Related Subjects:
| Sodium Physiology
| Hyponatraemia
| Hypernatraemia
| Diabetes Insipidus
Extracellular fluid (ECF) volume is determined by the balance
between sodium intake and renal excretion of sodium.
About
- Main extracellular cation at 135-145 mmol/L
- A chemical element with the symbol Na (from Latin: natrium)
- Atomic number 11.
- Soft, silvery-white, highly reactive metal
- Member of the alkali metals.
Key roles
- Critical for normal osmotic pressure
- Critical to the function of excitable cells
Detection of Blood Volume
- Low-Pressure Baroreceptors: Found in Thorax, on
the venous side of the circulation. Are capable of sensing contraction
and expansion of ECF volume. Lungs, great veins and atria can monitor plasma volume by virtue of large capacitance and distensibility.
- Arterial (High-Pressure) Baroreceptors: Aorta and carotid sinus participate in
the maintenance of ECF volume. In response to changes in
arterial pressure, pulse pressure profile, or vascular capacitance,
afferent impulses travel along the glossopharyngeal
and vagus nerve to an integrative site within the medulla,
where the central sympathetic outflow is tonically inhibited
- Intrarenal Sensing Mechanisms: The kidney contains several types of sensing mechanisms
capable of detecting alterations in ECF volume
Actual or sensed underfilling of the circulation.
- Causes thirst and water intake
- Stimulation of Renin Aldosterone Angiotensin causes sodium uptake in kidney
- Kidneys produce a concentrated small volume of urine
- Angiotensin and Sympathetic stimulation causes vasoconstriction
- Increased activity of sympathetic nerves, and release of circulating catecholamines
- Release of ADH with free water retention
Renal excretion
- Na is freely filtered and reabsorbed in the proximal tubule, distal tubule, and collecting duct
- Presence of Na in distal tubules is necessary to promote K excretion
- Reabsorption regulated by aldosterone and ANP
- Reabsorption in proximal tubule Coupled to the reabsorption
of other solutes
- Reabsorption in the Distal tubule is Coupled to the secretion of K
+ and H+
Pathology
- See hypo and hypernatraemia
Role
- Blood volume maintenance
- Water balance
- Cell membrane potential
- Acid-base balance
- Nerve conduction.
Osmolality
- Sodium and its anions make up about 90% of the total extracellular osmotically active solute.
- Serum osmolality (mOsm/kg H2O) = 2x (Na+ ) + (glucose)/18 + (BUN)/2.8
- For practical purposes, twice the Na+ concentration equals serum osmolality because urea and glucose are responsible for less than 5% of the osmotic pressure.
Control
- Transmembrane potential depends on balance of intra/extracellular K
- Differences in Na/K maintained by Na-K ATPase pump
- The Na/K ATPAse pumps 2 K in and 3 Na out giving a net outward positive charge
Management
- We need intake of 100 mmol/day of Na
References
Revisions