Hello Future Biologists! Welcome to Hormones!
This chapter is all about how your body coordinates its actions using chemical messengers. We have already looked at the super-fast nervous system (like sending an instant text message). Now we will study the slower, but longer-lasting, method of control: the hormonal system!
Don't worry if the names sound complicated—we will break down these biological text messages one by one. Understanding hormones is key to understanding how your body maintains a stable internal environment (a concept called Homeostasis).
1. Defining Hormones and the Endocrine System (Core 1)
What Exactly is a Hormone?
Imagine you need to send a message from your school office (the gland) to a specific classroom (the target organ). You could shout (fast nervous system), or you could send a letter through the postal service (slower hormonal system).
A Hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
Here are the four key characteristics of hormones you must remember:
1. They are Chemical substances (often proteins or steroids).
2. They are produced by specialised structures called Endocrine Glands.
3. They are transported around the body via the Bloodstream.
4. They only act on specific cells or organs, known as Target Organs.
Quick Review: Endocrine Glands
Glands that produce hormones are called endocrine glands. They are sometimes called ductless glands because they secrete the hormones directly into the blood, rather than through a tube (duct).
Key Takeaway: Hormones are blood-borne chemical messengers that cause slower, but more lasting, changes in specific target organs.
2. Important Endocrine Glands and Their Hormones (Core 2, Supplement 5)
The syllabus requires you to know four specific glands and the hormones they secrete.
Glands, Hormones, and General Roles:
1. Adrenal Glands (Located above the kidneys)
• Hormone: Adrenaline
• Role: Prepares the body for immediate action ("fight or flight").
2. Pancreas (Near the stomach)
• Hormone: Insulin and Glucagon (Supplement 5)
• Role: Controls the concentration of sugar (glucose) in the blood.
3. Testes (Male reproductive organs)
• Hormone: Testosterone
• Role: Development and regulation of male secondary sexual characteristics (e.g., deepening voice, body hair).
4. Ovaries (Female reproductive organs)
• Hormone: Oestrogen
• Role: Development and regulation of female secondary sexual characteristics and control of the menstrual cycle.
Did you know? The pituitary gland, located in the brain, is often called the "master gland" because it produces hormones that control many other endocrine glands!
Key Takeaway: The specific glands we study—Adrenal, Pancreas, Testes, and Ovaries—are responsible for secreting specific hormones (Adrenaline, Insulin/Glucagon, Testosterone, Oestrogen) into the blood.
3. Adrenaline: The 'Fight or Flight' Hormone (Core 3, Supplement 6)
Adrenaline (sometimes called epinephrine) is a fantastic example of a hormone that causes rapid, but short-term, changes when your body needs to react immediately to a stressful or dangerous situation.
The Adrenaline Rush: What Happens?
When you are suddenly frightened (e.g., a car swerves in front of you) or excited, the nervous system sends an impulse to the adrenal glands. They quickly secrete Adrenaline into the blood. This prepares your body to either fight the danger or flee from it.
The role of adrenaline is to control metabolic activity by directing resources (like oxygen and glucose) to the muscles and brain, giving you an immediate energy boost.
Specific Effects of Adrenaline:
Adrenaline causes several physical changes, limited to:
• Increased Heart Rate: The heart beats faster to pump blood (containing oxygen and glucose) more quickly to the muscles. (Core 3b, Supplement 6b)
• Increased Breathing Rate: You breathe faster and deeper to increase the intake of oxygen, which is needed for respiration to release energy. (Core 3a)
• Increased Pupil Diameter: The pupils dilate (get wider) allowing more light to enter the eyes, improving vision. (Core 3c)
• Increased Blood Glucose Concentration: Adrenaline stimulates the liver to convert stored glycogen back into glucose and release it into the blood, providing fuel for muscle contraction. (Supplement 6a)
Common Mistake Alert! Students often think adrenaline makes you feel scared. It doesn't! It just prepares your body for maximum physical effort, whether or not you feel scared.
Key Takeaway: Adrenaline is released rapidly during stress and causes immediate effects like faster heartbeat, faster breathing, and higher blood sugar, ensuring muscles have maximum energy and oxygen.
4. Comparing Nervous and Hormonal Control (Core 4)
Coordination is achieved in the human body using two systems: the Nervous System (using electrical impulses) and the Hormonal System (using chemical messengers). It is vital to compare and contrast these two methods.
Speed of Action:
• Nervous Control: Very Fast (milliseconds). Electrical impulses travel extremely quickly along neurones.
• Hormonal Control: Relatively Slow (seconds or even minutes). Hormones rely on the blood to transport them, which is a much slower process.
Duration of Effect:
• Nervous Control: Short-lived. The effect stops almost immediately after the impulse ends.
• Hormonal Control: Long-lasting. Once hormones are in the blood, they take time to be broken down, so the effects persist (e.g., growth hormones cause permanent change).
Transmission Method:
• Nervous Control: Transmitted by electrical impulses along neurones and chemical neurotransmitters across synapses.
• Hormonal Control: Transmitted by chemical hormones carried throughout the body in the bloodstream.
Memory Aid: Think of the nervous system as a sudden lightning strike, and the hormonal system as the slow, steady growth of a tree.
| Feature | Nervous Control | Hormonal Control |
|---|---|---|
| Signal Type | Electrical Impulse and Neurotransmitters | Chemical Hormones |
| Speed | Very Fast | Slower |
| Duration | Short / Temporary | Longer / Persistent |
| Target Area | Specific cells (muscles, glands) | Widespread (affects any cell with receptors) |
(Note: Since tables are not allowed, we must format this information clearly using text and bolding.)
Quick Comparison Summary:
• Speed: Nervous is Fast; Hormonal is Slow.
• Duration: Nervous is Short; Hormonal is Long.
• Transport: Nervous uses Neurones; Hormonal uses Blood.
5. Hormones and Blood Glucose Control (The Insulin/Glucagon Link)
While the detailed maintenance of the internal environment (homeostasis) is a separate topic (14.4), understanding the basic action of insulin and glucagon is crucial since they are primary examples of hormones.
These two hormones work together in an antagonistic (opposing) pair, produced by the Pancreas, to keep your blood sugar levels stable (a required part of homeostasis).
Insulin: The Lowering Hormone
• When blood glucose is too high (e.g., after a sugary meal), the pancreas releases Insulin.
• Insulin signals the liver and muscle cells to take up glucose from the blood and convert it into glycogen (storage form).
• Result: Insulin decreases blood glucose concentration. (Core 14.4.2)
Glucagon: The Raising Hormone (Supplement 5)
• When blood glucose is too low (e.g., after exercise), the pancreas releases Glucagon.
• Glucagon signals the liver to break down stored glycogen back into glucose and release it into the blood.
• Result: Glucagon increases blood glucose concentration.
If you struggle with this: Think of Insulin as the hormone that says "in-sulin, take glucose in!" and Glucagon as the hormone that says "glucose-gone, need to raise it!"
Key Takeaway: Insulin and Glucagon are essential pancreatic hormones that maintain blood sugar balance.