Edexcel A Level (IAL) Biology -7.21 Hormones in Osmoregulation- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -7.21 Hormones in Osmoregulation- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -7.21 Hormones in Osmoregulation- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 7.21 understand how the pituitary gland and osmoreceptors in the hypothalamus, combined with the action of antidiuretic hormone (ADH), bring about negative feedback control of mammalian plasma concentration and blood volume
Negative Feedback Control of Plasma Concentration and Blood Volume
🌟 Introduction
Mammals keep their blood plasma concentration and blood volume steady using a negative feedback loop. The key players are:
- Hypothalamus (osmoreceptors)
- Pituitary gland
- ADH (antidiuretic hormone)
- Kidneys
🧬 Main Concepts
1. Osmoreceptors in the Hypothalamus
- Sensitive to changes in blood water concentration.
- If blood becomes too concentrated (low water), they shrink.
- If blood becomes too dilute (high water), they swell.
2. Role of the Pituitary Gland
- Hypothalamus signals the posterior pituitary.
- Pituitary releases ADH depending on water balance signals.
3. ADH (Antidiuretic Hormone)
- Travels in blood to the kidneys.
- Makes collecting ducts more permeable to water.
- More ADH means more water reabsorbed, less urine.
- Less ADH means less reabsorption, more urine.
🔁 Negative Feedback Mechanism
Case 1: Plasma too concentrated (low blood volume)
- Osmoreceptors detect low water.
- Hypothalamus sends message to pituitary.
- More ADH released.
- Collecting ducts reabsorb more water.
- Blood becomes dilute again.
- System returns to normal.
Case 2: Plasma too dilute (high blood volume)
- Osmoreceptors detect too much water.
- Pituitary reduces ADH release.
- Collecting ducts become less permeable.
- More water lost in urine.
- Blood concentration rises back to normal.
📌 Summary Table
| Condition | ADH Level | Kidney Response | Effect on Blood |
|---|---|---|---|
| Blood concentrated | High | More water reabsorbed | Dilutes plasma |
| Blood dilute | Low | Less water reabsorbed | Concentrates plasma |
🧠 Quick Recap
Osmoreceptors monitor blood concentration.
Pituitary adjusts ADH release.
ADH controls water reabsorption in kidneys.
System works as a negative feedback loop to stabilise plasma concentration and blood volume.
Trick: “High concentration = High ADH” (both high).
Osmoreceptors monitor blood concentration.
Pituitary adjusts ADH release.
ADH controls water reabsorption in kidneys.
System works as a negative feedback loop to stabilise plasma concentration and blood volume.
Trick: “High concentration = High ADH” (both high).