Lecture: 21 Sodium

Human Kidney Physiology - Sodium

  • Presentation by: Dr. James Scholey, Professor of Medicine at the University of Toronto

Overview of Sodium in the Body

  • Sodium is essential for maintaining normal blood volume, blood pressure, and organ perfusion.
  • Evolutionally, early humans adapted to low sodium environments; currently, high sodium intake is common and may lead to hypertension.
  • Normal Sodium Intake:
    • Minimum required for life: 10-20 mmol/day (200-450 mg/day)
    • Average in North America: 150 mmol/day (3500 mg/day)
    • Recommended max: 100 mmol/day (2300 mg/day)
    • Recommended intake: 70 mmol/day (1600 mg/day)

Sodium Handling and Regulation by the Kidney

  • Filtered Sodium:

    • The kidney glomerulus filters approximately 150-170 liters of fluid and 22,500 mmoles of sodium daily.
    • >99% of water and sodium are reabsorbed by the kidneys.

  • Nephron Segments:

    • Proximal Tubule: 70% sodium reabsorption
    • Loop of Henle: 20-30% sodium reabsorption
    • Distal Tubule: 5-10%
    • Collecting Duct: 1-3%

Sodium Loss and Gain

  • Loss:

    • Excessive sweating (Na+ = 35 mmol/L)
    • Diarrhea (Na+ = 100-120 mmol/L)
    • Vomiting (Gastric Na+ = 10 mM)
    • Blood loss, osmotic diuresis, diuretic use, and reduced intake.

  • Gain:

    • Primarily from dietary sources: processed foods, fast foods, and high-sodium snacks.

Sensing Sodium Levels

  • The body senses sodium status via several mechanisms:
    • Arterial Baroreceptors: Located in the carotid body and aortic arch, relay volume status to the central nervous system.
    • Atrial Stretch Receptors: Located in the heart, respond to changes in blood volume.
    • Afferent Arteriole: Responds to changes in glomerular filtration.

Renin-Angiotensin System (RAS)

  • The RAS is a key player in the regulation of blood pressure and sodium balance:
    • Renin production is stimulated by:
    • Low arterial pressure (afferent arteriolar stretch)
    • SNS activation (baroreceptors in carotid and aorta)
    • Low dietary sodium intake.
    • Angiotensin II Actions:
    • Enhances sodium reabsorption especially in the proximal tubule via Na-H exchanger (NHE3)
    • Increases activity of sodium chloride co-transporter (NCC) in the distal convoluted tubule.

Collecting Duct Function

  • The collecting duct is responsible for reabsorbing 1-3% of filtered sodium.
    • Transport Protein: Epithelial sodium channel (ENaC)
    • Regulated by Aldosterone: Increases number and activity of sodium channels, enhancing sodium reabsorption while promoting potassium secretion.

Hormonal Regulation of Sodium

  • Aldosterone: Increases sodium retention in the kidney by enhancing sodium transport mechanisms.
  • Atrial Natriuretic Peptide (ANP): Released in response to atrial stretch; acts to inhibit sodium reabsorption in the collecting duct.

Case Studies

  • Cholera in Haiti: Highlighted how severe sodium loss from diarrhea leads to low extracellular fluid volume, necessitating intravenous sodium and water replenishment.
  • Oral Rehydration Solution for cholera includes sodium chloride, glucose, potassium chloride, and sodium citrate.

Sodium Imbalance Implications

  • The body takes time (days) to adjust sodium excretion based on intake changes, which can impact extracellular fluid volume.
  • Conditions of sodium excess (e.g., high sodium intake, dehydration) lead to physiological responses to mitigate imbalances.