Handling Of Sodium Potassium

Application of Filtration, Reabsorption, and Secretion to Specific Ions

Sodium is a critical ion for monitoring because:
- Regulating sodium aids in regulating plasma volume.
- It plays a vital role in extracellular fluid osmotic activity, as sodium retains water.
It is essential to note that while water follows sodium, the reverse is not true.

Relative concentrations of sodium:
- Extracellular: Approximately 145 milliequivalents.
- Intracellular: Approximately 15 milliequivalents.

Sodium is freely filtered by the glomerulus.
The filtration membrane does not obstruct sodium:
- Glomerular capillaries.
- Basement membrane.
- Slits created by podocytes.
Once sodium enters Bowman's space, its concentration matches that of plasma sodium.

Approximately 70% of substances, including sodium, are reabsorbed in the proximal tubule.
Apical membrane of proximal tubule cells allows passive movement of sodium into cells due to concentration gradient.
- Passive transport occurs because sodium concentration is higher in tubular fluid than in intracellular fluid.

Secondary Active Transport (Cotransport):
- Coupling reabsorption of substances with sodium movement.
- Moves substances from lower concentration in tubular fluid to higher concentration inside cells.
Counter Transport:
- Sodium can also facilitate the transport of hydrogen ions in exchange for sodium.

Sodium reabsorption creates an osmotic gradient leading to water reabsorption.
Fluid within the proximal tubule remains iso-osmotic with interstitial fluid due to concurrent reabsorption of both sodium and water.

Glucose utilizes sodium for its reabsorption:
- Glucose is present in lower concentrations in the tubular lumen compared to intracellular levels.
- Sodium movement down its gradient powers glucose cotransport across the apical membrane.
After sodium-driven transport, glucose passes to the interstitial fluid via passive transport through the basolateral membrane.
Water follows glucose due to its high osmotic activity, enhancing water reabsorption.

Transport Maximum: When plasma glucose levels rise above a certain point, glucose transporters are overwhelmed.
- Glucose filtration increases with plasma glucose levels, initially resulting in full reabsorption.
- Once the transport maximum is reached, excess glucose appears in urine, indicating renal glucose threshold.
Conditions such as uncontrolled diabetes lead to elevated glucose in the urine, known as osmotic diuresis:
- Water is lost due to glucose's osmotic activity.

In the thick ascending limb of the loop of Henle, about 25% of sodium is reabsorbed.
Unique cotransporter mechanism:
- Sodium, chloride, and potassium are cotransported.
Water is not reabsorbed here due to the tight junctions between cells.

In the distal nephron (distal tubule and collecting duct), around 5% of sodium is reabsorbed.
Sodium movement mechanism:
- Sodium moves passively into the cells, rising concentrations inside the tubular fluid.
- Active transport via the sodium-potassium pump enables sodium to cross the basolateral membrane.
Regulation of sodium reabsorption occurs here through hormonal influence, primarily aldosterone.

Like sodium, potassium is freely filtered at the glomerulus.
Concentrations:
- Extracellular level: Low in tubular fluid, similar to plasma.
- Intracellular level: High concentration within cells.

Contrary to sodium, potassium is primarily reabsorbed not through transcellular means but via solvent drag:
- As sodium and water are reabsorbed, the concentration gradient for potassium increases, allowing passive movement through leaky tight junctions.
- Approximately 70% of potassium reabsorption occurs in the proximal tubule via this process.
In the thick ascending limb, secondary active transport via the sodium-potassium-chloride cotransporter accounts for an additional 25% reabsorption of potassium.

Regulation of potassium occurs at the distal tubule and collecting duct by:
- Sodium-potassium pump maintaining intracellular potassium levels high.
- Potassium can move from inside the cell back to the tubular lumen, resulting in potassium secretion into the urine.
This potassium secretion can be adjusted based on the body's needs, similarly regulated by aldosterone.