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Understanding Selective Reabsorption in Nephron

Aug 17, 2024

Selective Reabsorption in the Nephron

Overview

  • Different parts of the nephron reabsorb different substances:
    • Proximal Convoluted Tubule (PCT): Reabsorbs all glucose and amino acids, most of the water, some salts, and some urea.
    • Loop of Henle: Reabsorbs some water and salts.
    • Collecting Duct: Reabsorbs water.
  • Understanding the process, especially in the PCT, is crucial for exams (6-8 marks in Paper 4).

Focus on PCT

  • Structure: PCT is a tube with a lumen and a wall, surrounded by blood capillaries. Its wall is one cell thick, facilitating easy reabsorption.
  • Components of PCT Cells:
    • Microvilli: Increase surface area and face the lumen.
    • Basal Membrane: Faces the capillaries and contains sodium-potassium pumps and transport proteins for glucose and amino acids.
    • Tight Junctions: Prevent leakage and ensure selective reabsorption through cell membranes.
    • Mitochondria: Provide ATP for active transport.

Reabsorption Process

  1. Active Transport:
    • Sodium-potassium pump uses ATP to actively transport sodium ions into the capillaries, creating a concentration gradient.
  2. Co-Transport:
    • Sodium ions diffuse into the cell, pulling glucose and amino acids via co-transporters.
    • All glucose and amino acids are reabsorbed from the lumen into the cell.
  3. Facilitated Diffusion:
    • Glucose and amino acids move into the blood through transport proteins in the basal membrane.
  4. Osmosis:
    • Water is reabsorbed due to an increase in water potential in the PCT lumen.
  5. Passive Reabsorption of Urea:
    • Urea is passively reabsorbed through simple diffusion.

Adaptations of PCT

  • Tight Junctions: Ensure reabsorption through cell membranes.
  • Microvilli and Basal Membrane: Increase surface area for carrying co-transporters and transport proteins.
  • Mitochondria: Ensure high ATP production for active transport.

Summary of Processes

  • Sodium-potassium pumps create a sodium ion gradient.
  • Co-transport of glucose and amino acids with sodium ions occurs.
  • Facilitated diffusion moves these solutes into the blood.
  • Water follows solutes by osmosis, and some urea is reabsorbed passively.

These notes encapsulate the process of selective reabsorption in the PCT and its physiological adaptations, crucial for understanding nephron function and preparing for related exam questions.

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