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This 12 page Class Notes was uploaded by Jess Graff on Monday May 2, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 4 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 05/02/16
BMS 508.03 4/25/2016 Chapter 26 (cont) Urinary System (cont) The Kidneys • Macula Densa • Epithelial cells of DCT, near renal corpuscle • Tall cells with densely clustered nuclei • Juxtaglomerular Cells • Smooth muscle fibers in wall of afferent arteriole • Associated with cells of macula densa • Together with macula densa forms juxtaglomerular complex (JGC) • The Collecting System • The distal convoluted tubule opens into the collecting system • Individual nephrons drain into a nearby collecting duct • Several collecting ducts: • Converge into a larger papillary duct • Which empties into a minor calyx • Transports tubular fluid from nephron to renal pelvis • Adjusts fluid composition • Determines final osmotic concentration and volume of urine Renal Phsyiology • The Goal of Urine Production • To maintain homeostasis • By regulating volume and composition of blood • Including excretion of metabolic waste products • 3 Organic Waste Products • Urea • Creatinine • Uric acid • Organic Waste Products • Are dissolved in bloodstream • Are eliminated only while dissolved in urine • Removal is accompanied by water loss • The Kidneys • Usually produce concentrated urine • 1200–1400 mOsm/L (four times plasma concentration) • Kidney Functions • To concentrate filtrate by glomerular filtration • Failure leads to fatal dehydration • Absorbs and retains valuable materials for use by other tissues • Sugars and amino acids • Basic Processes of Urine Formation • Filtration • Reabsorption • Secretion • An Overview of Renal Function • Water and solute reabsorption • Primarily along proximal convoluted tubules • Active secretion • Primarily at proximal and distal convoluted tubules • Long loops of juxtamedullary nephrons and collecting system • Regulate final volume and solute concentration of urine • Filtration • Hydrostatic pressure forces water through membrane pores • Small solute molecules pass through pores • Larger solutes and suspended materials are retained • Occurs across capillary walls • As water and dissolved materials are pushed into interstitial fluids • In some sites, such as the liver, pores are large • Plasma proteins can enter interstitial fluids • At the renal corpuscle • Specialized membrane restricts all circulating proteins • Reabsorption and Secretion • At the kidneys, it involves: • Diffusion • Osmosis • Channel-mediated diffusion • Carrier-mediated transport • Types of Carrier-Mediated Transport • Facilitated diffusion • Active transport • Cotransport • Countertransport • Characteristics of Carrier-Mediated Transport • A specific substrate binds to carrier protein that facilitates movement across membrane • A given carrier protein usually works in one direction only • Distribution of carrier proteins varies among portions of cell surface • The membrane of a single tubular cell contains many types of carrier proteins • Carrier proteins, like enzymes, can be saturated • Transport maximum (T ) and the Renal Threshold m • If nutrient concentrations rise in tubular fluid: • Reabsorption rates increase until carrier proteins are saturated • Concentration higher than transport maximum: • Exceeds reabsorptive abilities of nephron • Some material will remain in the tubular fluid and appear in the urine • Determines the renal threshold • Renal Threshold • Is the plasma concentration at which: • A specific compound or ion begins to appear in urine • Varies with the substance involved • Renal Threshold for Glucose • Is approximately 180 mg/dL • If plasma glucose is greater than 180 mg/dL: • T mf tubular cells is exceeded • Glucose appears in urine • Glycosuria • Renal Threshold for Amino Acids • Is lower than glucose (65 mg/dL) • Amino acids commonly appear in urine • After a protein-rich meal • Aminoaciduria • Ways of Expressing Osmotic Concentration • Osmolarity • Total number of solute particles per liter • Expressed in osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L) • Body fluids have osmotic concentration of about 300 mOsm/L • Other Measurements • Ion concentrations • In milliequivalents per liter (mEq/L) • Concentrations of large organic molecules • Grams or milligrams per unit volume of solution (mg/dL or g/dL) • Cortical and Juxtamedullary Nephrons • Nephron loop in cortical nephron • Is short • Does not extend far into medulla • Nephron loop in juxtamedullary nephron • Is long • Extends deep into renal pyramids • Functions in water conservation and forms concentrated urine Glomerular Filtration • The Process of Glomerular Filtration • Involves passage across a filtration membrane • 3 components of membrane 1. Capillary endothelium 2. Dense layer 3. Filtration slits • Glomerular Capillaries • Are fenestrated capillaries • Have pores 60–100 nm diameter • Prevent passage of blood cells • Allow diffusion of solutes, including plasma proteins • The Dense Layer • Is more selective • Allows diffusion of only: • Small plasma proteins • Nutrients • Ions • The Filtration Slits • Are the finest filters • Have gaps only 6–9 nm wide • Prevent passage of most small plasma proteins • Filtration Pressures • Glomerular filtration is governed by the balance between: • Hydrostatic pressure (fluid pressure) • Colloid osmotic pressure (of materials in solution) on either side of capillary walls • Hydrostatic Pressure • Glomerular hydrostatic pressure is blood pressure in glomerular capillaries • Tends to push water and solute molecules • Out of plasma • Into the filtrate • Is significantly higher than capillary pressures in systemic circuit • Due to arrangement of vessels at glomerulus • Glomerular Blood Vessels • Blood leaving glomerular capillaries • Flows into an efferent arteriole with a diameter smaller than afferent arteriole • Efferent arteriole produces resistance • Requires relatively high pressures to force blood into it • Capsular Hydrostatic Pressure (CsHP) • Opposes glomerular hydrostatic pressure • Pushes water and solutes • Out of filtrate • Into plasma • Results from resistance to flow along nephron and conducting system • Averages about 15 mm Hg • Net Hydrostatic Pressure (NHP) • Is the difference between: • Glomerular hydrostatic pressure and capsular hydrostatic pressure • Colloid Osmotic Pressure • Is the osmotic pressure resulting from the presence of suspended proteins • Blood colloid osmotic pressure (BCOP) • Tends to draw water out of filtrate and into plasma • Opposes filtration • Averages 25 mm Hg • Net Filtration Pressure (NFP) • Is the average pressure forcing water and dissolved materials: • Out of glomerular capillaries • Into capsular spaces • At the glomerulus is the difference between: • Hydrostatic pressure and blood colloid osmotic pressure across glomerular capillaries • The Glomerular Filtration Rate (GFR) • Is the amount of filtrate kidneys produce each minute • Averages 125 mL/min • About 10 percent of fluid delivered to kidneys • Leaves bloodstream • Enters capsular spaces • Creatinine Clearance Test • Is used to estimate GFR • A more accurate GFR test uses inulin • Which is not metabolized • Filtrate • Glomeruli generate about 180 liters of filtrate per day • 99% is reabsorbed in renal tubules • Filtration Pressure • Glomerular filtration rate depends on filtration pressure • Any factor that alters filtration pressure alters GFR • Control of the GFR • 3 interacting levels of control • Autoregulation (local level) • Hormonal regulation (initiated by kidneys) • Autonomic regulation (by sympathetic division of ANS) • Autoregulation of the GFR • Maintains GFR despite changes in local blood pressure and blood flow • By changing diameters of afferent arterioles, efferent arterioles, and glomerular capillaries • Reduced blood flow or glomerular blood pressure triggers: • Dilation of afferent arteriole • Dilation of glomerular capillaries • Constriction of efferent arterioles • Rise in renal blood pressure • Stretches walls of afferent arterioles • Causes smooth muscle cells to contract • Constricts afferent arterioles • Decreases glomerular blood flow • Hormonal Regulation of the GFR • By hormones of the: • Renin–angiotensin-aldosterone system • Natriuretic peptides (ANP and BNP) • The Renin–Angiotensin-Aldosterone System • 3 stimuli cause the juxtaglomerular complex (JGC) to release renin • Decline in blood pressure at glomerulus due to decrease in blood volume, fall in systemic pressures, or blockage in renal artery or tributaries • Stimulation of juxtaglomerular cells by sympathetic innervation • Decline in osmotic concentration of tubular fluid at macula densa • The Renin–Angiotensin-Aldosterone System: Angiotensin II Activation • Constricts efferent arterioles of nephron • Elevating glomerular pressures and filtration rates • Stimulates reabsorption of sodium ions and water at PCT • Stimulates secretion of aldosterone by adrenal cortex • Stimulates thirst • Triggers release of antidiuretic hormone (ADH) • Stimulates reabsorption of water in distal portion of DCT and collecting system • The Renin–Angiotensin-Aldosterone System: Angiotensin II • Increases sympathetic motor tone • Mobilizing the venous reserve • Increasing cardiac output • Stimulating peripheral vasoconstriction • Causes brief, powerful vasoconstriction • Of arterioles and precapillary sphincters • Elevating arterial pressures throughout body • The Renin–Angiotensin-Aldosterone System • Aldosterone • Accelerates sodium reabsorption in DCT and cortical portion of collecting system • Hormonal Regulation of the GFR • Increased Blood Volume • Automatically increases GFR • To promote fluid loss • Hormonal factors further increase GFR • Accelerating fluid loss in urine • Natriuretic Peptides • Are released by the heart in response to stretching walls due to increased blood volume or pressure • Atrial natriuretic peptide (ANP) is released by atria • Brain natriuretic peptide (BNP) is released by ventricles • Trigger dilation of afferent arterioles and constriction of efferent arterioles • Elevate glomerular pressures and increase GFR • Autonomic Regulation of the GFR • Mostly consists of sympathetic postganglionic fibers • Sympathetic activation • Constricts afferent arterioles • Decreases GFR • Slows filtrate production • Changes in blood flow to kidneys due to sympathetic stimulation • May be opposed by autoregulation at local level Reabsorption and Secretion • Reabsorption • Recovers useful materials from filtrate • Secretion • Ejects waste products, toxins, and other undesirable solutes • Reabsorption and Secretion • Occur in every segment of nephron • Except renal corpuscle • Relative importance changes from segment to segment • Reabsorption and Secretion at the PCT • PCT cells normally reabsorb 60–70 percent of filtrate produced in renal corpuscle • Reabsorbed materials enter peritubular fluid • And diffuse into peritubular capillaries • 5 Functions of the PCT • Reabsorption of organic nutrients • Active reabsorption of ions • Reabsorption of water • Passive reabsorption of ions • Secretion • Sodium Ion Reabsorption • Is important in every PCT process • Ions enter tubular cells by: • Diffusion through leak channels • Sodium-linked cotransport of organic solutes • Countertransport for hydrogen ions • The Nephron Loop and Countercurrent Multiplication • Nephron loop reabsorbs about 1/2 of water and 2/3 of sodium and chloride ions remaining in tubular fluid • By the process of countercurrent exchange