Sex Hormones in HumansCambridge IGCSE Biology: Revision notes
Section 1
What is a hormone and how do endocrine glands work?
A hormone is a chemical substance produced by a gland and carried by the blood that alters the activity of specific target organs.
Endocrine glands are ductless glands that secrete hormones directly into the bloodstream. Key endocrine glands and their hormones include:
| Gland | Hormone | Key Role |
|---|---|---|
| Adrenal glands | Adrenaline | 'Fight or flight' response |
| Pancreas | Insulin and glucagon | Blood glucose regulation |
| Testes | Testosterone | Male sexual characteristics and reproduction |
| Ovaries | Oestrogen | Female sexual characteristics and reproduction |
Hormones circulate in the blood and only affect cells with the correct target receptors. This specificity ensures precise control of body functions.
Examiners test whether you can name the gland, the hormone it produces, and its function. Always state all three when discussing endocrine control.
Think of hormones as postal messages sent through the bloodstream – they are released by a gland (post office), travel via blood (postal system), and only get 'read' by target cells (houses with the correct address receptors).
Section 2
How does adrenaline prepare the body for 'fight or flight'?
Adrenaline is secreted by the adrenal glands in response to stressful or emergency situations. It prepares the body to either face a threat or escape from it.
Adrenaline causes the following immediate effects:
- Increased breathing rate – more oxygen is delivered to muscles
- Increased heart rate – blood is pumped faster to muscles and the brain
- Increased pupil diameter – allows more light into the eye for better vision
- Increased blood glucose concentration – provides more energy for muscle contraction
These changes happen within seconds, allowing rapid physical response. Adrenaline also diverts blood away from the digestive system and towards skeletal muscles, which is why you may lose appetite during stress.
When you suddenly see a car approaching, adrenaline is released within milliseconds. Your heart rate increases to 120+ bpm, your pupils dilate, breathing becomes faster and deeper, and blood glucose rises. This is the 'fight or flight' response in action.
Students often forget that adrenaline increases blood glucose concentration. Remember: adrenaline mobilises glucose reserves to fuel muscle action, so blood glucose always rises during stress.
Section 3
How do nervous and hormonal control differ?
The body uses two main control systems: the nervous system and the hormonal system (endocrine system). These differ significantly in speed and duration of effect:
| Feature | Nervous Control | Hormonal Control |
|---|---|---|
| Speed of action | Very fast (milliseconds) | Slow (seconds to minutes) |
| Duration of effect | Short-lived (seconds) | Long-lasting (minutes to hours) |
| Transmission method | Electrical impulses via nerve fibres | Chemical hormones via blood |
| Type of response | Precise, localised responses | Widespread, general responses |
| Examples | Reflex actions, pupil reflex | Adrenaline response, glucose regulation |
In emergencies, the nervous system acts first (immediate reaction), while the hormonal system maintains the response. Many body functions involve both systems working together; for example, the sight of danger triggers a nerve impulse that causes the adrenal glands to release adrenaline.
When comparing nervous and hormonal control, always structure your answer as: nervous is faster/shorter, hormonal is slower/longer. Use comparative language like 'whereas' or 'in contrast' to score full marks.
The nervous system is like a telephone call – instant but stops immediately when you hang up. The hormonal system is like a letter in the post – slower to arrive but has lasting effects once received.
Section 4
What is the role of adrenaline in metabolic control?
Adrenaline plays a crucial role in metabolic control by increasing the availability of energy during stress or emergency situations.
The two main metabolic effects of adrenaline are:
-
Increases blood glucose concentration – Adrenaline stimulates the breakdown of glycogen (stored glucose) in the liver and muscles, releasing glucose into the bloodstream. This provides rapid energy for muscle contraction and brain function.
-
Increases heart rate – A faster heart rate ensures that glucose-rich blood is transported quickly to muscles and the brain, enabling them to work harder.
These effects work together: adrenaline mobilises glucose reserves and simultaneously increases blood flow to deliver this glucose where it is needed most. This is an example of metabolic control – the regulation of chemical reactions in the body to maintain energy supply during demand.
Once the stressful situation passes, adrenaline levels fall, the heart rate returns to normal, and excess glucose is taken up by cells or converted back to glycogen.
During a stressful exam, adrenaline is released. It breaks down liver glycogen, raising blood glucose from 5 mmol/dm³ to 7 mmol/dm³. Simultaneously, heart rate increases from 70 to 100 bpm, delivering this glucose-rich blood to the brain to improve concentration and memory recall.
Section 5
What is glucagon and how does it regulate blood glucose?
Glucagon is a hormone produced by the pancreas (specifically by alpha cells in the islets of Langerhans) that raises blood glucose concentration.
Glucoset secretion and function:
- Glucagon is secreted when blood glucose concentration falls below the normal range (below approximately 4 mmol/dm³)
- It acts on the liver and muscles to stimulate the breakdown of glycogen into glucose
- Glucose is released into the bloodstream, raising blood glucose concentration back to normal
- Once blood glucose returns to normal, glucagon secretion stops
Glucoaget and insulin (produced by beta cells) work antagonistically to maintain blood glucose homeostasis:
| Hormone | Produced by | Effect on blood glucose | When secreted |
|---|---|---|---|
| Insulin | Beta cells (pancreas) | Lowers | High blood glucose |
| Glucagon | Alpha cells (pancreas) | Raises | Low blood glucose |
This antagonistic relationship ensures blood glucose remains within a narrow, safe range (approximately 4–7 mmol/dm³) regardless of food intake or physical activity.
Examiners often ask about the complementary roles of insulin and glucagon. Always state that they work antagonistically – one lowers glucose while the other raises it – to maintain homeostasis.
Students sometimes confuse which hormone does what. Remember: Glucagon = Glucose up (both start with G). Insulin = Inhibit (lowers glucose). Use this mnemonic to avoid mixing them up.
Must Know
- A hormone is a chemical substance produced by a gland, carried by blood, and alters the activity of specific target organs
- Key endocrine glands: adrenal glands (adrenaline), pancreas (insulin and glucagon), testes (testosterone), ovaries (oestrogen)
- Adrenaline causes 'fight or flight' responses: increased breathing rate, heart rate, pupil diameter, and blood glucose concentration
- Nervous control is fast and short-lived (milliseconds to seconds); hormonal control is slow and long-lasting (seconds to hours)
- Adrenaline's role in metabolic control: increases blood glucose concentration by breaking down glycogen and increases heart rate to deliver glucose to muscles and brain
- Glucagon secreted by pancreas raises blood glucose; insulin lowers it – they work antagonistically to maintain homeostasis
That's the notes covered.
Carry on to the next subtopic.