Structure and Function of Renal Tube

• What are the key mechanisms by which glomerular filtrate is altered to produce urine?

  1. Selective Reabsorption – Movement of essential substances (e.g., water, glucose, ions) from filtrate back into blood.
  2. Secretion – Active transport of substances (e.g., H+, K+, drugs) into tubule.
  3. Osmotic Gradient & Water Reabsorption – Countercurrent mechanisms & ADH control water movement.
  4. Final Modification in Collecting Duct – Regulated by aldosterone and ADH for urine concentration.
• What is the role of the proximal convoluted tubule (PCT) in modifying glomerular filtrate?

  • Major site of reabsorption (~65%)
  • Reabsorbs:
  • 100% glucose & amino acids (via Na+-dependent co-transport)
  • ~65% Na+ & water (via active transport & osmosis)
  • HCO₃⁻ (bicarbonate) for pH balance
  • Secretes: H+ ions, NH₄⁺ (ammonium), some drugs
• How does the loop of Henle contribute to urine concentration?

  • Descending limb: Permeable to water, impermeable to solutes → Water exits, filtrate becomes concentrated.
  • Ascending limb: Impermeable to water, actively pumps Na+, K+, Cl⁻ → Filtrate becomes dilute.
  • Creates osmotic gradient essential for water reabsorption in collecting duct.
• What is the function of the distal convoluted tubule (DCT) in urine formation?

  • Fine-tunes ion and water balance
  • Regulated by hormones:
  • Aldosterone → Increases Na+ reabsorption & K+ secretion
  • Parathyroid hormone → Increases Ca²⁺ reabsorption
  • Secretes H+ for acid-base balance
• How does the collecting duct regulate final urine concentration?

  • ADH (Vasopressin) → Increases aquaporins → More water reabsorbed → Concentrated urine
  • Aldosterone → Enhances Na+ reabsorption → Water follows
  • Urea recycling contributes to medullary osmotic gradient
• What are the primary functions of the renal tubule in relation to the glomerular filtrate?

  The renal tubule reabsorbs essential substances (e.g., water, ions, glucose) back into the blood and secretes waste products (e.g., H+, K+, drugs) into the filtrate for excretion.
• Which parts of the renal tubule are primarily responsible for reabsorption and secretion?

  • Proximal convoluted tubule (PCT): Major site of reabsorption and some secretion
  • Loop of Henle: Water and ion reabsorption
  • Distal convoluted tubule (DCT) & Collecting duct: Fine-tuning of reabsorption and secretion, hormone-regulated
• What percentage of filtrate is reabsorbed in the PCT, and what are the key substances?

  ~65-70% of filtrate is reabsorbed, including:

  • Water (via osmosis)
  • Glucose & amino acids (via secondary active transport with Na⁺)
  • Na⁺, Cl⁻, K⁺, HCO₃⁻ (various transporters)
• How is glucose reabsorbed in the PCT?

  SGLT (sodium-glucose linked transporter) transports glucose into tubule cells with Na⁺, followed by facilitated diffusion into capillaries via GLUT transporters.
•  What substances are secreted into the PCT?

  • H+ ions (for acid-base balance)
  • NH₄⁺ (ammonium)
  • Drugs & toxins (e.g., penicillin)
• How does the loop of Henle contribute to reabsorption?

  • Descending limb: Water reabsorption (permeable to water, not solutes)
  • Ascending limb: Na⁺, K⁺, Cl⁻ reabsorption (impermeable to water, maintains medullary gradient)
• Is there secretion in the loop of Henle?

  Minimal secretion occurs here, mostly urea in the thin ascending limb.
• How does the DCT fine-tune reabsorption?

  • Na⁺ reabsorption (regulated by aldosterone)
  • Ca²⁺ reabsorption (regulated by PTH)
  • Water reabsorption (regulated by ADH)
• What is secreted in the DCT?

  • H+ ions (acid-base balance)
  • K+ ions (via aldosterone-regulated Na+/K+ exchange)
• How does the collecting duct regulate urine composition?

  • Reabsorbs water (ADH-dependent)
  • Secretes H+ and K+
  • Urea recycling contributes to medullary concentration gradient.
• Which hormones regulate renal tubule function?

  • Aldosterone → Increases Na+ reabsorption & K+ secretion in DCT/CD
  • ADH → Increases water reabsorption in the collecting duct
  • PTH → Increases Ca²⁺ reabsorption in DCT
• What is the main difference between active and passive transport in the renal tubule?

  • Active transport requires ATP to move substances against their concentration gradient.
  • Passive transport occurs down a concentration or electrochemical gradient without ATP.
• What are the key locations of active transport in the renal tubule?

  • Proximal tubule: Na⁺ reabsorption via Na⁺/K⁺ ATPase.
  • Loop of Henle (ascending limb): Na⁺/K⁺/2Cl⁻ cotransporter.
  • Distal tubule & collecting duct: Na⁺ reabsorption regulated by aldosterone.
• How does Na⁺/K⁺ ATPase drive active transport in the renal tubule?

  • Located on the basolateral membrane of tubule cells.
  • Pumps 3 Na⁺ out and 2 K⁺ in, creating a sodium gradient.
  • This gradient drives secondary active transport of glucose, amino acids, and other solutes.
• What is secondary active transport in the proximal tubule?

  • Uses Na⁺ gradient from Na⁺/K⁺ ATPase.
  • Na⁺-glucose symporter (SGLT) brings Na⁺ and glucose into the cell.
  • Glucose then exits via facilitated diffusion (GLUT transporters).
• What is passive reabsorption, and where does it occur?

  • Solutes move down their concentration gradient without ATP.
  • Proximal tubule: Water follows Na⁺ via osmosis.
  • Thin descending loop of Henle: Water reabsorbed by osmosis.
  • Distal tubule & collecting duct: Urea reabsorbed passively in medulla.
• How is chloride (Cl⁻) reabsorbed in the renal tubule?

  • Passive: Follows Na⁺ via electrical gradient.
  • Active: Na⁺/K⁺/2Cl⁻ cotransporter in the thick ascending limb.
• Why does water not leave the ascending limb of the loop of Henle?

  • Thick ascending limb is impermeable to water.
  • Active transport of Na⁺, K⁺, and Cl⁻ makes the medulla hypertonic.
  • This sets up an osmotic gradient for water reabsorption in the collecting duct.
• How does ADH (vasopressin) influence passive transport in the collecting duct?

  • ADH inserts aquaporins in the collecting duct.
  • Increases water permeability.
  • Water moves passively down the osmotic gradient into medullary interstitium.
• What transport mechanisms contribute to urine concentration in the renal tubule?

  • Active: Na⁺ reabsorption in the thick ascending limb.
  • Passive: Water reabsorption in the descending limb and collecting duct (with ADH).
  • Passive urea reabsorption in the medulla contributes to hyperosmolarity.
• What is micro puncture and how is it used to study renal transport?

  • A fine glass pipette is used to collect tubular fluid from specific nephron segments.
  • Allows measurement of ion and solute concentrations at different points.
  • Helps determine where active vs. passive transport occurs.
• How does in vivo renal clearance help study transport mechanisms?

  • Measures the rate of excretion of substances (e.g., inulin for GFR, PAH for renal plasma flow).
  • Comparing filtered, reabsorbed, and secreted amounts reveals active and passive transport processes.
• How is radiolabelled tracer analysis used in renal transport studies?

  • Radioactive isotopes (e.g., ³²P-labeled phosphate) track movement of substances across membranes.
  • Helps distinguish active vs. passive transport based on movement patterns.
• What role does patch-clamp electrophysiology play in studying renal transport?

  • Measures ion channel activity in renal tubule cells.
  • Identifies how ions like Na⁺ and K⁺ move through channels (active vs. passive).
  • Used to study the effect of hormones and drugs on transport.
• What are the main structural components of the renal tubule?

  • Proximal Convoluted Tubule (PCT)
  • Loop of Henle (Descending & Ascending limbs)
  • Distal Convoluted Tubule (DCT)
  • Collecting Duct (CD)
• What is the structure and function of the proximal convoluted tubule (PCT)?

  • Structure: Lined with cuboidal epithelial cells with a brush border (microvilli) for increased surface area.
  • Function: Major site of reabsorption (glucose, amino acids, Na⁺, Cl⁻, water, bicarbonate) and secretion (H⁺, NH₄⁺, drugs, toxins).
• How does the descending limb of the Loop of Henle function?

  • Structure: Thin, simple squamous epithelium, highly permeable to water.
  • Function: Water reabsorption via osmosis, concentrating the filtrate (↑ osmolarity). Impermeable to solutes.
•  What is the structure and function of the ascending limb of the Loop of Henle?

  • Thin Ascending Limb: Simple squamous epithelium, impermeable to water, allows passive Na⁺ and Cl⁻ reabsorption.
  • Thick Ascending Limb: Cuboidal cells, actively transports Na⁺, K⁺, Cl⁻ out, creating a dilute filtrate (↓ osmolarity).
• How does the distal convoluted tubule (DCT) contribute to renal function?

  • Structure: Cuboidal epithelium, few microvilli, hormone-sensitive.
  • Function: Fine-tunes ion balance:
  • Reabsorbs Na⁺ & Cl⁻ (regulated by aldosterone).
  • Secretes H⁺ & K⁺ (acid-base balance).
  • Reabsorbs Ca²⁺ (regulated by PTH).
• What is the function of the collecting duct (CD)?

  • Structure: Composed of principal cells (respond to ADH & aldosterone) and intercalated cells (acid-base regulation).
  • Function:
  • Water reabsorption (regulated by ADH).
  • Na⁺ reabsorption & K⁺ secretion (aldosterone).
  • H⁺ & HCO₃⁻ secretion (acid-base balance)
•  Name the four main sections of the Loop of Henle.

  1. Thin descending limb
  2. Thin ascending limb
  3. Thick ascending limb
  4. Medullary interstitium (not part of the loop but essential for function)
• What is the permeability of the descending limb?

  Highly permeable to water but impermeable to solutes (Na⁺, Cl⁻).
• What is the permeability of the ascending limb?

  Impermeable to water but actively transports Na⁺ and Cl⁻ into the medullary interstitium.
• How does the Loop of Henle contribute to urine concentration?

  Through the counter-current multiplier, it creates a hyperosmotic medulla, allowing water reabsorption from the collecting duct via osmosis.
• What is the counter-current multiplier?

  A mechanism in which the Loop of Henle creates a concentration gradient in the medulla by actively transporting Na⁺ and Cl⁻ into the interstitium while preventing water movement in the ascending limb.