# Homeostasis

> CIE A-Level Biology · CIE 9700 (2022-2024)
> Source: https://www.owlsprep.com/study/cie-9700-u15-overview/
> Weight: n/a

This unit explores how organisms maintain a stable internal environment despite external and internal changes, a fundamental requirement for cellular function and survival. We examine key homeostatic regulatory mechanisms in mammals.

**Prerequisites:** [Cell membrane structure and function](https://www.owlsprep.com/study/cie-9700-u04-cell-membranes/); [Hormonal cell signalling](https://www.owlsprep.com/study/cie-9700-u14-coordination-response/)

## Learning objectives

- Explain core homeostatic principles and the role of negative feedback in maintaining a stable internal environment
- Describe mammalian regulatory mechanisms for body temperature, blood water potential, and blood glucose concentration
- Relate mammalian kidney structure to its roles in osmoregulation and excretion
- Predict effects of hormonal imbalances on homeostatic function

## Unit at a Glance

All homeostatic systems follow the same core pattern of receptors, coordination centres, effectors, and negative feedback to reverse deviations from a set point. This unit progresses from general foundational principles to specific, clinically relevant regulatory systems, ending with a detailed look at the kidney, the primary organ for osmoregulation.

Understanding homeostatic mechanisms helps explain how disruptions like diabetes or dehydration cause illness, and forms a critical foundation for advanced study of human and animal physiology.

This unit is split into 5 connected sub-topics that progress from general principles to specific organ systems:
- [Homeostasis principles](https://www.owlsprep.com/study/cie-9700-u15-homeostasis-principles/) — Introduces core concepts including set points, receptors, effectors, and negative vs positive feedback.
- [Temperature regulation](https://www.owlsprep.com/study/cie-9700-u15-temperature-regulation/) — Explains thermoregulation in endotherms, including physiological mechanisms of heat gain and loss.
- [Osmoregulation](https://www.owlsprep.com/study/cie-9700-u15-osmoregulation/) — Covers control of blood water potential by the hypothalamus and anti-diuretic hormone (ADH).
- [Blood glucose regulation](https://www.owlsprep.com/study/cie-9700-u15-blood-glucose-regulation/) — Explains the roles of insulin, glucagon, and the liver in maintaining stable blood glucose levels.
- [Kidney structure and function](https://www.owlsprep.com/study/cie-9700-u15-kidney-structure-and-function/) — Breaks down kidney anatomy, ultrafiltration, selective reabsorption, and osmoregulatory function.

## Common pitfalls

- **Wrong:** Confusing positive and negative feedback as the main homeostatic mechanism
  - Why it fails: Positive feedback amplifies change away from the set point rather than reversing it
  - Correct: Remember that nearly all homeostatic systems rely on negative feedback to maintain stability
- **Wrong:** Mixing up the effect of ADH on kidney water reabsorption
  - Why it fails: Many students incorrectly recall ADH decreases water reabsorption
  - Correct: ADH increases permeability of collecting ducts to water, increasing reabsorption into the blood
- **Wrong:** Confusing the roles of insulin and glucagon
  - Why it fails: Students often mix which hormone raises and lowers blood glucose concentration
  - Correct: Insulin lowers blood glucose; glucagon raises blood glucose

## Cheatsheet

| Concept / Term | Key Summary |
| --- | --- |
| Negative feedback | Core homeostatic mechanism that reverses deviation from a set point |
| Positive feedback | Amplifies change away from set point; rare in homeostasis (e.g. labour contractions) |
| ADH | Released by posterior pituitary; increases water reabsorption in kidney collecting ducts |
| Insulin | Released by pancreatic β-cells; lowers blood glucose concentration |
| Glucagon | Released by pancreatic α-cells; raises blood glucose concentration |
| Endotherm | Organism that regulates body temperature internally via physiological mechanisms |
| Ultrafiltration | Filtration of small molecules from blood into Bowman's capsule under high pressure |
| Osmoregulation | Control of blood water potential to maintain stable cell water content |

## What's next

Start with the first sub-topic of this unit to master the core foundational principles of homeostasis, which all the specific regulatory mechanisms covered later in the unit rely on. Once you complete all sub-topics in Homeostasis, you will progress to the next unit on Inheritance, where you will explore genetic variation and patterns of inheritance in eukaryotes.

- [Homeostasis principles](https://www.owlsprep.com/study/cie-9700-u15-homeostasis-principles/)
- [Osmoregulation](https://www.owlsprep.com/study/cie-9700-u15-osmoregulation/)

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