Edexcel iGCSE Biology-3.25B -3.27B Nephron Function- Study Notes- New Syllabus
Edexcel iGCSE Biology-3.25B -3.27B Nephron Function- Study Notes- New syllabus
Edexcel iGCSE Biology-3.25B -3.27B Nephron Function- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
3.25B describe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate
3.26B understand how water is reabsorbed into the blood from the collecting duct
3.27B understand why selective reabsorption of glucose occurs at the proximal convoluted tubule
Ultrafiltration in the Bowman’s Capsule
🌱 Introduction
Ultrafiltration = the process where small molecules are filtered out of the blood under pressure in the glomerulus → Bowman’s capsule.
This is the first step of urine formation in the nephron.
🔹 How Ultrafiltration Happens
- Blood enters glomerulus through the renal artery (high pressure as it branches directly from aorta).
- Glomerulus: knot of capillaries with thin, permeable walls.
- Pressure forces small molecules (water, glucose, amino acids, salts, urea) → out of plasma into Bowman’s capsule.
- Large molecules (proteins, blood cells, platelets) are too big → stay in blood.
- This pressure-driven filtering = ultrafiltration.
🔹 Composition of the Glomerular Filtrate
The fluid that enters Bowman’s capsule (glomerular filtrate) contains:
- Water
- Glucose
- Amino acids
- Urea (to be excreted)
- Mineral ions/salts
It does not contain:
- Proteins (too large)
- Blood cells (too large)
📊 Summary Table
| Step | Process | What passes through |
|---|---|---|
| 1. Blood in glomerulus | High pressure | Forces plasma out |
| 2. Filter through walls | Capillaries + basement membrane | Acts as a sieve |
| 3. Enters Bowman’s capsule | Glomerular filtrate | Water, glucose, amino acids, urea, salts |
| 4. Stays in blood | Too large molecules | Proteins, RBCs, WBCs, platelets |
⚡ Quick Recap
Ultrafiltration = blood under pressure filters out small molecules into Bowman’s capsule.
Filtrate = water + glucose + amino acids + salts + urea.
No proteins or blood cells in filtrate.
👉 Think: “Useful + waste small stuff passes, big stuff stays”.
Water Reabsorption from the Collecting Duct
🌱 Introduction
After filtration, the nephron has to reabsorb useful substances.
One of the most important = water, to maintain body’s water balance.
This happens mainly in the collecting duct.
How Water is Reabsorbed
- Filtrate enters the collecting duct from the distal convoluted tubule.
- Surrounding the duct = tissue fluid of medulla is very concentrated (due to the loop of Henle).
- This creates a water potential gradient → water moves out of duct by osmosis.
- Water passes into surrounding capillaries, returning to the blood.
Role of ADH (Antidiuretic Hormone)
- ADH (from the pituitary gland) controls how much water is reabsorbed.
- When body needs to conserve water (sweating, dehydration):
→ More ADH released → collecting duct walls more permeable → more water reabsorbed → concentrated urine. - When water levels are high:
→ Less ADH released → duct walls less permeable → less water reabsorbed → dilute urine.
📊 Summary Table
| Condition | ADH level | Collecting duct permeability | Urine produced |
|---|---|---|---|
| Dehydrated | High | More permeable | Concentrated, small volume |
| Hydrated (excess water) | Low | Less permeable | Dilute, large volume |
⚡ Quick Recap
Collecting duct → water reabsorbed by osmosis into blood capillaries.
Controlled by ADH hormone.
High ADH = concentrated urine.
Low ADH = dilute urine.
👉 Mnemonic: ADH = “Always Drinking Hormone” → tells your kidneys how much water to keep!
Selective Reabsorption of Glucose in the Proximal Convoluted Tubule (PCT)
🌱 Introduction
After ultrafiltration in the Bowman’s capsule, the filtrate contains useful substances (like glucose, amino acids, water, salts) as well as wastes (urea).
The body cannot afford to lose glucose, as it’s the main source of energy.
Therefore, glucose is selectively reabsorbed in the proximal convoluted tubule (PCT).
Why Selective Reabsorption of Glucose Happens in the PCT
- Glucose is essential for respiration → provides ATP for all cell activities.
- It must not be lost in urine → otherwise body would waste energy and nutrients.
- The PCT is the best place because:
- Specialised epithelial cells with microvilli → very large surface area.
- Cells packed with mitochondria → ATP for active transport.
- Allows 100% of glucose (in healthy individuals) to be reabsorbed into blood.
How Glucose is Reabsorbed
- Sodium-potassium pumps in PCT cells create a concentration gradient.
- Glucose is reabsorbed by active transport and co-transport with sodium ions.
- Glucose diffuses into surrounding capillaries → returns to bloodstream.
📊 Summary Table
| Feature of PCT | Adaptation | Role in Glucose Reabsorption |
|---|---|---|
| Microvilli | Increases surface area | More absorption of glucose |
| Many mitochondria | Supply ATP | Energy for active transport |
| Close to capillaries | Short diffusion distance | Quick transfer to blood |
| Special transport proteins | Sodium-glucose co-transport | Moves glucose efficiently |
⚡ Quick Recap
Glucose = vital fuel → must be reabsorbed.
PCT is adapted with microvilli + mitochondria.
Process: Active transport + co-transport with sodium ions.
Result: 100% of glucose reabsorbed into blood (normally).
👉 Mnemonic: PCT = Picks up Critical Treasure (glucose)