Edexcel iGCSE Biology-3.24 Nephron Structure- Study Notes- New Syllabus

Structure of a Nephron

📝 Introduction

• The nephron is the functional unit of the kidney.
• It is the microscopic structure where blood is filtered and urine is formed.
• Each kidney contains about 1 to 1.5 million nephrons.
• Every nephron works continuously to remove waste products while returning useful substances to the blood.
• A nephron is made up of several specialised parts, and each part performs a specific function.

🌱 What is a Nephron?

• A nephron is a long, thin tube with different specialised regions.
• It begins with a filtering unit and ends with a collecting duct.
• As the filtered fluid passes through the nephron, useful substances are reabsorbed and waste products remain in the urine.

🧩 Main Parts of a Nephron

The nephron consists of the following main parts:

• Glomerulus
• Bowman’s capsule
• Proximal convoluted tubule (PCT)
• Loop of Henle
• Distal convoluted tubule (DCT)
• Collecting duct

1. Glomerulus

Structure

• The glomerulus is a ball-shaped network of tiny blood capillaries.
• It is located at the beginning of the nephron.
• It receives blood from the renal artery.

Function

• It is the site of filtration.
• Blood enters the glomerulus under high pressure.
• Small molecules are forced out of the blood through the capillary walls.

Small molecules filtered

• Water
• Glucose
• Amino acids
• Urea
• Mineral salts

Substances that remain in the blood

• Red blood cells
• White blood cells
• Platelets
• Plasma proteins

These substances are too large to pass through the filter.

2. Bowman’s Capsule

Structure

• Bowman’s capsule is a cup-shaped structure that surrounds the glomerulus.
• It forms the first part of the nephron.

Function

• Collects the fluid filtered from the glomerulus.
• Passes the filtered fluid into the proximal convoluted tubule.

The glomerulus and Bowman’s capsule work together as the filtration unit of the nephron.

3. Proximal Convoluted Tubule (PCT)

Structure

• The PCT is the first coiled tube after Bowman’s capsule.
• Its lining contains many microvilli.
• The cells also contain many mitochondria.

Why are Microvilli Present?

Microvilli increase the surface area for absorption.

Function

This is the main site of selective reabsorption.

The PCT reabsorbs:

• All glucose
• All amino acids
• Most water
• Most mineral salts

These useful substances move back into the surrounding blood capillaries.

4. Loop of Henle

Structure

• The Loop of Henle is a U-shaped section of the nephron.
• It has:
  • A descending limb
  • An ascending limb

Function

• The Loop of Henle helps control the amount of water and salts in the body.
• It:
  • Reabsorbs water.
  • Reabsorbs mineral salts.
  • Helps produce concentrated urine when the body needs to conserve water.

This is important for osmoregulation.

5. Distal Convoluted Tubule (DCT)

Structure

• The DCT is the second coiled tube of the nephron.
• It comes after the Loop of Henle.

Function

• The DCT makes the final adjustments to the composition of the filtrate.
• It:
  • Reabsorbs some mineral salts.
  • Helps maintain the correct ion balance in the blood.
  • Works under the control of hormones.

This fine-tunes the composition of the urine before it enters the collecting duct.

6. Collecting Duct

Structure

• The collecting duct is the final part of the nephron.
• Several nephrons empty into one collecting duct.

Function

• The collecting duct:
  • Collects urine from many nephrons.
  • Reabsorbs water if needed.
  • Its permeability is controlled by ADH (Antidiuretic Hormone).

If more ADH is present

• More water is reabsorbed.
• Urine becomes more concentrated.

If less ADH is present

• Less water is reabsorbed.
• Urine becomes more dilute.

The urine then flows into the renal pelvis, enters the ureter, and finally reaches the urinary bladder.

🌟 How a Nephron Forms Urine

The nephron forms urine through three main processes:

1. Filtration

• Occurs in the glomerulus and Bowman’s capsule.
• Small molecules are filtered from the blood.

2. Selective Reabsorption

• Occurs mainly in the proximal convoluted tubule.
• Useful substances are returned to the blood.

3. Water Regulation

• Occurs in the Loop of Henle, distal convoluted tubule, and collecting duct.
• The amount of water in the urine is adjusted according to the body’s needs.

📊 Parts of a Nephron and Their Functions

📊 Flow Through a Nephron

⚡ Quick Recap
Bowman’s capsule + Glomerulus → filter blood.
PCT → take back glucose, amino acids, water.
Loop of Henle → concentrates urine.
DCT → fine-tunes salt levels.
Collecting duct → final water control, urine sent out.
👉 Flow: Glomerulus → Bowman’s capsule → PCT → Loop of Henle → DCT → Collecting duct.