Lecture 4: Renal Physiology and Urine Production

Key Processes in Nephron Function

Filtration and Reabsorption

• Filtration Rate: High rate; volume of filtrate in capsular space exceeds plasma volume in 30 minutes.
• Reabsorption: Nephron reabsorbs 99% of the filtrate.
  • Proximal Convoluted Tubule: Major site of reabsorption; characterized by microvilli increasing surface area.
• Solute Reabsorption Mechanisms:
  • Active Transport: Requires ATP or energy from another ion gradient.
  • Passive Transport: Solutes move down their gradient without energy input.
• Water Reabsorption: Follows solutes via osmosis across semi-permeable membranes.
• Small Protein Recovery: Through pinocytosis, proteins are returned to the blood.

Secretion

• Function: Helps maintain blood pH by secreting hydrogen ions, preventing acidity.
• Elimination: Secretion helps eliminate ammonium ion, creatinine, and potassium.

Transport Processes

• Cellular Pathways:
  • Paracellular: Between cells.
  • Transcellular: Through cells.
• Reabsorption Pathways:
  • Active transport via sodium-potassium pumps; diffusion into interstitial fluid.

Water Reabsorption

• Obligatory Reabsorption: Water follows solutes (e.g., sodium), primarily in proximal convoluted tubule.
• Facultative Reabsorption: Occurs in the collecting duct influenced by antidiuretic hormone (ADH, vasopressin).
  • ADH Effect: Promotes aquaporin insertion in principal cells, enhancing water reabsorption back into blood.

Transport Mechanisms

• Active Transport:
  • Primary: Direct use of ATP (e.g., sodium-potassium ATPase pumps).
  • Secondary: Utilizes energy from ion gradients.
    • Symporters: Ions move in the same direction.
    • Antiporters: Ions move in opposite directions.
  • Transport Maximum: Upper limit of speed for symporters and antiporters.

Solute and Water Reabsorption

• Sodium Reabsorption: Drives reabsorption of water and other solutes.
  • Sodium symporters and antiporters manage sodium and solute reabsorption.
  • Bicarbonate and water are also reabsorbed.
• Glucose and Amino Acids: Reabsorbed in proximal convoluted tubule; reabsorption dependent on sodium.
• Chloride Reabsorption: Passive paracellular reabsorption; influences reabsorption of other positively charged ions.

Pathophysiology

• Glucosuria: Occurs when blood glucose levels exceed transport maximum, often due to diabetes or genetic disorders.

Bicarbonate Reabsorption and Ammonia Secretion

• Bicarbonate Reabsorption: Linked to sodium reabsorption; helps buffer blood.
• Ammonia Production and Secretion:
  • From deamination of glutamine in proximal tubule cells.
  • Ammonia binds hydrogen ions, forming ammonium ions, which are secreted.