Lecture Human anatomy and physiology - Chapter 25: The urinary system (part b) include: Tubular reabsorption; sodium reabsorption; reabsorption of nutrients, water, and ions; reabsorptive capabilities of renal tubules and collecting ducts;. | 25 The Urinary System: Part B Tubular Reabsorption A selective transepithelial process All organic nutrients are reabsorbed Water and ion reabsorption are hormonally regulated Includes active and passive process Two routes Transcellular Paracellular Tubular Reabsorption Transcellular route Luminal membranes of tubule cells Cytosol of tubule cells Basolateral membranes of tubule cells Endothelium of peritubular capillaries Tubular Reabsorption Paracellular route Between cells Limited to water movement and reabsorption of Ca2+, Mg2+, K+, and some Na+ in the PCT where tight junctions are leaky Figure Active transport Passive transport Peri- tubular capillary 2 4 4 3 3 1 1 2 4 3 Filtrate in tubule lumen Transcellular Paracellular Paracellular Tight junction Lateral intercellular space Capillary endothelial cell Luminal membrane Solutes H2O Tubule cell Interstitial fluid Transcellular Basolateral membranes 1 Transport across the luminal membrane. 2 Diffusion through the cytosol. 4 Movement through the interstitial fluid and into the capillary. 3 Transport across the basolateral membrane. (Often involves the lateral intercellular spaces because membrane transporters transport ions into these spaces.) Movement via the transcellular route involves: The paracellular route involves: • Movement through leaky tight junctions, particularly in the PCT. Sodium Reabsorption Na+ (most abundant cation in filtrate) Primary active transport out of the tubule cell by Na+-K+ ATPase in the basolateral membrane Na+ passes in through the luminal membrane by secondary active transport or facilitated diffusion mechanisms Sodium Reabsorption Low hydrostatic pressure and high osmotic pressure in the peritubular capillaries Promotes bulk flow of water and solutes (including Na+) Reabsorption of Nutrients, Water, and Ions Na+ reabsorption provides the energy and the means for reabsorbing most other substances Organic nutrients are reabsorbed by secondary active transport Transport . | 25 The Urinary System: Part B Tubular Reabsorption A selective transepithelial process All organic nutrients are reabsorbed Water and ion reabsorption are hormonally regulated Includes active and passive process Two routes Transcellular Paracellular Tubular Reabsorption Transcellular route Luminal membranes of tubule cells Cytosol of tubule cells Basolateral membranes of tubule cells Endothelium of peritubular capillaries Tubular Reabsorption Paracellular route Between cells Limited to water movement and reabsorption of Ca2+, Mg2+, K+, and some Na+ in the PCT where tight junctions are leaky Figure Active transport Passive transport Peri- tubular capillary 2 4 4 3 3 1 1 2 4 3 Filtrate in tubule lumen Transcellular Paracellular Paracellular Tight junction Lateral intercellular space Capillary endothelial cell Luminal membrane Solutes H2O Tubule cell Interstitial fluid Transcellular Basolateral membranes 1 Transport across the luminal membrane. 2 Diffusion through the cytosol. 4 .
