Sodium and Water
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+
Function of Sodium
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.Osmolarity
Changes in sodium concentration affect osmoreceptors and will affect ADH and thirst mechanisms. Osmoreceptors will activate with a 1-2% change in osmolality.Acid-Base balance
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.
Regulation of Sodium and Water
Regulation of any system is typically a balance between input and output:
- Sodium intake is essentially unregulated
- Therefore, sodium concentration is a function of:
- Sodium elimination
- Sodium reabsorption
- Water homeostasis
Control of total body water is a major mechanism to regulate sodium concentration.
Sodium 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
- GIT
- Urine
- Adjust renal elimination is the main mechanism to regulate sodium concentration
Can be performed in two ways:
- Adjust renal elimination is the main mechanism to regulate sodium concentration
Sodium Reabsorption
Given that:
- 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
Water Homeostasis
Body water homeostasis involves:
- Sensors
Osmoreceptors present in the:- Macula densa
- Circumventricular organs
Subfornical organ and the vascular organ of the lamina terminalis.- Change in cellular volume secondary to changes in osmolality alter hormone secretion.
- Effectors
Predominantly hormonal:
References
- Brandis K. The Physiology Viva: Questions & Answers. 2003.
- CICM September/November 2014
- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
- National Research Council. Recommended Dietary Allowances. 10th Ed. 1989. National Academies Press.