The Endocrine SystemAQA GCSE Biology: Revision notes
Section 1
What is the endocrine system and how does it work?
The endocrine system is a system of glands that secrete hormones directly into the blood. Unlike exocrine glands (which release substances through ducts), endocrine glands are ductless and release hormones that travel through the bloodstream to target cells throughout the body.
Hormones are chemical messengers that regulate and coordinate bodily functions over longer timescales than the nervous system. Once secreted, hormones circulate in the blood until they reach their target cells, which have specific receptors on their surface that recognise and bind to the hormone.
Think of hormones as postal letters: they're created at the gland (post office), released into the bloodstream (postal system), and only open and work when they reach the correct address (target cell with matching receptor).
Section 2
What are the main endocrine glands and which hormones do they produce?
The major endocrine glands and their hormones are:
| Gland | Location | Main Hormones | Function |
|---|---|---|---|
| Pituitary | Base of brain | FSH, LH, ADH, growth hormone | Controls other glands; regulates growth, water balance, reproduction |
| Thyroid | Neck | Thyroxine (T4) | Regulates metabolic rate and growth |
| Adrenal | On top of kidneys | Adrenaline, cortisol | Fight-or-flight response; stress response |
| Pancreas | Below stomach | Insulin, glucagon | Regulate blood glucose levels |
| Ovaries | Female pelvis | Oestrogen, progesterone | Regulate menstrual cycle; develop female sexual characteristics |
| Testes | Male scrotum | Testosterone | Develop male sexual characteristics; produce sperm |
Each gland produces specific hormones suited to its role in maintaining homeostasis and coordinating body functions.
Examiners expect you to know the location of each gland and the specific hormones it produces. Learn the table format – you may be asked to match glands to hormones or describe their functions.
Students often confuse insulin and glucagon – remember: insulin lowers blood glucose (and is produced when glucose is in the blood), whilst glucagon raises it.
Section 3
How do nervous and hormonal coordination differ?
The nervous system and endocrine system are both forms of coordination, but they differ significantly in how they operate:
| Feature | Nervous System | Hormonal System |
|---|---|---|
| Speed | Very fast (milliseconds) | Slow (seconds to minutes) |
| Duration | Short-lasting; effects end quickly when signal stops | Long-lasting; effects persist until hormone is broken down |
| Specificity | Highly specific; signals target precise neurons and cells | Less specific; hormones affect all cells with matching receptors |
| Type of signal | Electrical impulses; neurotransmitters | Chemical messengers (hormones) |
| Transmission | Through nerves and synapses | Through the bloodstream |
Summary: The nervous system is fast and precise, ideal for rapid responses (e.g. reflexes). The hormonal system is slower but longer-lasting, ideal for maintaining steady states and coordinating slow metabolic processes (e.g. growth, reproduction).
Examiners often ask students to compare these systems using the three key criteria: speed, duration, and specificity. Use the table structure to organise your answer clearly.
When you touch a hot surface: the nervous system causes your hand to jerk away in milliseconds (fast, short-lasting); the hormonal system then releases adrenaline, which keeps your heart rate elevated for minutes (slow, long-lasting).
Section 4
What is the pituitary gland's role as the 'master gland'?
The pituitary gland is often called the 'master gland' because it controls many other endocrine glands in the body through the hormones it secretes.
The pituitary gland has two parts:
- Anterior pituitary (front): Produces hormones that regulate other glands, including FSH and LH (which control the ovaries and testes) and growth hormone
- Posterior pituitary (back): Produces ADH, which regulates water reabsorption in the kidneys
How it controls other glands:
- The pituitary detects conditions in the body (often via signals from the hypothalamus, which sits just above it)
- It secretes hormones like FSH, LH, and thyroid-stimulating hormone (TSH)
- These hormones travel in the blood and stimulate other endocrine glands (ovaries, testes, thyroid) to produce their own hormones
- The other glands' hormones feed back to the pituitary, which adjusts its own hormone secretion – this is called negative feedback
This hierarchical control system ensures that hormone levels remain in balance and that the body responds appropriately to changing conditions.
When explaining pituitary control, examiners want to see you describe the chain of command: hypothalamus → pituitary → target gland → hormone release. Mention negative feedback to show understanding of how balance is maintained.
The pituitary is like the managing director of a company: it receives information and sends out instructions to department heads (other glands), who then carry out the work (produce their own hormones).
Section 5
How do hormonal effects relate to target cells?
Hormones only affect target cells – those with the correct receptors on their surface. This provides specificity to hormonal control despite hormones being distributed throughout the body via the bloodstream.
Key principle: A hormone will travel in the blood past many cells, but will only have an effect on cells that have the matching receptor protein. Think of it as a lock-and-key mechanism:
- The hormone is the key
- The receptor on the target cell is the lock
- Only when they match does the hormone bind and activate the cell
Examples:
- Insulin is produced by the pancreas and circulates in the blood, but only muscle, fat, and liver cells have insulin receptors, so only these cells respond by taking up glucose
- Testosterone circulates throughout the body but only affects cells in male reproductive tissues, skin, and muscle that have testosterone receptors
- Thyroxine from the thyroid affects metabolic rate in many cells because these cells have thyroxine receptors
This receptor-based mechanism explains why the endocrine system, though slower than the nervous system, can still achieve a degree of specificity in its effects.
Use the lock-and-key analogy in exam answers when explaining why hormones only affect certain cells. This demonstrates clear understanding of the receptor mechanism.
Must Know
- The endocrine system consists of ductless glands that secrete hormones directly into the blood; hormones are chemical messengers that regulate body functions
- The main endocrine glands are: pituitary (controls other glands), thyroid (thyroxine), adrenal (adrenaline), pancreas (insulin/glucagon), ovaries (oestrogen/progesterone), and testes (testosterone)
- Nervous system is fast and specific; hormonal system is slow and long-lasting – use this comparison to answer questions about coordination methods
- The pituitary gland is the 'master gland' because it controls other endocrine glands via hormone secretion; it receives signals from the hypothalamus and uses negative feedback to maintain hormone balance
- Hormones only affect target cells that have matching receptors (lock-and-key mechanism); this provides specificity despite hormones being distributed via the bloodstream
- Learn the locations and hormones of all six main glands for identification questions
That's the notes covered.
Carry on to the next subtopic.