67. Physiology - Distal Na+, K+ Handling
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Questions and Answers

Which transporters are primarily responsible for sodium chloride reabsorption in the distal tubule cells?

  • Sodium/proton exchanger and K+/Na+ transporter
  • Sodium-glucose co-transporter and Na+/Ca2+ exchanger
  • Sodium-Potassium-ATPase and Na+/Cl- co-transporter (correct)
  • Na+/H+ antiporter and Na+/K+ pump

Which cell type is primarily responsible for hydrogen ion secretion in the distal nephron?

  • Intercalated cells (correct)
  • Principal cells
  • Podocytes
  • Mesangial cells

What is the primary distinction between titratable acids and ammonium excretion?

  • Titratable acid excretion requires the presence of fixed buffers, whereas ammonium excretion does not. (correct)
  • Titratable acids are related to renal metabolism, while ammonium is primarily a waste product.
  • Titratable acids involve bicarbonate, while ammonium does not.
  • Ammonium is produced in the proximal tubule, titratable acids are produced in the collecting duct.

How does total body potassium excess influence hydrogen ion handling by the nephron?

<p>It reduces hydrogen ion secretion. (D)</p> Signup and view all the answers

Which factor is NOT involved in modulating distal nephron sodium reabsorption?

<p>Urea concentration (C)</p> Signup and view all the answers

How does inhibition of carbonic anhydrase affect the formation of H+ ions?

<p>It decreases the secretion of H+ ions. (A)</p> Signup and view all the answers

What is the lowest possible pH that can be established in the urine due to primary active transport in the distal tubule?

<p>4.4 (A)</p> Signup and view all the answers

Which factor favors the secretion of H+ ions in the renal tubule?

<p>Increased negativity of tubular fluid (TF) (B)</p> Signup and view all the answers

What is a significant error in understanding renal acid-base handling?

<p>Confusing H+ excretion with H+ secretion (A)</p> Signup and view all the answers

During the secretion of H+ ions, which components in tubular fluid lead to the formation of titratable acid?

<p>HPO42- and H2PO4- (D)</p> Signup and view all the answers

What is the primary function of distal convoluted tubule cells?

<p>Reabsorption of Na+ and Cl- (B)</p> Signup and view all the answers

Which of the following best describes the basolateral membrane potential of distal convoluted tubule cells?

<p>-80 mV (D)</p> Signup and view all the answers

The Na,Cl co-transporter in distal convoluted tubule cells is sensitive to which type of diuretic?

<p>Thiazide diuretics (D)</p> Signup and view all the answers

What maintains the low intracellular Na+ concentration in distal convoluted tubule cells?

<p>Na,K ATPase (A)</p> Signup and view all the answers

Which cell type in the distal nephron is responsible for secreting K+?

<p>Principal cells (B)</p> Signup and view all the answers

Which ions are reabsorbed in the distal convoluted tubules through the Na,Cl co-transporter?

<p>Na+ and Cl- (C)</p> Signup and view all the answers

What is the process through which water reabsorption is primarily regulated in the distal nephron?

<p>Sodium absorption (C)</p> Signup and view all the answers

What role does Cl- play in the reabsorption process in distal convoluted tubules?

<p>Cl- is reabsorbed passively down its transcellular electrical gradient (B)</p> Signup and view all the answers

What is the primary process by which the kidney excretes acid in the form of phosphates and ammonium?

<p>Secretion of H+ that meets phosphates or ammonia (C)</p> Signup and view all the answers

How does the kidney replace the bicarbonate (HCO3-) consumed in buffering acids from protein metabolism?

<p>Through excretion of acids and reabsorption of bicarbonate (B)</p> Signup and view all the answers

What role does carbonic anhydrase play inside renal intercalated cells?

<p>It facilitates the formation of bicarbonate from CO2 and H2O (D)</p> Signup and view all the answers

What is the result of the bicarbonate that is formed in renal intercalated cells?

<p>It enters the bloodstream as 'new' HCO3- (D)</p> Signup and view all the answers

Why is the replacement of bicarbonate important for maintaining acid-base balance?

<p>To prevent the accumulation of acids produced by metabolism (A)</p> Signup and view all the answers

What happens to CO2 produced during acid buffering in the extracellular fluid (ECF)?

<p>It is expelled through the lungs (A)</p> Signup and view all the answers

In the buffering process, what does H+ react with to produce either phosphates or ammonia in the renal tubular fluid?

<p>Bicarbonate (A)</p> Signup and view all the answers

What is meant by 'new' HCO3- formed in the kidneys?

<p>Bicarbonate not previously present in the kidney (C)</p> Signup and view all the answers

What happens to renin levels when there is an increased NaCl load to the macula densa?

<p>Renin levels decrease (B)</p> Signup and view all the answers

How does a low glomerular filtration rate (GFR) affect Na+ reabsorption during hemorrhage?

<p>Na+ reabsorption decreases (B)</p> Signup and view all the answers

What is the primary role of the kidney in relation to blood pressure and volume?

<p>To regulate Na+ reabsorption (D)</p> Signup and view all the answers

What occurs when there is an increased Na+ load delivered to the principal cell?

<p>Na+ reabsorption increases (D)</p> Signup and view all the answers

Which factor might override the effects of aldosterone on Na+ reabsorption?

<p>Increased Na+ load (B)</p> Signup and view all the answers

What is the potential consequence of high Na+ reabsorption in the distal nephron?

<p>Increased K+ secretion (D)</p> Signup and view all the answers

Why might the kidney struggle to predict the net result of simultaneous signals from Na+ load and aldosterone?

<p>The stimuli can provide opposing messages (B)</p> Signup and view all the answers

What physiological phenomenon is compared to the kidney's processing of conflicting inputs?

<p>Decision-making under pressure (C)</p> Signup and view all the answers

What is the primary role of aldosterone in the kidney regarding potassium and sodium?

<p>It stimulates both sodium reabsorption and potassium secretion. (D)</p> Signup and view all the answers

Which buffers are primarily utilized in the distal nephron for H+ secretion?

<p>Ammonia, phosphate, and bicarbonate (D)</p> Signup and view all the answers

How is bicarbonate (HCO3-) reabsorbed in the distal nephron?

<p>In exchange for chloride ions at the basolateral membrane. (C)</p> Signup and view all the answers

What is the role of the H+-ATPase in the distal nephron?

<p>It allows the reclamation of bicarbonate and excretion of H+. (B)</p> Signup and view all the answers

What happens when H+ is secreted into the tubular fluid?

<p>It binds to bicarbonate, forming carbon dioxide and water. (B)</p> Signup and view all the answers

Which statement about the handling of HCO3- in the distal nephron is true?

<p>The last HCO3- is reclaimed through ATPase activity without traversing the apical membrane. (C)</p> Signup and view all the answers

What is the lowest urine pH that can be achieved due to urinary acidification?

<p>4.4 (B)</p> Signup and view all the answers

Which of the following components is not involved in the buffering process within the distal nephron?

<p>Urea (C)</p> Signup and view all the answers

Flashcards

Distal tubule sodium reabsorption

The process of reclaiming sodium ions from the filtrate in the distal tubule.

Principal cells in distal tubule

Epithelial cells that are involved in sodium, potassium transport, and water balance.

Intercalated cells

Distal tubule and collecting duct cells crucial in acid/base balance.

Ammonium excretion mechanism

Way the kidney excretes excess acid. Ammonium is produced and excreted in urine.

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Potassium balance & hydrogen secretion

Potassium balance affects how hydrogen ions are secreted, influencing body pH.

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Distal Tubule Cell Function

Reabsorbs sodium (Na+) and chloride (Cl-) ions.

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Na,K ATPase Role

Maintains low intracellular sodium (Na+) levels crucial for sodium reabsorption.

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Principal Cell Function

Reabsorbs sodium (Na+) and water (H2O); secretes potassium (K+).

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Intercalated Cell Function

Secretes hydrogen (H+) and potassium (K+); some types reabsorb K+.

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Thiazide-Sensitive Co-transporter

A type of sodium-chloride co-transporter (NCC) found in the distal tubule, targeted by thiazide diuretics.

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Distal Convoluted Tubule

Part of the nephron, distal to the macula densa, involved in fine-tuning solute and water reabsorption and secretion.

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Electrochemical Gradient

The combination of a concentration gradient and an electrical gradient that drives movement across cell membranes.

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Proximal Tubule Role

Handles bulk of solute and water reabsorption/secretion in the nephron.

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Na+ Load Effect on Principal Cells

Increased sodium load in the tubular fluid directly increases sodium reabsorption by principal cells, independent of the renin-angiotensin-aldosterone system.

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Conflicting Signals in Kidney

The kidney's tubules can receive opposing signals regarding sodium reabsorption, such as low GFR (reducing sodium load) and high aldosterone (increasing reabsorption).

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GFR and Na+ Reabsorption Relationship

A low glomerular filtration rate (GFR) leads to a decreased delivery of sodium to the distal nephron, potentially resulting in decreased sodium reabsorption.

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Aldosterone and Na+ Reabsorption

Aldosterone increases sodium reabsorption in the distal tubules, counteracting potential reductions in sodium reabsorption due to low GFR.

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Sodium Reabsorption Priority

The kidney prioritizes maintaining blood pressure and volume, often favoring sodium reabsorption even if it means increased potassium loss.

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High GFR and Distal Tubule

A high glomerular filtration rate (GFR) leads to more sodium being delivered distally, increasing sodium reabsorption but possibly increasing potassium loss.

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Macula Densa and Renin

Increased sodium chloride load to the macula densa decreases renin production, which in turn lowers angiotensin II and aldosterone, decreasing sodium reabsorption.

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Principal Cell and Increased Na Load

Increased sodium load directly increases sodium reabsorption in principal cells, regardless of other hormonal influences.

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Hyperkalemia

A condition where there is an abnormally high level of potassium in the blood.

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Aldosterone's role in hyperkalemia

When aldosterone levels are high, it stimulates sodium reabsorption and potassium secretion, leading to increased potassium excretion in urine.

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Distal nephron's duties

The distal nephron is responsible for: 1. Reclaiming all filtered bicarbonate (HCO3-) 2. Excreting excess hydrogen ions (H+) generated by metabolism.

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H+ secretion mechanism

Hydrogen ions (H+) are actively transported into the tubular fluid via H+-ATPase and exchanged for potassium (H,K-ATPase).

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Urinary acidification

H+ secretion creates a large pH gradient, allowing urine to become acidic (pH as low as 4.4).

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Distal nephron buffers

Bicarbonate (HCO3-), phosphates (HPO42- , H2PO4-), and ammonia (NH3) act as buffers in the distal nephron, helping to maintain pH balance.

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HCO3- reclamation in distal nephron

The last remaining HCO3- in the tubular fluid is reclaimed by combining with secreted H+ to form CO2 and H2O, which are then reabsorbed.

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Distal nephron's role in pH regulation

The distal nephron plays a crucial role in maintaining acid-base balance by fine-tuning HCO3- reabsorption and H+ excretion, ensuring proper pH levels in the body.

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H+ Secretion Inhibition

Inhibition of carbonic anhydrase activity slows the formation of H+ ions, reducing their secretion and leading to acidosis.

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Filtered vs. Excreted H+

The kidney secretes much more H+ than it excretes. Most secreted H+ reacts with bicarbonate in the tubular fluid, forming CO2 and water, which are reabsorbed.

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Titratable Acid

H+ ions react with phosphate buffers (HPO42- and H2PO4-) in the tubular fluid, forming titratable acid (H2PO4- and H3PO4) that is excreted in urine.

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Ammonium Excretion

Ammonia (NH3) in the tubular fluid reacts with H+ to form ammonium ions (NH4+) which are excreted in urine.

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H+ Secretion Location

While the proximal tubule handles the bulk of H+ secretion, the distal tubule establishes a significant pH gradient by actively transporting H+ into the tubular fluid.

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How does the kidney remove acid (H+)?

The kidney gets rid of excess acid (H+) by producing bicarbonate (HCO3-) and secreting it into the blood. It achieves this by two processes: excreting acid in the form of phosphates and ammonium, while reabsorbing HCO3-. This ensures that H+ is removed from the body and proper pH is maintained.

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What is the role of intercalated cells in acid excretion?

Intercalated cells are specialized cells in the distal tubule and collecting duct that actively secrete H+ ions into the tubular fluid. They also reabsorb HCO3- ions back into the bloodstream, playing a crucial role in regulating blood pH.

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How does the kidney replace HCO3- used up in buffering?

The kidney replaces lost HCO3- by secreting H+ ions into the tubular fluid, where they bind to phosphates or ammonia. This forms acidic compounds that are trapped and excreted in the urine. Simultaneously, the kidney reabsorbs newly generated HCO3- back into the blood, effectively replacing the lost HCO3-.

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What is the difference between HCO3- reabsorption and HCO3- generation?

HCO3- reabsorption simply means the kidney reclaims HCO3- already present in the filtrate back into the bloodstream. HCO3- generation involves the kidney actively creating NEW HCO3- from other molecules, which is then transported into the blood.

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How does the kidney get rid of the acid (H+) produced by protein metabolism?

The kidney's main strategy is to eliminate this acid by generating new HCO3- through the processes described above (excretion of phosphates & ammonium, and HCO3- reabsorption). This prevents the body from becoming too acidic.

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Why is the kidney's role in acid-base balance crucial?

The kidney is essential for maintaining the body's pH balance. The kidney's ability to regenerate HCO3- and eliminate acid ensures that the body's fluids remain within a narrow, healthy pH range. This is vital for the proper functioning of enzymes, cells, and overall metabolism.

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Study Notes

Learning Objectives

  • Distal tubule and collecting duct function in sodium chloride reabsorption
  • Distal tubule cell transporters for sodium and potassium
  • Principal cell channels for sodium and potassium transport
  • Inhibition of transporters affecting diuresis
  • Mechanisms modulating distal nephron sodium reabsorption/potassium secretion/reabsorption

Acid Handling

  • Cell types involved in hydrogen ion secretion/reabsorption in distal tubule and collecting duct
  • Differentiation between titratable acid and ammonium excretion
  • Distinction between acid excretion and secretion
  • Calculation of net acid excretion
  • Source of renal ammonium and its processing by nephron segments
  • Factors affecting hydrogen ion secretion and excretion (pCO2, pH, carbonic anhydrase, filtered buffers, electrical potential)

Relationship Between Potassium and Hydrogen

  • Effect of potassium secretion/reabsorption on hydrogen ion secretion
  • Concept of total body potassium deficit/excess and its effects on hydrogen ion handling
  • Impact of body pH on total body potassium content and plasma potassium concentration

Overview of Tubular Solute Handling

  • Sodium (Na+) reabsorption and potassium (K+) secretion/reabsorption in early and distal nephron segments
  • Quantitative and regulatory aspects of water reabsorption/secretion in early and distal nephron segments
  • Role of H+ in regulating urinary pH in distal nephron
  • Proximal tubule's role in handling the bulk of solute/water

Distal Convoluted Tubule

  • Main function: Na+ and Cl- reabsorption
  • Impermeable to water
  • Membrane potential: basolateral -80 mV (K+ potential), apical -55 mV (net, Na+ and K+ potentials)
  • Sodium chloride co-transporter (NCC)
  • Na,K ATPase and chloride channels on basolateral membrane

Na,Cl Co-transporter

  • Inhibition by thiazide diuretics for hypertension treatment

Principal Cell

  • Reabsorption of Na+, Cl-, and H₂O
  • Secretion of K+
  • Basolateral Na,K-ATPase and K+ channels
  • Apical Na+ channels (ENaC)
  • Low intracellular Na+ and negative PD facilitate Na+ entry
  • High intracellular K+ facilitates K+ secretion

Paracellular Pathway

  • Chloride driven by lumen negative transcellular voltage

Factors Affecting Distal Na+ Reabsorption and K+ Secretion

  • Aldosterone: synthesized in adrenal cortex, increases in response to angiotensin II, ↑Na+ reabsorption, ↑K+ secretion, regulated by blood pressure and volume alterations
  • Na+ load (delivery): ↑Na+ load ↑ Na+ reabsorption and ↑K+ secretion, ↓Na+ load ↓Na+ reabsorption and ↓K+ secretion.
  • Non-reabsorbable anions (e.g., phosphate, ketones, penicillins): more negative tubular fluid, inhibit Na+ reabsorption and promote K+ secretion.
  • Distal tubular fluid flow rate: ↑flow rate ↑bending of luminal cilia, ↑secretion of K+, independent of other factors.

Effect of Na+ Load

  • Independent of renin-Ang II-aldosterone system
  • Increased Na+ load leads to increased Na+ reabsorption and K+ secretion.
  • Influence of GFR changes.
  • Effect of disturbances in body volume status

Cellular Dilemma (Principal Cell)

  • Conflicting inputs affecting principal cell function
  • Role of opposing messages in regulatory mechanisms of the kidney

Non-Reabsorbable Anions

  • Phosphate, ketones, penicillins, bicarbonate
  • Impact on transtubular potential difference (PD) and Na+ & K+ handling.

Distal Tubular Flow Rate

  • Increased flow bends cilium, leading to opening of Ca2+-sensitive K+ channels
  • Higher plasma [K+], ↑K+ secretion
  • Increased flow rate promotes K+ secretion

Two More Roles for Kidney

  • Reclamation of all filtered HCO₃
  • Excretion of H⁺ generated by metabolism
  • Luminal active transport, electrical gradient, and H+/K+ exchange.

Five Factors Affecting H+ Secretion

  • Partial pressure of CO₂
  • Cell pH
  • Carbonic anhydrase activity
  • Amounts of filtered/secreted buffers
  • Electrical potential difference

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Description

This quiz explores the critical functions of the distal tubule and collecting duct in sodium chloride reabsorption and acid-base balance. It covers mechanisms of potassium and hydrogen ion transport, including the role of various transporters and factors influencing their activity. Assess your understanding of renal processes essential for maintaining electrolyte and acid-base homeostasis.

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