# Human Physiology — Hormones and Homeostasis (IB)
Hormones coordinate slower, longer-lasting responses compared to the nervous system. IB Biology (Topics 6.5–6.6) covers the endocrine system, blood glucose regulation, the roles of thyroxine and leptin, and the principles of homeostatic control through negative feedback.
1. Endocrine System vs Nervous System
| Feature | Endocrine | Nervous |
|---|---|---|
| Signal type | Chemical (hormones) | Electrical (nerve impulses) |
| Transmission | Blood | Neurons |
| Speed | Slower (seconds–hours) | Fast (milliseconds) |
| Duration | Long-lasting | Short-lived |
| Target | Widespread (any cell with receptor) | Specific (particular muscle/gland) |
2. Blood Glucose Regulation
Maintaining blood glucose within narrow limits is essential — glucose is the primary fuel for cells, and extremes can be dangerous.
When Blood Glucose Is TOO HIGH
- Beta (β) cells of the islets of Langerhans in the pancreas detect high glucose
- They secrete insulin
- Insulin stimulates:
- Uptake of glucose into cells (especially liver and muscle) via GLUT4 transporters
- Conversion of glucose to glycogen (glycogenesis) in the liver
- Increased cellular respiration
- Blood glucose falls back to normal
When Blood Glucose Is TOO LOW
- Alpha (α) cells of the islets of Langerhans detect low glucose
- They secrete glucagon
- Glucagon stimulates:
- Breakdown of glycogen to glucose (glycogenolysis) in the liver
- Release of glucose into the blood
- Blood glucose rises back to normal
Negative Feedback
This is an example of negative feedback — a deviation from the set point triggers responses that reverse the change.
Type I vs Type II Diabetes
| Feature | Type I | Type II |
|---|---|---|
| Cause | Autoimmune destruction of β cells | Insulin resistance / insufficient insulin |
| Onset | Childhood | Usually adulthood |
| Treatment | Insulin injections | Diet, exercise, medication |
| Insulin produced? | Little or none | Some (but cells don't respond) |
3. Thyroxine
- Produced by the thyroid gland
- Regulates metabolic rate (the speed of chemical reactions in cells)
- Contains iodine — iodine deficiency leads to goitre (enlarged thyroid)
- Controlled by negative feedback involving the hypothalamus and pituitary:
- Hypothalamus releases TRH (thyrotropin-releasing hormone)
- TRH stimulates the anterior pituitary to release TSH (thyroid-stimulating hormone)
- TSH stimulates the thyroid to produce thyroxine
- High thyroxine levels inhibit TRH and TSH release → less thyroxine produced
- Low thyroxine levels → less inhibition → more TRH and TSH → more thyroxine
Thyroxine and Negative Feedback — An Example
4. Leptin
- A hormone secreted by adipose (fat) cells
- Amount of leptin correlates with the amount of fat stored
- Acts on the hypothalamus to:
- Reduce appetite (inhibit hunger)
- Increase energy expenditure
- Plays a role in long-term energy balance and body weight regulation
- Obesity: some obese individuals may have leptin resistance (high leptin levels but the hypothalamus doesn't respond normally)
- Leptin deficiency (rare genetic condition) → extreme hunger and obesity
5. Homeostasis
Homeostasis is the maintenance of a relatively constant internal environment despite changes in external conditions.
Components of a Homeostatic Control System
- Receptor/Sensor: detects a change from the set point (stimulus)
- Control centre: processes information and initiates a response (e.g., hypothalamus, medulla)
- Effector: carries out the response (e.g., muscle, gland)
Negative Feedback
- The response opposes the change, returning the variable to the set point
- Examples: blood glucose regulation, thermoregulation, thyroxine levels
Positive Feedback
- The response amplifies the change (moves further from set point)
- Less common; used in specific situations:
- Childbirth: oxytocin stimulates uterine contractions → more oxytocin released → stronger contractions
- Blood clotting: activated platelets attract more platelets
- Action potential: Na⁺ influx opens more Na⁺ channels
Worked Example
Question: Explain how blood glucose levels are regulated after eating a carbohydrate-rich meal. (4 marks)
Solution: After eating, carbohydrates are digested and glucose is absorbed into the blood, causing blood glucose levels to rise. The β cells of the islets of Langerhans in the pancreas detect this increase and secrete insulin into the blood. Insulin binds to receptors on target cells (liver, muscle, fat cells) and stimulates the uptake of glucose and its conversion to glycogen (glycogenesis) for storage. This causes blood glucose levels to fall back to normal. Once normal levels are restored, insulin secretion decreases — this is an example of negative feedback.
Practice Questions
- Compare the endocrine and nervous systems. (3 marks)
- Explain the roles of insulin and glucagon in blood glucose regulation. (4 marks)
- Describe how thyroxine secretion is controlled by negative feedback. (4 marks)
- Explain the role of leptin in regulating body weight. (3 marks)
- Distinguish between negative and positive feedback, giving one example of each. (4 marks)
Answers
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Summary
- The endocrine system uses hormones in the blood for slower, longer-lasting responses.
- Blood glucose: insulin (lowers) and glucagon (raises) — negative feedback via pancreatic islet cells.
- Thyroxine: controls metabolic rate; regulated by TRH → TSH → thyroxine negative feedback loop.
- Leptin: from fat cells; reduces appetite; leptin resistance contributes to obesity.
- Homeostasis: maintained by negative feedback (opposes change); positive feedback amplifies change (e.g., childbirth).