Renal Physiology Lecture 1: Process of Renal reabsorption

Question 1

What happens to most filtered fluid?

Answer 1

• ~99% filtered fluid is reabsorbed
• Only 1.5L urine produced

Question 2

Purpose of filtration

Answer 2

• Substantial filtration allows removal of wastes & control of fluid and electrolyte balance
• Conserves critical nutrients & solutes (such as glucose)

Question 3

Overview of Renal Reabsorption

Answer 3

Active transport of solutes out of filtrate and into interstitium

• Water follows by osmosis

Active transport of Na+ from the lumen to interstitium

• Creates transepithelial concentration & electrochemical gradient → lumen becomes more negative than interstitium
• Anions follow the positively charged sodium ions
• Tubular filtrate becomes dilute as solutes leave
  
  • Facilitates water movement from lumen to interstitium
  • Concentrates remaining solutes infiltrate
    
    • Favours movement down a concentration gradient into the interstitium

Question 4

What are the two routes for solutes to travel across the tubular epithelium?

Answer 4

• Transcellular:
  
  • Transport through tubular epithelial cells
• Paracellular:
  
  • Diffusion between epithelial cells → via tight junctions

Question 5

Active Transport of Sodium

Answer 5

• Sodium enters tubular epithelial cell through various membrane proteins
  
  • Apical membrane features antiporters and symporters
  • Sodium Hydrogen exchangers (NHE) and Epithelial sodium channels (ENac) contribute to sodium reabsorption
• Sodium ions flow down the electrochemical gradient
• Sodium concentration in the epithelial cell is low due to the action of basolateral Na-K-ATPase

Question 6

Sodium-linked Secondary Active Transport

Answer 6

• Facilitates sodium-linked secondary active transport of substances against their concentration gradient
  
  • Na moving down electrochemical gradient uses the SGLT protein to pull glucose into the cell against its concentration gradient
  • responsible for others including aa, other ions, metabolites
• Glucose diffuses out of basolateral side via GLUT
• Basolateral Na-K-ATPase maintains low intracellular Na

Question 7

Passive Reabsorption

Answer 7

Substances can undergo passive reabsorption

• i.e. urea → metabolic waste product
  
  • Diffusion through epithelial cell junctions down a concentration gradient
  • Reabsorption of sodium and other solutes drives water reabsorption from the lumen
  • Concentrates urea in the lumen and allows passive reabsorption of urea in to renal interstitium

Question 8

Endocytosis of Protein during reabsorption

Answer 8

Proteins get broken down via enzymes in lysosomes

• Very small amount of protein does enter the tubules
  
  • conserved by reabsorption at proximal tubule
• Receptor-mediated endocytosis conserves the protein at apical membrane
  
  • A negligible amount of protein in the final urine
• Intracellular degradation of proteins to amino acids via enzymes
  
  • Transported into the renal interstitium

Question 9

Saturation of Renal Transport

Answer 9

• If all available transporters are occupied, transport maximum for a substance is reached
  
  • Excess solute is not reabsorbed
  • Will be excreted in the final urine

Question 10

Peritubular Reabsorption of Solute & Water via Bulk Flow

Answer 10

Capillary exchange also governed by the Starling equilibrium

• Low peritubular capillary hydrostatic pressure, higher capillary colloid osmotic pressure
  
  • Favours movement of fluid and solute into peritubular capillaries by bulk flow