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Regulation of Water Balance Vivek Bhalla, MD Division of Nephrology Stanford University School of Medicine September 14th, 2015 Learning Objectives (3) 1. A rise in tonicity does what? – Stimulates ADH, thirst 2. How is the kidney primed to concentrate urine? – Countercurrent multiplication – Countercurrent exchange 3. How that concentrating mechanism is utilized to maintain water balance? – What does ADH do to the kidney? “Big Picture” Hypothalamus “Big Picture” Hypothalamus Hypothalamus •Paraventricular Neurons •Supraoptic Nuclei ADH Release Kidney Anatomy Hyperosmotic Medulla •Tubular Lumen •Tubular epithelial Cells •Interstitium •Endothelial cells – vasa recta •Blood within the vasa recta Kidney Anatomy •Tubular Lumen •Tubular epithelial Cells •Interstitium •Endothelial cells – vasa recta •Blood within the vasa recta Kidney Anatomy Hyperosmotic Medulla How is this gradient established? Hyperosmotic Medulla Counter Current Multiplication Counter Current Multiplication Countercurrent Multiplication Countercurrent Exchange ABC- Artery Vein Collecting Duct Countercurrent Exchange Too Much or Too Little ADH % of water remaining In the lumen Relative to the original glomerular filtrate Aquaporins – Water Channels Aquaporins – Water Channels Urinary Concentration Take Home Points (3) 1. ADH release and thirst are regulated by tonicity 2. Countercurrent multiplication and exchange ready the medulla for ADH-dependent water reabsorption 3. ADH stimulates Aquaporin 2 insertion in the luminal (apical) membrane along the collecting duct
