Lecture Notes: Filtering and Reabsorption in the Nephron

Introduction

• Presenter: Miss A Stroke
• Focus: Year 13 topic on filtering and reabsorption in the nephron.
• Main Topics:
  • Structure of the nephron
  • Function of various parts: renal capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting ducts
  • Ultra filtration process
  • Selective reabsorption

Overview of the Kidney

• Function: Filtering and osmoregulation
• Structure: Nephrons located in the medulla
  • Long tubules surrounded by capillaries
  • Approx. 1 million nephrons per kidney

Structure of the Nephron

• Components:
  • Afferent arteriole leading to glomerulus (capillaries)
  • Renal capsule (Bowman's capsule)
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting duct
• Function: Creation of urine by filtering blood

Urine Composition

• Contains excess water, dissolved salts, urea, small substances
• Does not contain proteins, blood cells, or glucose in a healthy individual
  • Proteins and blood cells are too large to filter
  • Glucose is reabsorbed in the proximal convoluted tubule

Filtration and Reabsorption Process

Step 1: Ultrafiltration in the Glomerulus

• Process:
  • Blood enters through afferent arteriole
  • High pressure forces small molecules and water into renal capsule (glomerular filtrate)
  • Large proteins and blood cells remain in blood

Step 2: Proximal Convoluted Tubule (PCT)

• Selective Reabsorption:
  • 85% of filtrate reabsorbed
  • Adaptations: microvilli (increased surface area), mitochondria (energy for active transport)
  • Process:
    • Sodium ions actively transported out of PCT
    • Sodium ions diffuse back in with glucose via cotransporter proteins
    • Glucose diffuses into the bloodstream

Step 3: Loop of Henle

• Function: Sodium ion gradient maintenance
• Structure:
  • Ascending limb (thick walls, impermeable to water)
  • Descending limb (thin walls, permeable to water)
• Process:
  • Sodium ions actively transported out, creating low water potential
  • Water diffuses out of descending limb by osmosis
  • At the base of the ascending limb, some sodium ions diffuse out

Step 4: Distal Convoluted Tubule (DCT) and Collecting Ducts

• Function: Further reabsorption
• Process:
  • Water diffuses out due to concentrated medulla environment
  • Remaining liquid forms urine

Example Application Question

• Question: Differences in loop of Henle length between desert animals and humans
• Explanation: Longer loop in desert animals for increased water reabsorption
  • Larger surface area for ion transport
  • More water reabsorbed into blood, concentrated urine

Summary

• Nephron consists of renal capsule, PCT, loop of Henle, DCT, and collecting ducts surrounded by capillaries.
• Glomerular filtrate created in renal capsule, glucose and water reabsorbed in PCT.
• Sodium ion gradient in loop of Henle facilitates water reabsorption.
• Further reabsorption occurs in DCT and collecting ducts.