Electrolytes
 

Physiology of Sodium

 
 
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Overview

  • Sodium is the principle cation in extracellular fluid. Its extracellular concentration is normally 12 mmol/L, while its extracellular concentration is approximately 140 mmol/L.
    • Roles of Sodium

    • Maintenance of blood volume
    • Maintenance of cellular osmotic pressure gradients
    • Maintance of membrane potentials, and activation of action potentials
    • Nerve conduction
    • Intake

    • Dietary sodium comes from many different sources. Average dietary salt intake is much higher than recommended levels for most western people.
    • Excretion

    • Sodium is primarily excreted by the kidneys; it is freely filtered through the glomerulus and reabsorbed within the proximal convoluted tubule. Sodium is also excreted through sweat, tears, and faeces.

Sodium Homeostasis

  • Pressure natriuresis - increased blood volume and/or pressure will stimulate increased sodium excretion through increased glomerular filtration.
  • Renin-angiotensin-aldosterone system (RAAS) - reduced perfusion within the juxtaglomerular apparatus stimulates renin release, which in turn stimulates the production of angiotensin I. Angiotensin I is converted to angiotensin II by ACE. Angiotensin II modulates pressure natriuresis and increases tubular sodium resorption, both directly and indirectly by stimulating aldosterone.
  • Aldosterone - once secreted by the adrenal cortex will stimulate increased active reabsorption of sodium.
  • Antidiuretic hormone (ADH) - the pituitary gland secretes ADH in response to reduced serum water concentration, which then acts on the distal convoluted tubule to salvage water and concentrate urine.
Last updated on April 15th, 2020
 
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