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This lesson introduces the endocrine system — a network of glands that produce hormones to coordinate and control processes throughout the body. Understanding the difference between the nervous system and the endocrine system is a key requirement of Edexcel GCSE Biology (1BI0) Topic 7.
The endocrine system is a collection of glands that produce chemicals called hormones. These hormones are released directly into the bloodstream and are carried to target organs, where they produce an effect.
Key definitions:
Exam Tip: If asked to define a hormone, always include three elements: (1) it is a chemical messenger, (2) it is carried in the blood, and (3) it acts on a target organ. Missing any of these can lose marks.
The body has two major communication systems. The nervous system uses electrical impulses along neurones, while the endocrine system uses chemical hormones in the blood. Both coordinate body functions, but they work in very different ways.
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Signal type | Electrical impulses | Chemical hormones |
| Transmission route | Along neurones (nerve cells) | Via the bloodstream |
| Speed | Very fast (milliseconds) | Slower (seconds to minutes to reach target) |
| Duration of effect | Short-lived; stops when impulse stops | Longer-lasting; can persist for hours, days or longer |
| Area affected | Very precise; targets a specific effector | Can be widespread; affects any organ with the right receptors |
| Example | Pulling hand away from a hot object (reflex) | Growth, puberty, blood glucose regulation |
Exam Tip: A common question asks you to compare the nervous and endocrine systems. Learn this table thoroughly — use the mnemonic STDA (Signal, Transmission, Duration, Area) to remember the four key comparison points.
The pituitary gland is a small, pea-sized gland located at the base of the brain. It is often called the "master gland" because it produces hormones that control many other glands in the body.
Hormones produced by the pituitary gland include:
| Hormone | Target / Function |
|---|---|
| FSH (follicle-stimulating hormone) | Stimulates egg maturation in the ovaries; stimulates oestrogen production |
| LH (luteinising hormone) | Triggers ovulation (release of an egg from the ovary) |
| ADH (antidiuretic hormone) | Controls water reabsorption in the kidneys |
| Growth hormone | Stimulates growth of bones and tissues |
| TSH (thyroid-stimulating hormone) | Stimulates the thyroid gland to produce thyroxine |
The endocrine system contains many glands located throughout the body. You need to know the location and function of each for the Edexcel specification.
| Gland | Location | Hormone(s) Produced | Key Function |
|---|---|---|---|
| Pituitary gland | Base of the brain | FSH, LH, ADH, growth hormone, TSH | Controls other glands; master gland |
| Thyroid gland | In the neck, around the trachea | Thyroxine | Controls the basal metabolic rate (the speed of chemical reactions in the body) |
| Adrenal glands | On top of each kidney | Adrenaline | Prepares the body for fight or flight |
| Pancreas | Below the stomach | Insulin and glucagon | Regulates blood glucose concentration |
| Ovaries (females) | In the pelvic region | Oestrogen and progesterone | Control the menstrual cycle and female secondary sexual characteristics |
| Testes (males) | In the scrotum | Testosterone | Controls male secondary sexual characteristics (e.g. deepening voice, muscle development, body hair) |
When a gland releases a hormone:
Because hormones travel in the blood, they take longer to act than nerve impulses. However, their effects tend to last much longer.
Exam Tip: Remember that hormones are not "used up" instantly. They circulate until they are broken down by the liver, which is why their effects can last for hours or even longer.
Imagine the body from top to bottom. The key glands are positioned as follows:
Exam Tip: In the exam you may be asked to label glands on a diagram of the human body. Make sure you can identify each gland by its location. A common mistake is confusing the adrenal glands (on the kidneys) with the kidneys themselves.
| Term | Definition |
|---|---|
| Endocrine system | A collection of glands that produce hormones |
| Hormone | A chemical messenger carried in the blood to a target organ |
| Target organ | The organ where a hormone has its effect |
| Pituitary gland | The master gland at the base of the brain |
| Gland | An organ that produces and releases hormones or other substances |