PowerPoint Lecture Slides prepared by Janice Meeking Mount Royal College 25 C H A P T E R The Urinary System Part B Copyright 2010 Pearson Education Inc 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 Copyright 2010 Pearson Education Inc Tubular Reabsorption Transcellular route Luminal membranes of tubule cells Cytosol of tubule cells Basolateral membranes of tubule cells Endothelium of peritubular capillaries Copyright 2010 Pearson Education Inc 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 Copyright 2010 Pearson Education Inc Movement via the transcellular route involves 1 Transport across the luminal membrane 2 Diffusion through the cytosol 3 Transport across the basolateral membrane Often involves the lateral intercellular spaces because membrane transporters transport ions into these spaces 4 Movement through the interstitial fluid and into the capillary Tight junction Lateral intercellular space Filtrate in tubule lumen Tubule cell Interstitial fluid Capillary endothelial cell Peri tubular capillary The paracellular route involves Movement through leaky tight junctions particularly in the PCT H2O Luminal membrane 1 1 Solutes Copyright 2010 Pearson Education Inc 2 3 4 Paracellular Transcellular Transcellular 3 2 3 4 4 Paracellular Basolateral membranes Active transport Passive transport Figure 25 13 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 Copyright 2010 Pearson Education Inc Sodium Reabsorption Low hydrostatic pressure and high osmotic pressure in the peritubular capillaries Promotes bulk flow of water and solutes including Na Copyright 2010 Pearson Education Inc 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 maximum Tm reflects the number of carriers in the renal tubules available When the carriers are saturated excess of that substance is excreted Copyright 2010 Pearson Education Inc Reabsorption of Nutrients Water and Ions Water is reabsorbed by osmosis obligatory water reabsorption aided by water filled pores called aquaporins Cations and fat soluble substances follow by diffusion Copyright 2010 Pearson Education Inc Nucleus Tubule cell Interstitial fluid Peri tubular capillary 3Na 2K 1 3Na 2K K Filtrate in tubule lumen Na Glucose Amino acids Some ions Vitamins H2O Lipid soluble substances Cl Ca2 K and other ions urea 2 3 4 5 6 1 At the basolateral membrane Na is pumped into the interstitial space by the Na K ATPase Active Na transport creates concentration gradients that drive 2 Downhill Na entry at the luminal membrane 3 Reabsorption of organic nutrients and certain ions by cotransport at the luminal membrane 4 Reabsorption of water by osmosis Water reabsorption increases the concentration of the solutes that are left behind These solutes can then be reabsorbed as they move down their concentration gradients 5 Lipid soluble substances diffuse by the transcellular route 6 Cl and other anions K and urea diffuse by the paracellular route Tight junction Cl Paracellular route Primary active transport Secondary active transport Passive transport diffusion Transport protein Ion channel or aquaporin Copyright 2010 Pearson Education Inc Figure 25 14 Reabsorptive Capabilities of Renal Tubules and Collecting Ducts PCT Site of most reabsorption 65 of Na and water All nutrients Ions Small proteins Copyright 2010 Pearson Education Inc Reabsorptive Capabilities of Renal Tubules and Collecting Ducts Loop of Henle Descending limb H2O Ascending limb Na K Cl Copyright 2010 Pearson Education Inc Reabsorptive Capabilities of Renal Tubules and Collecting Ducts DCT distal collect tubule and collecting duct Reabsorption is hormonally regulated Ca2 PTH Water ADH Na aldosterone and ANP Copyright 2010 Pearson Education Inc Reabsorptive Capabilities of Renal Tubules and Collecting Ducts Mechanism of aldosterone Targets collecting ducts principal cells and distal DCT channels ATPases Promotes synthesis of luminal Na and K Promotes synthesis of basolateral Na K Copyright 2010 Pearson Education Inc Tubular Secretion Reabsorption in reverse K H NH4 creatinine and organic acids move from peritubular capillaries or tubule cells into filtrate Disposes of substances that are bound to plasma proteins Copyright 2010 Pearson Education Inc Tubular Secretion Eliminates undesirable substances that have been passively reabsorbed e g urea and uric acid Rids the body of excess K Controls blood pH by altering amounts of H or HCO3 in urine Copyright 2010 Pearson Education Inc Regulation of Urine Concentration and Volume Osmolality Number of solute particles in 1 kg of H2O Reflects ability to cause osmosis Copyright 2010 Pearson Education Inc Regulation of Urine Concentration and Volume Osmolality of body fluids Expressed in milliosmols mOsm The kidneys maintain osmolality of plasma at 300 mOsm using countercurrent mechanisms Copyright 2010 Pearson Education Inc Countercurrent Mechanism Occurs when fluid flows in opposite directions in two adjacent segments of the same tube Filtrate flow in the loop of Henle countercurrent multiplier Blood flow in the vasa recta countercurrent exchanger Copyright 2010 Pearson Education Inc Countercurrent Mechanism Role of countercurrent mechanisms Establish and maintain an osmotic gradient 300 mOsm to 1200 mOsm from renal cortex through the medulla Allow the kidneys to vary urine concentration Copyright 2010 Pearson Education Inc Cortex Medulla Copyright 2010 Pearson Education Inc Figure 25 15 Countercurrent Multiplier Loop of Henle Descending limb Freely permeable to H2O which passes out of the filtrate into the hyperosmotic medullary interstitial fluid Filtrate osmolality increases to 1200 mOsm Copyright 2010 Pearson Education Inc Countercurrent Multiplier Loop of Henle Ascending limb Impermeable to H2O Selectively permeable to solutes Na and Cl are passively reabsorbed in the thin segment actively reabsorbed in the thick segment