Describe the function, distribution, regulation and physiological importance of sodium
, chloride, potassium, magnesium, calcium and phosphateions
Normal total body Na+ is 60mmol.kg-1 70% of which is exchangeable. Total body Na+ is distributed as:
- 50% in ECF
Sodium is the dominant extracellular cation.
- Typical ECF [Na+] of 140mmol.L-1.
- 45% in bone
- 5% in ICF
A minor intracellular cation.
- ICF [Na+] varies with cell type, but is typically 12-20mmol.L-1.
- Concentration is kept low by the action of the 2Na+-3K+ ATPase exchange pump and the low permeability of the cellular membrane to Na+
Regulation of ECF volume
Principal ECF cation. Changes in sodium levels cause compensatory fluid shifts. Loss of sodium content will result in hypotension/hypovolaemia, with consequent baroreceptor stimulation and activation of the RAAS. Baroreceptors will activate with a 7-10% change in volume.
Changes in sodium concentration affect osmoreceptors and will affect ADH and thirst mechanisms. Osmoreceptors will activate with a 1-2% change in osmolality.
Na+-H+ exchange pumps in the kidney are stimulated in acidosis.
Resting Membrane Potential
Alterations in sodium concentration will affect intracellular potassium to a similar degree, which will alter the RMP.
Total body Na+ content is regulated to within 2%. Regulation of any system is typically a balance between input and output, however sodium intake is essentially unregulated. Sodium is therefore regulated by adjusting methods of elimination.
Sodium is eliminated in:
- Sweat and GIT
Obligatory and not amenable to regulation.
- Acclimatisation to hot environments improves the efficiency of sweating by reducing its tonicity, reducing sodium loss
Regulation of sodium is predominantly via adjusting renal elimination, and is mediated primarily by aldosterone.
Urinary Elimination of Sodium
Urinary elimination can be adjusted in two ways:
Changes in sodium reabsorption
This is the main mechanism for controlling sodium in euvolaemia.
- Normal glomerular filtrate is ~180L.day-1
- The dominant osmole in glomerular filtrate is sodium
- Normal urine output is ~1.5L-1
The majority of filtered sodium must be reabsorbed. This is called bulk reabsorption and occurs in the PCT and LOH:
- 60% of total reabsorption is by the Na+-K+ ATPase pump in the PCT
- 30% of total reabsorption is by the Na+-K+-2Cl- co-transporter in the LOH
The remaining 10% of sodium reabsorption occurs in the DCT and CT. As it is under the influence of aldosterone, it is the component which is important in regulation. Aldosterone increases Na+ reabsorption by increasing the number or activity of these pumps:
- Na+-Cl- pumps in the DCT
- Na+-K+ ATPase pumps in principal cells of the DCT
- Na+-H+ pumps in intercalated cells of the CT