Demonstrate understanding of how an animal maintains a stable internal environment
What this domain tests
This domain assesses how an animal maintains a stable internal environment despite external change. You need to explain negative feedback, receptors, control centres, effectors, and specific mechanisms such as thermoregulation, osmoregulation and blood glucose regulation.
Core rules and concepts
- Homeostasis is maintenance of a relatively constant internal environment around a set point.
- Negative feedback reverses a change: a receptor detects a deviation, a control centre processes it, and effectors restore the set point.
- Thermoregulation: sweating and vasodilation cool the body; shivering and vasoconstriction warm it.
- Blood glucose: after a meal, beta cells in the pancreas secrete insulin, which increases glucose uptake and glycogen storage in liver and muscle. Between meals, alpha cells secrete glucagon, which promotes glycogen breakdown and gluconeogenesis.
- Osmoregulation: ADH from the pituitary increases water reabsorption in the kidney collecting ducts; aldosterone increases sodium reabsorption.
- The nephron filters blood and reabsorbs useful substances.
Worked example
After a carbohydrate meal, blood glucose rises. Pancreatic beta cells detect the rise and secrete insulin. Insulin binds to liver and muscle cells, increasing glucose uptake and converting glucose to glycogen. Blood glucose falls towards the set point. This is negative feedback because the response reverses the initial rise.
Traps that cost marks
Describing positive feedback as the main mechanism; confusing insulin and glucagon; saying ADH reduces urine volume without explaining water reabsorption; ignoring the receptor-control-effector sequence.