Sodium distribution

Extracellular vs intracellular

Extracellular (plasma) concentration
= 140 mmol/L

Intracellular concentration
= 10 mmol/L

Interstitial concentration
= 140 mmol/L

NB:

[KB2:p13]

Interstitial concentration of protein is lower than plasma concentration

According to Gibbs-Donnan equilibrium
--> Sodium concentration in ISF should be lower

In fact, this is the case
--> Sodium concentration in the plasma water (i.e. without proteins) is about 6 or 7 mmol/L higher than ISF
--> But overall concentration is roughly the same

Body sodium distribution

[KB2:p14]

Total body Na+
= 60mmol/kg

• Extracellular fluid = 50%
• Bone = 45%
• Intracellular fluid = 5%

NB:

• Exchangeable Na+ is 70% of total sodium
  * Measured using ²⁴Na+
• Non-exchangeable sodium is mostly in bone crystal

NB:

Also see Daily balance of sodium and water