B15: The Endocrine System
Hormones and the endocrine system
📋 Key Concepts
The endocrine system: A system of glands that secrete hormones directly into the bloodstream. Hormones are chemical messengers that travel in the blood to target organs where they produce an effect.
Key Terms
- endocrine system
- hormones
- pituitary gland
- thyroid gland
- adrenal gland
- pancreas
- insulin
- glucagon
- negative feedback
- Type 1 diabetes
- Type 2 diabetes
📝 Key Endocrine Glands
| Gland | Hormone(s) Produced | Function |
|---|---|---|
| Pituitary gland | ADH, FSH, LH, growth hormone | "Master gland" — controls many other glands and body functions. Produces ADH to control water balance. |
| Thyroid gland | Thyroxine | Controls the body's basal metabolic rate (how fast chemical reactions happen). Also important for growth and development. |
| Adrenal gland | Adrenaline | Prepares the body for "fight or flight" — increases heart rate, breathing rate, and blood flow to muscles. Released in response to stress/danger. |
| Pancreas | Insulin and glucagon | Regulates blood glucose concentration. Insulin lowers blood glucose; glucagon raises it. |
| Ovaries (female) | Oestrogen, progesterone | Control the menstrual cycle and development of female secondary sexual characteristics. |
| Testes (male) | Testosterone | Controls development of male secondary sexual characteristics and sperm production. |
Example 1
A person is in a dangerous situation. Name the hormone that is released and describe its effects on the body.
Solution:
Adrenaline is released by the adrenal glands. Its effects include: increased heart rate, increased breathing rate, increased blood flow to muscles, dilation of pupils, and increased conversion of glycogen to glucose in the liver. All of these prepare the body for "fight or flight".
📝 Hormones vs Nervous System
| Feature | Nervous System | Endocrine System (Hormones) |
|---|---|---|
| Signal type | Electrical impulses | Chemical (hormones) |
| Speed of action | Very fast | Slower |
| Duration of effect | Short-lived | Long-lasting |
| Method of travel | Along neurones (nerves) | In the bloodstream |
| Area of effect | Targeted (specific area) | Widespread (can affect many organs) |
| Example | Reflex action (pulling hand from heat) | Adrenaline causing increased heart rate |
Example 2
Explain why the body uses hormones rather than the nervous system to control the menstrual cycle.
Solution:
The menstrual cycle is a long-lasting process (approximately 28 days) that needs sustained, gradual changes in hormone levels. Hormones are long-lasting and widespread, making them suitable. The nervous system produces short-lived, rapid, targeted responses, which would not be appropriate for the slow, continuous regulation needed for the menstrual cycle.
📝 Negative Feedback
Negative feedback: A mechanism that maintains stable conditions (homeostasis). When a level rises above normal, the body responds to bring it back down. When a level falls below normal, the body responds to bring it back up. This keeps conditions within a narrow range.
How negative feedback works:
- A change from the normal level is detected
- The body responds to reverse the change
- The response reduces or counteracts the original change
- When the level returns to normal, the corrective mechanism is switched off
Example 3
Explain how negative feedback controls the level of thyroxine in the blood.
Solution:
If thyroxine levels rise above normal, the pituitary gland detects this and reduces the production of thyroid-stimulating hormone (TSH). With less TSH, the thyroid gland produces less thyroxine, and levels fall back to normal.
If thyroxine levels fall below normal, the pituitary gland releases more TSH, which stimulates the thyroid to produce more thyroxine, bringing levels back to normal.
📝 Blood Glucose Regulation
Blood glucose concentration must be kept within a narrow range. The pancreas monitors blood glucose levels and responds by releasing insulin or glucagon.
When blood glucose is TOO HIGH (after eating):
- The pancreas detects the rise and releases insulin into the blood
- Insulin causes the liver to convert excess glucose into glycogen for storage
- Insulin also causes body cells to take up more glucose from the blood
- Blood glucose concentration decreases back to normal
When blood glucose is TOO LOW (between meals or during exercise):
- The pancreas detects the fall and releases glucagon into the blood
- Glucagon causes the liver to convert stored glycogen back into glucose
- Glucose is released into the blood
- Blood glucose concentration increases back to normal
Blood glucose regulation:
High blood glucose → Pancreas releases insulin → Liver converts glucose → glycogen → Blood glucose falls
Low blood glucose → Pancreas releases glucagon → Liver converts glycogen → glucose → Blood glucose rises
High blood glucose → Pancreas releases insulin → Liver converts glucose → glycogen → Blood glucose falls
Low blood glucose → Pancreas releases glucagon → Liver converts glycogen → glucose → Blood glucose rises
Example 4
A person eats a large meal. Explain how their blood glucose concentration is returned to normal.
Solution:
After eating, blood glucose concentration rises above normal. The pancreas detects this increase and releases insulin into the blood. Insulin stimulates the liver to convert excess glucose into glycogen for storage. Insulin also causes body cells to take up more glucose from the blood for respiration. As a result, blood glucose concentration falls back to the normal range.
📝 Diabetes
Type 1 diabetes: The pancreas cannot produce enough (or any) insulin. Blood glucose concentration rises dangerously after eating. Treated by injecting insulin before meals and monitoring blood glucose. The cause is not related to lifestyle.
Type 2 diabetes: The body cells no longer respond properly to insulin (insulin resistance), or the pancreas does not produce enough insulin. Often linked to obesity and lifestyle factors. Treated by controlling diet, exercise, and sometimes medication or insulin injections.
| Feature | Type 1 Diabetes | Type 2 Diabetes |
|---|---|---|
| Cause | Pancreas produces little or no insulin | Body cells become resistant to insulin, or insufficient insulin produced |
| Typical age of onset | Usually childhood or teenage years | Usually adulthood (increasingly in younger people) |
| Risk factors | Not lifestyle-related (autoimmune) | Obesity, poor diet, lack of exercise, genetics |
| Treatment | Insulin injections | Diet, exercise, medication, sometimes insulin |
❓ Practice Questions
Q1: Name the gland that produces insulin and glucagon, and explain the effect of each hormone on blood glucose concentration.
Q2: Compare the nervous system and the endocrine system in terms of speed, duration and method of signal transmission.
Q3: Explain what negative feedback is and how it helps maintain blood glucose concentration within a normal range.
Q4: Describe the differences between Type 1 and Type 2 diabetes, including their causes and treatments.
Q5: A person exercises vigorously. Explain how glucagon helps maintain their blood glucose concentration during exercise.
✅ Answers
- The pancreas produces both insulin and glucagon. Insulin lowers blood glucose by causing the liver to convert glucose to glycogen and stimulating cells to take up glucose. Glucagon raises blood glucose by causing the liver to convert glycogen back to glucose, which is released into the blood.
- Nervous system: fast, short-lived, electrical impulses travel along neurones, targeted effect. Endocrine system: slower, long-lasting, chemical hormones travel in the bloodstream, widespread effect.
- Negative feedback is a mechanism that reverses a change from the normal level. When blood glucose rises too high, insulin is released to bring it down. When blood glucose falls too low, glucagon is released to bring it up. This keeps blood glucose within a narrow range. Once the level returns to normal, the corrective mechanism reduces.
- Type 1: pancreas produces little or no insulin (autoimmune), usually starts in childhood, treated with insulin injections, not linked to lifestyle. Type 2: cells become resistant to insulin or pancreas produces too little, usually starts in adulthood, linked to obesity/lifestyle, treated with diet, exercise and medication (sometimes insulin).
- During exercise, muscles use glucose for respiration, so blood glucose falls. The pancreas detects the low blood glucose and releases glucagon. Glucagon causes the liver to convert stored glycogen back into glucose, which is released into the blood. This raises blood glucose back to normal.
🎯 Exam Tips
- Learn the six key glands and their hormones — this is a common recall question
- The pituitary gland is the "master gland" — it controls many other glands
- Insulin and glucagon have OPPOSITE effects — insulin lowers glucose, glucagon raises glucose
- Both insulin and glucagon are produced by the PANCREAS
- The LIVER is where glucose ↔ glycogen conversion happens (not the pancreas)
- Type 1 = no insulin production; Type 2 = insulin resistance — don't confuse these
- When explaining negative feedback, always describe what happens when the level is too high AND too low
- Negative feedback REVERSES the change — remember this key idea
🔢 Maths Skills
Mathematical Skills
Interpreting hormone graphs: When reading hormone level graphs for the menstrual cycle or blood glucose regulation, identify peaks, troughs and the timing of changes. Read values from the y-axis at specific time points on the x-axis. Calculate the change in concentration by subtracting the initial value from the peak value. Compare the shapes of different hormone curves — note where one hormone rises as another falls (e.g. oestrogen inhibiting FSH shows as an inverse relationship on the graph).
⚠️ Common Misconceptions
Watch Out!
1. Hormones act instantly. Wrong: hormones produce an immediate response, like nerve impulses. Correct: hormones travel in the blood and act more slowly than nerve impulses — they take time to reach target organs and produce effects that are longer-lasting but not instant.
2. Insulin cures diabetes. Wrong: insulin injections cure diabetes permanently. Correct: insulin manages blood glucose levels in Type 1 diabetes but does not cure the condition — treatment must continue for life.
✍️ 6-Mark Question
Extended Answer
6 marks: Compare the nervous and endocrine systems.
The nervous system uses electrical impulses that travel along neurones to carry information quickly to specific target areas, producing rapid but short-lived responses. The endocrine system uses chemical messengers (hormones) that travel in the bloodstream to target organs, producing slower but longer-lasting and more widespread effects. For example, a reflex action (nervous) is very fast and localised, such as pulling your hand away from heat. In contrast, adrenaline (endocrine) takes longer to have an effect but causes widespread changes including increased heart rate and breathing rate that persist for some time. Both systems work together to coordinate the body's responses, but the nervous system is better for rapid, targeted responses while the endocrine system is better for slow, sustained regulation such as controlling the menstrual cycle or blood glucose.
Mark scheme: 1 mark — electrical vs chemical; 1 mark — fast vs slow; 1 mark — short-lived vs long-lasting; 1 mark — targeted vs widespread; 1 mark — correct named example for each; 1 mark — conclusion comparing when each is more appropriate.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A patient's blood glucose was monitored over 6 hours. At 0 hours, glucose was 5.0 mmol/L (normal). After eating a sugary meal at 1 hour, glucose rose to 8.5 mmol/L. By 2 hours it had fallen to 6.2 mmol/L and by 3 hours it was back to 5.0 mmol/L. A second patient (with Type 1 diabetes) ate the same meal: their glucose rose from 7.0 mmol/L to 14.0 mmol/L at 1 hour and was still 11.5 mmol/L at 3 hours. Explain the differences between the two patients' data, referencing the hormones involved and the mechanisms of blood glucose regulation.
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🎬 Video Resources
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