Na+ Reabsorption: PCT, Loop, DCT

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Questions and Answers

In the proximal convoluted tubule (PCT), which of the following mechanisms primarily drives $Na^+$ transport across the basolateral membrane?

  • Facilitated diffusion, moving $Na^+$ along its electrochemical gradient.
  • Primary active transport via the $Na^+/K^+$ ATPase pump, moving $Na^+$ against its electrochemical gradient. (correct)
  • Passive diffusion, driven by the concentration gradient of $Na^+$.
  • Co-transport with glucose and amino acids, utilizing a common carrier protein.

What is the significance of the brush border in the proximal convoluted tubule (PCT) concerning $Na^+$ reabsorption?

  • It increases the surface area to enhance facilitated diffusion of $Na^+$ from the lumen into the cell. (correct)
  • It facilitates passive diffusion of $Na^+$ across the basolateral membrane.
  • It decreases the surface area for $Na^+$ reabsorption.
  • It actively secretes $Na^+$ into the tubular lumen.

Which segment of the nephron is impermeable to sodium?

  • Thin Descending Limb of the Loop of Henle (correct)
  • Thick Ascending Limb of the Loop of Henle
  • Distal Convoluted Tubule
  • Proximal Convoluted Tubule

What specific characteristic of the thick ascending limb of the loop of Henle contributes to the hypotonicity of the fluid leaving this segment?

<p>Active reabsorption of $Na^+$, $K^+$, and $Cl^-$ without water reabsorption. (A)</p> Signup and view all the answers

In the distal convoluted tubule (DCT) and collecting tubules, what is the primary mechanism by which aldosterone increases $Na^+$ reabsorption?

<p>By stimulating the $Na^+/K^+$ ATPase pump activity and increasing the number of $Na^+$ channels. (A)</p> Signup and view all the answers

How does the reabsorption of $Na^+$ in the PCT contribute to the reabsorption of other substances?

<p>It creates an osmotic gradient that drives water reabsorption, indirectly concentrating other solutes for reabsorption. (D)</p> Signup and view all the answers

If a drug inhibits the $Na^+/K^+$ ATPase pump in the proximal tubule, what would be the most likely direct effect on $Na^+$ and water reabsorption?

<p>Decreased $Na^+$ reabsorption, leading to increased water excretion. (A)</p> Signup and view all the answers

A patient is diagnosed with a condition that impairs the function of the $Na^+/K^+$/2Cl- cotransporter in the thick ascending limb of the loop of Henle. Which of the following direct consequences would be expected?

<p>Decreased reabsorption of sodium, potassium, and chloride, leading to increased excretion and dilute urine. (C)</p> Signup and view all the answers

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Flashcards

PCT $Na^+$ Reabsorption

About 70% of filtered sodium is reabsorbed here.

Luminal $Na^+$ Transport in PCT

Sodium moves from the lumen to the cell via concentration and electrical gradients.

Basolateral $Na^+$ Transport in PCT

Sodium is moved to the interstitial fluid via $Na^+$-$K^+$ ATPase.

Role of $K^+$ in $Na^+$ Reabsorption

Leaking $K^+$ maintains negativity inside cells, promoting more $Na^+$ entry.

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Results of $Na^+$ Reabsorption in PCT

Facilitates reabsorption of water, glucose, amino acids, and bicarbonate.

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Thin Descending Limb

Not permeable to sodium; leads to hypertonic tubular fluid.

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Thin Ascending Limb

Passively reabsorbs $Na^+$

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Thick Ascending Limb

Actively reabsorbs 20% of $Na^+$ via $Na^+$-2$Cl^-$-$K^+$ cotransporter; impermeable to water.

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Study Notes

  • Renal handling of Na+ is important for maintaining fluid balance and electrolyte balance
  • The body handles sodium reabsorption in three key areas: PCT, Loop of Henle and DCT & collecting tubules

Na+ Reabsorption in PCT (70%)

  • 70% of Na+ load is reabsorbed
  • At the luminal border, Na+ transports from the lumen to inside cells by facilitated diffusion under the influence of:
    • Concentration gradient
    • Electrical gradient, lumen -3 mv and inside cell -70 mv
  • A large surface area of brush border of PCT helps with this process
  • Na+ crosses to interstitial fluid at the baso-lateral border by primary active transport against its electrochemical gradient by Na+ - K+ ATPase activity
    • For each 3 Na+ pumped out only 2 K+ ions are carried in
  • After entering the cell K+ ions diffuse back again to the interstitium, concentration gradient and high permeability of cell membrane helps
    • Maintains the intracellular negativity in relation to luminal fluid which increases Na+ entry to the cell (helps the facilitated diffusion)
  • Na+ reabsorption result in:
    • Reabsorption of 70% of water due to the high osmolality created by Na+ reabsorption
    • Active co-transport of glucose, amino acids, HCO3- and other organic acids
    • Passive diffusion of Cl- (in 2nd half of PCT due to Cl- concentration)

Na+ Reabsorption in the Loop of Henle (20%)

  • 20% of sodium reabsorption occurs

Thin Descending Part

  • The only part of the nephron where Na+ is not reabsorbed
  • Freely permeable to water leading to hypertonic tubular fluid

Thin Ascending Part

  • The only part of the nephron where Na+ reabsorption occurs passively

Thick Ascending Part

  • Active reabsorption of 20% of Na+ by co-transport protein carrier (1 Na+, 2 Cl- and 1 K+)
  • This part is poorly permeable to water, which results in the fluid leaving this thick part being hypotonic

Na+ Reabsorption in the DCT & Collecting Tubules (10%)

  • 10% of Na+ is actively reabsorbed in exchange with H+ or K+ with the help of Aldosterone hormone

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