Hormones
A hormone is defined by three key characteristics:
- It is a chemical substance produced by a gland.
- It is carried by the blood to its destination.
- It alters the activity of one or more specific target organs.
Building on the concept of homeostasis, hormones play a vital role in maintaining internal balance (e.g., controlling blood glucose levels). Unlike nerve impulses which are localized and fast, hormonal responses are widespread and slower.
Hormone: A chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
Key terms to remember:
- Gland: An organ that produces and releases substances (like hormones).
- Target Organ: The specific organ that responds to the hormone because it has receptors for that hormone.
- Blood: The transport medium. This is why hormonal responses take longer than nervous responses.
Correct Understanding: The liver is a target organ for insulin. It receives insulin via the blood and responds by converting glucose into glycogen. The hormone itself is produced by the pancreas. Always check if the question asks for the producer (gland) or the receiver (target organ).
Why examiners accept this: The markscheme requires three specific components: production by a gland, transport by blood, and effect on target organs. Missing any one of these results in lost marks.
Correct Usage Example: "A hormone is a chemical substance produced by a gland, transported by the blood, which affects a target organ."
Incorrect Usage: "A hormone is a chemical that controls body functions." (This is too vague and misses the transport mechanism).
You must be able to identify specific glands in diagrams and state the hormones they secrete. The following table lists the core requirements:
| Gland | Location | Hormone Secreted |
|---|---|---|
| Adrenal glands | On top of the kidneys | Adrenaline |
| Pancreas | Behind the stomach | Insulin (and Glucagon*) |
| Testes | In the scrotum | Testosterone |
| Ovaries | In the pelvis | Oestrogen |
*Note: Glucagon is also secreted by the pancreas but is covered in the Supplement section below.
Identification Tip: In diagrams, look for:
- Adrenal glands: Small triangular caps on top of the kidneys.
- Pancreas: A leaf-shaped gland behind the stomach loop.
- Testes/Ovaries: Reproductive organs located lower in the body.
The effects of adrenaline are:
- Increased breathing rate: Allows more oxygen to enter the lungs.
- Increased heart rate: Pumps blood (and therefore glucose and oxygen) faster to the muscles.
- Increased pupil diameter: Allows more light into the eyes, improving vision.
These changes ensure that the muscles receive enough energy and oxygen to react quickly.
Correct Understanding: Adrenaline causes the pupil diameter to increase. This allows more light to enter the eye, which helps you see better in a dangerous situation. Remember: 'Fight or Flight' means getting ready to act, so you need better vision.
Why examiners accept this: Examiners look for specific physiological changes. Vague answers like 'it makes you excited' are not accepted. You must state the measurable physical change.
Correct Usage Example: "Adrenaline causes an increase in heart rate and an increase in pupil diameter."
Incorrect Usage: "Adrenaline makes the heart beat faster." (While true, 'increased heart rate' is the standard scientific phrasing preferred in markschemes).
| Feature | Nervous Control | Hormonal Control |
|---|---|---|
| Speed of Action | Fast (rapid) | Slow |
| Duration of Effect | Short-lived | Long-lasting |
Reasoning: Nervous control uses electrical impulses along neurones, which are very fast but stop immediately when the signal stops. Hormonal control relies on hormones traveling through the bloodstream, which takes time, and the chemicals remain in the blood for a longer period before being broken down.
- Nervous control is faster than hormonal control.
- The effects of hormonal control last longer than the effects of nervous control.
Glucagon is a hormone secreted by the pancreas. It plays a critical role in regulating blood glucose concentration.
When blood glucose levels are too low (hypoglycemia), glucagon is released. It travels to the liver, where it stimulates the breakdown of stored glycogen into glucose. This glucose is then released into the blood, raising the blood glucose concentration back to normal levels.
Key Point: Both insulin and glucagon are secreted by the pancreas, but they have opposite effects:
- Insulin: Lowers blood glucose (by converting glucose to glycogen).
- Glucagon: Raises blood glucose (by converting glycogen to glucose).
Correct Understanding: Insulin decreases blood glucose concentration. It is released when blood glucose is too high (e.g., after eating). Glucagon is the hormone that increases blood glucose concentration when it is too low.
Why examiners accept this: The markscheme specifically looks for glucagon and adrenaline. Insulin is the distractor option that lowers glucose.
Correct Usage Example: "The hormones that increase blood glucose concentration are glucagon and adrenaline."
Incorrect Usage: "Insulin increases blood glucose." (This is factually wrong and will result in zero marks).
- Increasing Blood Glucose Concentration: Adrenaline stimulates the liver to break down glycogen into glucose. This increases the blood glucose concentration, providing more fuel for respiration.
- Increasing Heart Rate: As mentioned in the Core section, this ensures that the oxygen and glucose produced are delivered rapidly to the muscles.
Why is this an advantage?
The increased glucose allows for more respiration in the muscle cells. Respiration releases energy (ATP), which is used for muscle contraction. This enables the body to react quickly and physically during a 'fight or flight' situation.