In-depth Notes: Tubular Reabsorption and Secretion in Renal Physiology

Lecture 44: Tubular Reabsorption and Secretion

Overview of Renal Tubules

• Basic Functions:
  • Creation of urine from glomerular filtrate through three basic processes:
  • Filtration
  • Reabsorption (occurs mainly in proximal convoluted tubule)
  • Secretion (occurs in proximal and distal convoluted tubules)

Learning Objectives and Key Concepts

1. Functions of Renal Tubule

   • Understand that the renal tubule helps in creating urine from glomerular filtrate, primarily through reabsorption and secretion.

2. Proximal Convoluted Tubule (PCT)

   • Major site for reabsorption of:
     • 60-70% of water
     • Nearly all organic substrates (e.g., glucose, amino acids)
     • 60-70% of sodium and chloride ions
   • Anatomy:
     • Lined with a leaky, simple cuboidal epithelium which is enhanced with microvilli, increasing surface area for absorption.

3. Transport Mechanism in PCT

   • Types of Transport Proteins:
     • Transcellular Route: Molecules cross through the epithelial cells (involving channels and carriers)
     • Paracellular Route: Molecules diffuse between cells through tight junctions
   • Reabsorption Process:
     • Active transport for nutrients (e.g., glucose co-transported with sodium via SGLT1/2) and passive movement for water.
     • Osmotic reabsorption occurs after solute reabsorption, pulling water into the interstitial fluid through aquaporins.

4. Diabetes Mellitus

   • Characterized by:
     • Hyperglycemia (high blood sugar)
     • Glycosuria (glucose in urine) leading to polyuria (excessive urination)
   • Mechanism: Saturation of glucose transporters in PCT due to an excess of glucose causes reduced water reabsorption, increasing urine output.
   • In untreatable conditions, urine becomes sweet due to excess glucose, a condition historically noted by physicians.

5. Secretion Processes

   • Substances Secreted by PCT:
     • H+, NH3/NH4+, creatinine, and organic ions (toxins/drugs)
   • Secretion pathways involve various transporters that enable the removal of waste products from blood to tubular fluid.
   • The full knowledge of respiratory and metabolic acidosis and alkalosis mechanisms related to H+ ion transport in kidney function.

Summary

• The PCT carries out a significant role in the absorption and secretion of organic molecules
• Understanding the function of renal tubules is crucial, particularly with respect to pathophysiological processes such as diabetes mellitus, which illustrates the importance of renal function in maintaining homeostasis and managing metabolic waste.