Lecture 29 - Reabsorption and Secretion I

Kidney Reabsorption & Secretion I
Tubular Reabsorption

  • Definition: Process returning most tubular contents to the blood.

  • Nature: Selective, involving both active (ATP-dependent) and passive transport.

  • Transport Methods:

    1. Transcellular Route: Substances pass across apical and basolateral membranes of epithelial cells, through the cytosol, into interstitial fluid, and then into peritubular capillaries.

    2. Paracellular Route: Substances move between adjacent cells via leaky tight junctions (common in PCT), especially for water, Ca²⁺, Mg²⁺, K⁺, Cl⁻, and some Na⁺.

Reabsorption of Sodium (Na⁺)

  • Na⁺ is the most abundant cation in filtrate.

  • Basolateral Membrane: Primary active transport via Na⁺-K⁺ ATPase pump.

  • Apical Membrane: Secondary active transport (e.g., cotransport with glucose/amino acids) or facilitated diffusion via channels.

Reabsorption of Water, Nutrients, and Ions

  • Water reabsorption is coupled to Na⁺ reabsorption, driven by osmosis.

  • Organic nutrients (glucose, amino acids, vitamins) are reabsorbed via secondary active transport with Na⁺.

Transport Maximum (Tm)

  • Definition: Limited capacity of transcellular transport systems for specific substances.

  • Clinical Relevance: If solute levels exceed Tm (e.g., hyperglycemia), excess is excreted in urine (e.g., glucose in diabetes mellitus).

Passive Tubular Reabsorption of Water

  • An osmotic gradient created by Na⁺ and solute reabsorption draws water into the blood.

  • Aquaporins: Facilitate water passage.

    • Obligatory: Always present in PCT.

    • Facultative: Inserted in collecting ducts only when ADH is present.

Passive Tubular Reabsorption of Solutes

  • After water reabsorption, remaining solutes become concentrated, allowing them (e.g., Ca²⁺, Mg²⁺, K⁺, Cl⁻, urea) to diffuse into peritubular capillaries, often via paracellular routes.

Reabsorption Summary by PCT

  • PCT reabsorbs: Nutrients, 70% of Na⁺ and water, various ions, and most uric acid; half of urea is reabsorbed.

Loop of Henle Specifics

  • Descending Limb: Permeable to H₂O only.

  • Ascending Limb: Permeable to solutes only (e.g., Na⁺-K⁺-2Cl⁻ symporter in thick segment).

Hormonal Control of Reabsorption and Secretion

  • Aldosterone: Increases Na⁺ reabsorption and K⁺ secretion in collecting ducts and DCT, regulating blood pressure.

  • Angiotensin II: Stimulates aldosterone release.

  • Atrial Natriuretic Peptide (ANP): Reduces Na⁺ reabsorption (opposite to aldosterone), decreasing blood volume/pressure.

  • Parathyroid hormone: Increases Ca²⁺ reabsorption in DCT and collecting ducts.

  • Antidiuretic Hormone (ADH): Increases water reabsorption in collecting ducts by inserting aquaporins, affecting blood osmolarity and volume.

H⁺ Secretion

  • Occurs in PCT and Collecting Duct via Na⁺/H⁺ exchangers.

  • H⁺ is generated from H₂CO₃ (catalyzed by carbonic anhydrase), influencing blood pH.

Bicarbonate Recovery

  • HCO₃⁻ does not cross the apical membrane directly but is crucial for maintaining acid-base balance, forming from CO₂ and H₂O.

Summary of Tubular Reabsorption and Secretion

  • Cortex Region: Reabsorbs 70% of filtrate (H₂O, Na⁺, HCO₃⁻, glucose, amino acids), regulated reabsorption of Na⁺ (aldosterone) and Ca²⁺ (PTH), and secretes H⁺, NH₄⁺, and drugs. Water adjustments by ADH.

  • Inner Medulla: Major roles for secretion and pH balance (H⁺, HCO₃⁻, NH₄⁺ adjustments).