B5 – Homeostasis & Response

🔧 Core Concepts

Area	What it’s about	Key question theme
Regulation	How cells/organisms keep internal conditions stable in a changing environment.	“Explain a feedback loop.”
Feedback Loops	Negative vs positive mechanisms that either dampen or amplify a change.	“Give an example of each.”
Nervous & Endocrine Systems	Two main ways the body signals and reacts.	“Compare neural and hormonal signalling.”
Homeostatic mechanisms	Temperature, blood glucose, pH, water balance, light, waste removal.	“What stops body temperature from rising during exercise?”
Transport & Circulation	Blood and lymph movement, oxygen transfer, filtration.	“Define the roles of arteries, veins, capillaries, alveoli.”
Clinical Links	Diabetes, hypertension, dehydration, heat stroke.	“Why are blood glucose levels monitored in diabetes?”

---

⚡ Regulation & Feedback Loops

🔄 Negative Feedback

• Goal – maintain a set point.
• 1️⃣ Sensor → 2️⃣ Control centre → 3️⃣ Effector → 4️⃣ Return road (feedback).
• Example – Body temperature:
  • Sensor: Thermoreceptors in skin & hypothalamus.
  • Control centre: Hypothalamus.
  • Effector: Sweat glands, blood vessels.
  • Return: Reduction in skin temperature → reset.

🚨 Positive Feedback

• Goal – amplify a response.
• Example – Childbirth:
  • Causing factor: Contractions → Nerve impulses → Release oxytocin → More contractions.

---

🧠 Nervous vs Endocrine Signalling

Feature	Nervous	Endocrine
Signal speed	✓ rapid (ms)	✗ slower (minutes–hours)
Target cells	✓ specific, rapid	✓ many cells, long duration
Signal carriers	✓ Neurotransmitters (GABA, dopamine)	✓ Hormones (insulin, adrenaline)
Transport medium	✓ Neuro‑axons	✗ Blood & lymph
Examples	Sensory input → reflex arc	Hormone release → appetite control
Key term	Receptor	Receptor

Exam cue: “Why does insulin act more slowly than adrenaline?” – discuss endocrine signalling.

---

🔬 Hormone Action

Step	What happens	Example
1️⃣ Secretion	Hormone released into bloodstream.	Insulin from pancreas.
2️⃣ Transport	Travels to target cell.	Adrenaline via blood.
3️⃣ Receptor binding	Hormone binds to specific receptor on the target cell.	Glucagon binds to receptors on liver.
4️⃣ Signal transduction	Alters gene expression or enzyme activity.	Insulin promotes glucose uptake.

🌡️ Heat & Cold Response

• Adrenaline → increased heat production (shivering).
• Thyroid hormones → metabolic rate.

---

❤️ Circulation & Gas Exchange

Structure	Function	Key Point
Arteries	Carry blood away from the heart.	High pressure, thick walls.
Veins	Carry blood toward heart.	Valves prevent back‑flow.
Capillaries	Exchange gases/products.	Microscopic, thin walls.
Alveoli	Gas‑exchange site of lungs.	Surfactant reduces surface tension.
Blood‑brain barrier	Protects brain from toxins.	Tight junctions, selective permeability.

Exam highlight: “What provides the main barrier between blood and brain?” – answer blood‑brain barrier.

---

🌿 Homeostatic Mechanisms

Theme	Mechanism	Factors involved	Clinical relevance
Body temperature	Sweating, vasodilation	Hypothalamus, sweat glands, blood vessels	Heat stroke
Blood glucose	Insulin & glucagon	Pancreas, liver, muscle	Diabetes mellitus
pH	Buffer systems (bicarbonate)	Blood, CO₂, kidneys	Acidosis, alkalosis
Water balance	Antidiuretic hormone (ADH)	Kidneys, brain	Dehydration
Light	Melatonin secretion	Pineal gland	Circadian rhythm
Waste removal	Lymphatic & renal	Lymph nodes, kidneys	Urine formation, infections
Oxygen & CO₂	Hemoglobin affinity	Blood, lungs	Hypoxia, hypercapnia

---

💡 Exam‑Style “Key Ideas”

1. Describe a negative feedback loop that involves the hypothalamus – start with a change in body temperature, list sensor, control centre, effector and return road.
2. Explain the difference between a reflex arc and hormone release – include speed of response, target cells and duration.
3. Why is the blood–brain barrier important during infection? – discuss selective permeability and neuroinflammation.
4. Illustrate how the body regulates pH when you gulp acidic soda. – describe buffering, renal compensation.

---

📚 Revision Resources

Resource	Type	Why use it
BBC Bitesize – Homeostasis and body processes	Video & quiz	Interactive, visually explains feedback.
AQA GCSE Biology study guide	PDF	Detailed notes + exam questions.
QuestionsBank	Past papers	Practice real exam wording.
Quizlet – Homeostasis flashcards	Digital cards	Test key terms like homeostasis, ADH, sympathetic.
Physics & Maths Tutor – Homeostasis	Mind map	Clear visual of loops.
YouTube – “How the body keeps things balanced”	Short video	Good for visual learners.

Tip: Create a “wheel diagram” of feedback loops (sensor → centre → effector → return) – handy for quick revision and diagram questions.

---

✅ Quick‑Check Checklist

 I can compare neural and hormonal response times.

 I understand the difference between negative and positive feedback.

 I can name the receptors involved in hormone action.

 I can describe the role of arteries, veins, capillaries, alveoli.

 I know how the body keeps pH balanced.

 I can explain why ADH is released during dehydration.