Lecture 18: Na+ and Water Balance

Key Topics in Na+ and Water Balance

• Concentrating Urine
  • Importance of Na+ and water balance in urine concentration.

Step 2: Tubular Reabsorption

• Definition: Quick reclamation of tubular contents back to the blood.
• Main Site: Predominantly in the Proximal Convoluted Tubule (PCT).
• Process: Selective transepithelial process.
• Reabsorption Highlights:
  • Almost all organic nutrients (e.g., glucose, amino acids) are reabsorbed.
  • Water and ion reabsorption is hormonally regulated.

Reabsorptive Capabilities of Renal Tubules and Collecting Ducts

• Proximal Convoluted Tubule:

  • Major reabsorption site: 65% of Na+ and water, virtually all nutrients, many ions, and nearly half of urea.

• Nephron Loop:

  • Descending Limb: Water can exit, solutes cannot.
  • Ascending Limb: Water cannot exit, solutes can (thin segment & thick segment specifics outlined).

• Distal Convoluted Tubule and Collecting Duct:

  • Hormonal Regulation:
  • Antidiuretic Hormone (ADH): Promotes water reabsorption through aquaporin insertion.
  • Aldosterone: Stimulates Na+ reabsorption, impacts blood pressure and K+ levels.
  • Atrial Natriuretic Peptide: Decreases blood volume/pressure by reducing Na+.
  • Parathyroid Hormone: Increases Ca2+ reabsorption in DCT.

Juxtaglomerular Apparatus

• Components: Juxtaglomerular (JG) cells (secreting renin for blood pressure) and macula densa (sensing NaCl content).
• Function: Regulates sodium balance and blood pressure.

Relationship Between Plasma Glucose and Glucose Handling

• Diabetes Impact: Uncontrolled diabetes leads to glucose exceeding transport maximum, resulting in glucose in urine.

Step 3: Tubular Secretion

• Process: Opposite of reabsorption; moving selected substances from peritubular capillaries into tubular fluid, mostly in DCT.
• Importance: Disposing of drugs/metabolites, eliminating undesirable substances, regulating K+ and blood pH (via H+ and HCO3−).

Regulation of Urine Concentration and Volume

• Main Function of Kidneys: To maintain body fluid osmotic concentration around 300 mOsm.
  • Osmolality Definition: Number of solute particles in 1 kg of H2O.
  • Mechanisms: Use of countercurrent mechanism.

Mechanism for Forming Dilute Urine

• Factors: Absence of ADH leads to large volumes of dilute urine (e.g., ~100 mOsm) due to decreased aquaporins.

• Explanation: Water reabsorption is minimal in overhydrated conditions, diluting urine further.

Mechanism for Forming Concentrated Urine

• Factors: Presence of maximal ADH results in small volumes of concentrated urine (~1200 mOsm).
  • Urea Role: Contributes to osmotic gradient.

Summary of Key Points

• Na+ reabsorption is active in all tubular segments except the descending limb.
• Water reabsorption occurs via osmosis and depends on Na+ reabsorption through aquaporin channels.
• Aquaporins are expressed in proximal tubules and are only present in collecting ducts in the presence of ADH.