Sodium Balance and Reabsorption Quiz

Sodium Balance

• When sodium intake is less than sodium excretion, the body is in a negative sodium balance, leading to sodium loss.
• When sodium intake is more than sodium excretion, the body is in a positive sodium balance, leading to sodium retention.

Sodium Reabsorption

• The majority of sodium reabsorption takes place in the proximal convoluted tubule (PCT), where 2/3 of sodium is reabsorbed.
• In the PCT, water reabsorption is linked to sodium reabsorption through an isosmotic mechanism.
• The thick ascending loop of Henle reabsorbs ¼ of the filtered sodium load without any water reabsorption.
• The final stage of sodium reabsorption in the distal convoluted tubule (DCT) and collecting duct (CD) is regulated by aldosterone.

Early Proximal Tubule

• Most essential solutes are reabsorbed in the early proximal tubule.

• Many secondary active transporters are utilized, driven by the sodium gradient.

• Cotransport mechanisms:

  • Sodium-glucose (SGLT)
  • Sodium-amino acid
  • Sodium-lactate, citrate, or phosphate

• Sodium moves into the cell down its electrochemical gradient coupled to these solutes (apical membrane)

• Sodium is removed from the cell via the Na+/K+ - ATPase.

• The solutes exit via facilitated diffusion.

• Sodium/Hydrogen (Na+/H+) exchanger and bicarbonate diffusion are key for acid/base balance.

Late Proximal Tubule

• This region primarily reabsorbs sodium chloride (NaCl).
• Sodium/Potassium ATPase (Na+/K+-ATPase) drives NaCl reabsorption.
• Thiazide diuretics act on this region, inhibiting NaCl absorption. Examples include:
  • Chlorothiazide
  • Hydrochlorothiazide
  • Metalozone

Late Distal Tubule & Collecting Duct

• Approximately 3% of sodium is reabsorbed in this region.
• Epithelial sodium channels (ENaCs) are present on the apical membrane.
• Sodium enters the channels down its electrochemical gradient and exits via the Na+/K+-ATPase (basolateral membrane).
• Potassium-sparing diuretics like amiloride and spironolactone inhibit sodium absorption in this region, leading to mild diuresis.
• Aldosterone increases sodium reabsorption in this region by:
  • Increasing ENaC channel production and insertion
  • Increasing Na+/K+-ATPase protein expression

Tubular Maximum (Tm)

• The maximal rate of solute transport is called Tm. Measured in mg/min.
• Saturation of transporters causes Tm.
• Below Tm, all filtered load is reabsorbed.
• Above Tm, solute is excreted.
• Tms are found in the proximal tubule.

Glucose Reabsorption

• Glucose is filtered across the glomerulus and reabsorbed in the proximal tubule through a two-step process:

  • Sodium-glucose cotransporter (SGLT)
  • Facilitated diffusion

• The energy driving this process comes from the sodium/potassium ATPase.

• The SGLT is a secondary active transport system that is saturated at a certain level.

• Splay is the portion of the titration curve where reabsorption approaches full saturation but is not quite there yet.

• Glucose excretion (glucosuria) occurs when the plasma glucose concentration exceeds the Tm.

• Heterogeneity of nephrons and differences in Tm across nephrons contribute to splay.

Glucosuria (Glucose Excretion)

• Normally, all filtered glucose is reabsorbed, with none excreted.
• Uncontrolled diabetes mellitus leads to increased plasma glucose concentrations, resulting in a filtered load that exceeds reabsorptive capacity. Glucose is then excreted.
• Genetic abnormalities in the SGLT (sodium-glucose cotransporter) can result in decreased Tm and glucose excretion.