still slightly below the start. At 30 minutes it is about 36.65 °C, not 37.0. The fall is not steady: the gradient is steeper between 5 and 15 minutes than near 20 minutes. The minimum occurs at 20 minutes, when the swimmer leaves the water, so the rise begins only after getting out.
Negative feedback works relative to the set point. With a set point of 39 °C, a core temperature of 37 °C is detected as too cold, so heat-gaining responses (shivering, vasoconstriction) are triggered, which is why people feel cold and shiver as a fever starts. Sweating and vasodilation happen when the temperature is above the set point. The temperature has not changed but the set point has, so there is a response. The hypothalamus coordinates one direction of response at a time.
The stimulus for insulin release is high blood glucose. Insulin lowers it, so the stimulus decreases and insulin secretion falls: the response reverses the change and then switches off. Insulin rising while glucose falls would overshoot into low blood glucose. Glucose rising despite insulin shows the response is not working. A constant rate would mean no feedback at all.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.