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.
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