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Kidney Filtration and Reabsorption Overview

Dec 5, 2024

Lecture Notes on Kidney Filtration and Reabsorption

Overview

  • Production of 180 liters of filtrate per day.
  • Reabsorption of 99% of filtrate; excretion of ~2 liters.
  • Sodium reabsorption: 99.5% reabsorbed.
  • Glucose should be fully reabsorbed in healthy individuals; no glucose in urine.
  • Urea: major waste product, some reabsorbed to help manage pH and osmotic gradient.

Types of Water Reabsorption

  • Obligatory Water Reabsorption:
    • Occurs in proximal convoluted tubule and loop of Henle (85%).
    • Cannot be altered by homeostatic mechanisms.
  • Facultative Water Reabsorption:
    • Occurs in distal convoluted tubule and collecting duct (15%).
    • Can be adjusted.

Mechanisms of Reabsorption

  • Water Reabsorption:

    • Always occurs by osmosis.
    • Sodium is the main solute that drives water reabsorption.
  • Sodium Reabsorption in Proximal Convoluted Tubule:

    • Involves sodium-potassium pump (requires ATP).
    • Sodium is actively transported out of the cell, potassium in.
    • Secondary active transport: sodium co-transports glucose into the cell.
    • Facilitated diffusion allows glucose to exit the cell.
  • Sodium Reabsorption in Loop of Henle:

    • Ascending limb is impermeable to water.
    • Sodium and chloride are reabsorbed through sodium-potassium-chloride co-transport.
    • Solutes stay in the medulla to create a high solute concentration for osmosis.
  • Sodium Reabsorption in Collecting Duct:

    • Sodium channels allow sodium to leave, some reabsorbed into blood.
    • High solute concentration in medulla aids water reabsorption.

Role of Antidiuretic Hormone (ADH) and Aquaporins

  • ADH regulates the number of aquaporins in collecting duct cells.
  • Increased ADH increases aquaporin numbers, leading to more water reabsorption.
  • Solute concentration in blood is the stimulus for ADH release.
  • Inverse relationship between aquaporins/ADH levels and urine volume.

Glucose Reabsorption

  • Sodium-glucose transporters in proximal convoluted tubule reabsorb glucose.
  • Fixed number of transporters; excess glucose results in glucose in urine when transport maximum is exceeded.

Secretion Process

  • Substances leave blood and enter filtrate to be excreted in urine.
  • Includes medications, drugs, and hydrogen ions (for pH regulation).

Important Concepts

  • Countercurrent multiplier system plays a role in maintaining osmotic gradient.
  • Diabetes insipidus is related to a deficiency of ADH, leading to excessive urination and dehydration.