Maintaining the Internal Environment
Core concepts
Homeostasis is the maintenance of a relatively stable internal environment despite external change. Candidates must understand negative feedback (a deviation triggers a response that reverses it) and positive feedback (amplifies change; rare, e.g. childbirth). Know receptors, the control centre, effectors and the response loop.
Study thermoregulation (hypothalamus; vasodilation, sweating, shivering, vasoconstriction), osmoregulation (kidney nephrons, ADH, aldosterone), and blood glucose regulation (insulin and glucagon from the pancreas, glycogen storage in liver and muscle). Understand how the nervous and endocrine systems coordinate, and the difference between hormonal (slower, longer-lasting) and nervous (fast, brief) control. Gas exchange and transport systems - lungs, alveoli, heart and blood vessels - illustrate maintaining oxygen and nutrient supply. Consider the role of the liver in detoxification and homeostasis.
Traps
- Negative feedback reverses a change; positive feedback amplifies it - mixing these up is a common error.
- Insulin lowers blood glucose; glucagon raises it. Check the direction every time.
- ADH increases water reabsorption, concentrating urine - it does not add water.
- Distinguish the stimulus, receptor, control centre, effector and response in a feedback loop.
Worked example
After a sugary meal, blood glucose rises → pancreatic beta cells detect it → insulin released → cells take up glucose and liver stores glycogen → glucose falls → insulin secretion reduces. Rule of thumb: always name the receptor, the hormone and the effector when describing a feedback loop.