Coordination & Response
Structures and Functions in Living Organisms · 7 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
Nervous system structure, reflex arcs, and hormones are all commonly examined.
A hormone is a chemical messenger produced by an endocrine gland and carried in the blood to target cells in another part of the body. Endocrine glands have no ducts; they secrete their hormones directly into the bloodstream.
The major endocrine glands and their key hormones are summarised below. The detailed mechanisms (such as how insulin lowers blood glucose, or how ADH controls water reabsorption) are covered in topic 11 Homeostasis and topic 9 Excretion respectively.
| Hormone | Where it is made | Main role |
|---|---|---|
| Adrenaline | Adrenal glands (one above each kidney) | "Fight-or-flight" response: prepares the body for sudden physical action. Raises heart rate, raises breathing rate, redirects blood to muscles, breaks down stored glycogen in the liver to release glucose |
| Insulin | Pancreas | Lowers blood glucose. Stimulates muscles and liver to take in glucose from the blood and store it as glycogen |
| Testosterone | Testes (in males) | Triggers male puberty: development of male reproductive organs, growth of facial/body hair, deepening of the voice, increased muscle mass |
| Oestrogen | Ovaries (in females) | Triggers female puberty: breast development, widening of the hips, body hair. Also rebuilds the uterus lining after each menstrual period |
| Progesterone | Ovaries | Maintains the thick uterus lining ready for a fertilised egg, and during pregnancy. Prevents further ovulation while a fertilised egg is implanted |
| ADH (antidiuretic hormone) | Pituitary gland (just under the brain) | Increases the permeability of the kidney's collecting ducts to water, allowing the body to reabsorb more water and produce less urine (see topic 9) |
| FSH (follicle-stimulating hormone) | Pituitary gland | Causes an egg to mature in the ovary each month; stimulates the ovary to release oestrogen |
| LH (luteinising hormone) | Pituitary gland | Triggers ovulation (the release of the mature egg); stimulates the ovary to release progesterone |
How adrenaline works (the "fight-or-flight" response)
The adrenal glands release adrenaline in response to fear, excitement, or sudden physical demand. Adrenaline causes a coordinated set of changes that prepare the body to either fight a threat or run from it:
- Heart rate increases. Blood is pumped to the muscles faster.
- Breathing rate and depth increase. More oxygen enters the blood; more CO₂ leaves.
- Pupils dilate. More light enters the eye, improving vision in low-light or distant scenes.
- Blood is redirected away from non-essential organs (such as the gut) and towards muscles. The arterioles supplying muscles dilate; those supplying the gut constrict.
- Glucose is released into the blood. Adrenaline triggers the liver to break down stored glycogen back into glucose, raising blood sugar so muscle cells have more fuel for respiration.
All of these effects begin within seconds of the adrenaline reaching its target cells and fade within minutes once the danger has passed.