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Chapter 18

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University of Toronto St. George

Chapter 18Urinary System Renal Function Functions of Urinary SystemThe primary functions of the kidney are 1 Regulation of plasma ionic composition including sodium potassium calciummagnesium chloride bicarbonate and phosphates 2 Regulation of plasma volume3 Regulation of plasma osmolarity4 Regulation of plasma hydrogen ion concentration5 Removal of metabolic waste products and foreign substancesBy regulating the plasma composition the kidneys ultimately control the volume and composition of all the bodys fluids Secondary kidney functions include 6 Secretion of hormones and enzymes a Erythropoietin stimulates erythrocyte production b Reninenzyme that converts angiotensinogen into angiotensin I 7 Activation of Vitamin D 3 8 Gluconeogenesis during periods of need Anatomy of Urinary System Review the anatomyBasic Renal Exchange ProcessesThree basic exchange processes of the renal nephrons are 1 Glomerular Filtration 2 Reabsorption3 SecretionGlomerular Filtration Filtration is driven by Starling forces hydrostatic and osmotic pressure gradients The filtrate resembles plasma except in lacking proteins present in plasma The glomerular filtrate passes three barriers to enter Bowmans capsule1 Capillary endothelial cell layerThese cells have fenestrations or pores thatincrease the movement of fluid through the cells by bulk flow2 Epithelial cell layer of Bowmans capsule These cells are called podocytes because they have foot processes that interdigitate and form slitpores 3 Basement membraneThe basement membrane is sandwiched between theprevious two layers and acts as the primary barrier to the filtration of proteins Together these three layers form the glomerular membrane or filtration barrier This membrane allows the bulk flow of proteinfree fluid from the blood into the lumen of Bowmans capsuleGlomerular Filtration PressureThe glomerular filtration pressure is the same thing as the net filtration rate and is due to the same Starling forces 1 Glomerular Capillary Hydrostatic PressureThis pressure is approximately 60 mm Hg The higher hydrostatic pressure is dueto the high resistance of the efferent arteriole2 Bowmans Capsule Oncotic PressureNormally this pressure is negligible because very little protein leaves thecapillaries and enters the capsule If we assume that the Bowmans capsule oncoticpressure is equal to 0 mm Hg the pressure favoring filtration is 60 mm Hg 3 Bowmans Capsule Hydrostatic PressureThis pressure opposes filtration and is typically about 15 mm Hg This pressure isless than the capillary hydrostatic pressure and filtration is favored4 Glomerular Oncotic PressureThe pressure is due to the presence of proteins in the capillaries The oncoticpressure is about 29 mm Hg and opposes filtration The net pressure opposing filtration at the renal corpuscle under normal conditions is 15 mm 29 mm 44 mm Hg Hg Hg
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