What Are Hormones?
Definition
Hormone
A chemical messenger released by glands that travels in the bloodstream to target organs, where it produces a specific response.
- Hormones are chemical messengers released by endocrine glands.
- They travel through the bloodstream to reach specific target cells in different organs.
- Only cells with the correct receptors respond, which ensures each hormone triggers the right effect.
- Hormones coordinate slow, long-lasting changes, such as growth, metabolism, and internal balance.
Definition
Receptor
A molecule on a target cell that detects and responds to a specific hormone.
Hint
Unlike nerves, which act quickly and locally, hormones regulate processes that need sustained control such as growth, metabolism, and blood composition.
Why Does the Body Need Hormones?
- Hormones ensure different organ systems work together smoothly.
- They adjust processes such as blood glucose levels, water balance, growth, and reproduction.
- Because hormones travel in the blood, one signal can coordinate responses across the whole body.
Example
- Insulin lowers blood glucose.
- Adrenaline prepares the body for sudden action.
- ADH controls water reabsorption in the kidneys.
What Is the Endocrine System?
Definition
Endocrine gland
A gland that makes and releases hormones directly into the bloodstream.
- The endocrine system is the network of glands that produce and release hormones.
- Major endocrine glands:
- Pituitary gland: regulates many other glands.; often called the “master gland.”
- Pancreas: controls blood glucose using insulin and glucagon.
- Adrenal glands: release adrenaline during stress.
- Thyroid gland: controls metabolic rate.
- Ovaries/testes: produce sex hormones for reproduction and development.
How Does the Body Use Hormones to Maintain Homeostasis?
- Hormones often work through negative feedback, a system that reverses changes to keep conditions stable.
- When a factor moves outside a safe range, hormones are released to bring it back to normal.
- When the factor returns to the correct level, hormone release decreases.
Example
- Just as we covered in homeostasis, blood glucose levels are a good example of negative feedback:
- High glucose → insulin released → glucose stored → levels return to normal
- Low glucose → glucagon released → glucose released from liver → levels rise
What Does Adrenaline Do?
- Adrenaline prepares the body for quick action during stress ("fight or flight").
- It increases heart rate, widens air passages, and releases stored glucose for immediate energy.
- These effects act quickly but last only a short time.
Common Mistake
- Adrenaline does not control long-term stress.
- It is a short-term emergency hormone.
How Do Hormones Control Growth and Development?
- Growth hormone stimulates cell division and protein synthesis.
- Sex hormones (oestrogen and testosterone) trigger puberty and the development of secondary sexual characteristics.
- These changes occur gradually because hormones act over long periods.
What Are the Key Differences Between Nerves and Hormones?
| Feature | Nervous System | Endocrine (Hormonal) System |
|---|---|---|
| Type of signal | Electrical impulses | Chemical signals (hormones) in the bloodstream |
| Speed | Very fast | Slower |
| Duration | Short-lasting | Long-lasting |
| Response nature | Precise and localized | Widespread but only affects target cells |
| Typical function | Immediate actions and rapid responses | Regulation of long-term processes and homeostasis |
Self review
- What makes a hormone affect only certain cells?
- How do insulin and glucagon act as a negative feedback pair?
- Why is ADH essential for water balance?
- Why are hormonal responses slower than nervous responses?
- How does adrenaline prepare the body for rapid action?
