Edexcel A Level (IAL) Biology -7.20 Selective Reabsorption in the Kidney- Study Notes- New Syllabus

Selective Reabsorption & the Countercurrent Multiplier

🌱 Introduction

After ultrafiltration, the filtrate entering the nephron contains useful substances like glucose, amino acids, ions, and water.
The nephron must reabsorb these.
Two main regions are involved:

• Proximal Convoluted Tubule (PCT)
• Loop of Henle

🧬 1. Selective Reabsorption in the Proximal Convoluted Tubule (PCT)

The PCT is the main site of reabsorption.
Around 70 percent of the filtrate is reabsorbed here.

🔍 Why the PCT is so effective

• Cells have microvilli, giving large surface area.
• Lots of mitochondria for ATP.
• Close contact with capillaries for rapid transport.
• Co-transport proteins and carrier proteins.

📌 What Gets Reabsorbed (and How)

Glucose and Amino Acids

• Reabsorbed by co-transport with sodium ions.
• Sodium moves down its gradient into the cell, pulling glucose or amino acids with it.
• Actively transported into the blood.

Sodium Ions

• Pumped out of PCT cells by active transport using the sodium-potassium pump.
• This maintains the gradient for all co-transport processes.

Water

• Follows by osmosis because the filtrate becomes more dilute as solutes leave.
• PCT is highly permeable to water.

Chloride Ions

• Follow passively due to electrical attraction to sodium.

Urea

• Small amount diffuses back, but most remains and is excreted.

🌀 2. Loop of Henle as a Countercurrent Multiplier

The Loop of Henle creates a salt concentration gradient in the medulla.
This gradient allows the collecting duct to reabsorb large amounts of water.

This process is called a countercurrent multiplier.

🧩 Key Idea

The descending and ascending limbs have opposite properties.

Descending limb

• Permeable to water
• Impermeable to ions

→ Water moves out by osmosis into the salty medulla.

Ascending limb

• Impermeable to water
• Pumps sodium and chloride ions out

→ Makes the medulla even saltier.

🔁 Why It’s Called “Countercurrent”

• The two limbs flow in opposite directions.
• This keeps a steep concentration gradient along the whole loop.

🔁 Why It’s a “Multiplier”

• Each small movement of ions is multiplied into a very strong medulla salt gradient.

💧 Final Outcome

Because the medulla becomes very salty, the collecting duct (later in the nephron) can reabsorb large amounts of water, especially when ADH is present.

This is essential for:

• Producing concentrated urine
• Preventing dehydration

📊 Summary Table