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.