Reabsorption and Regulation

NEPHRON IN A NUTSHELL

  • Process Overview
    • Blood enters the nephron and undergoes filtration and reabsorption to produce urine.
    • Waste products and excess substances are excreted as urine.
    • The nephron features several key components essential for filtration and reabsorption:
    • Bowman's Capsule
    • Glomerulus
    • Proximal Convoluted Tubules (PCT)
    • Descending Limb of the Loop of Henle
    • Distal Convoluted Tubules (DCT)
    • Ascending Limb of the Loop of Henle
    • Collecting Duct

COMPONENTS OF THE NEPHRON

  • Bowman's Capsule: Initiates the filtration process.
  • Glomerulus: A tangled network of capillaries where blood filtration occurs.
  • Proximal Convoluted Tubules (PCT):
    • Reclaims most tubular contents and returns them to the blood through a selective transepithelial process.
    • Around 65-70% of sodium and water reabsorption occurs here.
    • 100% glucose and amino acids are reabsorbed in PCT.
  • Descending Limb:
    • Highly permeable to water (20% reabsorption).
    • As water leaves, osmolarity of fluid increases.
  • Ascending Limb:
    • Impermeable to water.
    • Pumps out NaCl, leading to more dilute fluid.
  • Distal Convoluted Tubules (DCT):
      • Early: Similar to thick ascending limb, impermeable to water.
      • Late: Influenced by antidiuretic hormone (ADH) for osmolarity adjustments.
  • Collecting Duct:
    • Final site for urine processing and influenced by hormones (ADH and aldosterone) to regulate water and sodium reabsorption.

TUBULAR REABSORPTION

  • Definition: Tubular reabsorption is the process by which necessary substances and water are reclaimed from the filtrate back into the blood.

  • Mechanism:

    • Active and Passive Transport:
    • Primary active transport (i.e., Na-K ATPase).
    • Secondary active transport with co-transport for sugars and certain ions.
    • Passive transport for lipid-soluble substances and certain ions.

  • Affecting Components:

    • Aquaporins enhance water transport.
    • Ion channels and transport proteins facilitate the movement of sodium ions.

LOOP OF HENLE CHARACTERISTICS

  • Thin Descending Limb:
    • Water is reabsorbed, osmolarity increases.
  • Thin Ascending Limb:
    • Impermeable to water, primarily sodium and chloride ions are reabsorbed.
  • Thick Ascending Limb:
    • Active transport of sodium, potassium, and chloride.
    • Dilutes the fluid as it ascends.
  • Significance: Loop of Henle creates a concentration gradient essential for maximizing water reabsorption in the collecting duct.

DISTAL TUBULE

  • Early Distal Tubule: Similar to thick ascending limb, critical for sodium reabsorption without water permeation.
  • Late Distal Tubule:
    • Na+ reabsorption and K+ secretion modulated by aldosterone.
    • H+ reabsorption and K+ secretion occur in Type A intercalated cells, aiding in pH regulation.

COLLECTING DUCT

  • Function:
    • Processes urine, carries filtrate through medulla to the ureter.
    • Regulates water retention based on ADH presence.
    • Can reabsorb 20% more water from filtrate during hormonal signaling.

TUBULAR SECRETION

  • Definition: Reverse reabsorption process that eliminates waste.
  • Occurs Mainly In: Proximal Tubule.
  • Substances Secreted:
    • K+, H+, NH4+, creatinine, and organic acids and bases.
    • Some substances synthesized in tubule cells (e.g., HCO3−).

OVERVIEW OF REABSORPTION AND SECRETION

  • Cortex:
    • 65% of filtrate volume reabsorbed (H2O, Na+, HCO3−, glucose, amino acids).
    • Regulated Reabsorption: Na+ (aldosterone regulates), Ca2+ (parathyroid hormone).
    • Secretion of H+ and NH4+.
  • Outer Medulla:
    • Primarily water reabsorption.
  • Inner Medulla:
    • Regulated secretion (K+ by aldosterone) and urea handling.

REGULATION OF URINE CONCENTRATION AND VOLUME

  • Countercurrent Multiplier: Long nephron loops of juxtamedullary nephrons create an osmotic gradient through fluid flow and pump activity.
  • Properties include:
    • Filtrate flows in opposite directions through two parallel nephron sections.
  • Osmotic Gradient: Established with water reabsorption in the descending limb and salt pumping in the ascending limb.

COUNTERCURRENT EXCHANGE

  • Vasa Recta's Role:
    • Preserve osmotic gradient; highly permeable to fluids, allowing for balance without disrupting osmotic conditions.
  • Mechanism: Fluid flow in the vasa recta having countercurrent exchange with surrounding interstitial fluid.

HORMONAL REGULATION OF NEPHRON FUNCTION

  • Antidiuretic Hormone (ADH):
    • Promotes water reabsorption in response to elevated extracellular osmolarity by making the collecting ducts more permeable to water.
    • Stimulates aquaporin production enhancing water transport capabilities.
  • Renin-Angiotensin-Aldosterone System (RAAS):
    • Triggered by decreases in blood flow and pressure, leading to Na+ and water retention, vasoconstriction to elevate blood pressure.
  • Atrial Natriuretic Peptide (ANP):
    • Released in response to atrial stretch, inhibiting Na+ and water reabsorption, promoting diuresis.

RESPONSES TO VOLUME CHANGES

  • Volume Contraction:
    • Triggers various hormonal pathways to retain sodium and water through ADH and aldosterone.
  • Volume Expansion:
    • Promotes excretion of sodium and water through ANP and inhibited renin and aldosterone activity.