BIO 311D 1st Edition Lecture 22 Outline of Last Lecture I Forms of Nitrogenous wastes II Ammonia III Urea IV Uric acid V Excretory Processes VI Kidneys VII Loop of Henle Outline of Current Lecture I Nephron II Proximal Tubule III Descending Limb IV Ascending Limb V Distal Tubule VI Collecting Duct VII Solute Gradients and Water Conservation VIII Birds Reptiles and Marine Animals Current Lecture The nephron is organized for stepwise processing of blood filtrate The filtrate produced in Bowman s capsule contains salts glucose amino acids vitamins nitrogenous wastes and other small molecules The filtrate formed by the nephrons in the kidney is not the same as urine The filtrate is first refined and concentrated by the processes of forming the urine that leaves the body A Filtration and secretion B Reabsorption and secretion C Reabsorption and exertion D Filtration and reabsorption E Secretion and excretion From Blood Filtrate to Urine A Closer Look Proximal Tubule Reabsorption of ions water and nutrients takes place in the proximal tubule Molecules are transported actively and passively from the filtrate into the interstitial fluid and then capillaries Some toxic materials are actively secreted into the filtrate As the filtrate passes through the proximal tubule materials to be excreted become concentrated Descending Limb of the Loop of Henle Reabsorption of water continues through channels formed by aquaporin proteins Movement is driven by the high osmolarity of the interstitial fluid which is hyperosmotic to the filtrate The filtrate becomes increasingly concentrated More concentrated increase in concentration Ascending Limb of the Loop of Henle In the ascending limb of the loop of Henle salt but not water is able to diffuse from the tubule into the interstitial fluid The filtrate becomes increasingly dilute Less concentrated decrease in concentration Distal Tubule The distal tubule regulates the K and NaCl concentrations of body fluids The controlled movement of ions contributes to pH regulation The two solute model explaining urine production in the nephron states that A NaCl moves out of the nephron and into the interstitial fluid in the descending loop of henle B The fluid entering the distal convoluted tubule is more concentrated in NaCl than is the fluid entering the proximal convoluted tubule C The transport epithelium in the ascending loop of henle is relatively impermeable to water D All urea movements along the nephron are from the interstitial fluid into the tubule fluid E The ratio of NaCl to urea in interstitial fluid is about the same all along the length of the nephron Collecting Duct The collecting duct carries filtrate through the medulla to the renal pelvis One of the most important tasks is reabsorption of solutes and water Urine is hyperosmotic to body fluids Solute Gradients and Water Conservation The mammalian kidney s ability to conserve water is a key terrestrial adaptation Hyperosmotic urine can be produced only because considerable energy is expended to transport solutes against concentration gradients The two primary solutes affecting osmolarity are NaCl and urea Why does the solute concentration of the filtrate increase as it passes down the Loop of Henle Water is loss during the descending tubule Permeable to water but impermeable to salts Since water is loss the concentration will increase By what mechanism does the filtrate become more concentrated than the interstitial fluid Where does this process take place Countercurrent exchange or two solute model Process takes place in the loop of henle What is the advantage of being able to excrete a concentrated waste To conserve water Mammals The juxtamedullary nephron is key to water conservation in terrestrial animals Mammals that inhabit dry environments have long loops of Henle while those in fresh water have relatively short loops Birds and Other Reptiles Birds have shorter loops of Henle but conserve water by excreting uric acid instead of urea Other reptiles have only cortical nephrons but also excrete nitrogenous waste as uric acid The blood diet of vampire bats requires the urinary excretion of A Dilute urine immediately after drinking blood and then excretion of concentrated urine after digesting the abundant proteins in the blood B Many more sodium ions than are in the urine of other small predatory mammals C Many fewer potassium ions than are in the urine of other small predatory mammals D Many fewer bicarbonate ions and many more hydrogen ions than are in the urine of other small predatory mammals Freshwater Fishes and Amphibians Freshwater fishes conserve salt in their distal tubules and excrete large volumes of dilute urine Kidney function in amphibians is similar to freshwater fishes Amphibians conserve water on land by reabsorbing water from the urinary bladder Marine Bony Fishes Marine bony fishes are hypoosmotic compared with their environment Their kidneys have small glomeruli and some lack glomeruli entirely Filtration rates are low and very little urine is excreted Hormonal circuits link kidney function water balance and blood pressure Mammals control the volume and osmolarity of urine The kidneys of the South American vampire bat can produce either very dilute or very concentrated urine This allows the bats to reduce their body weight rapidly or digest large amounts of protein while conserving water Antidiuretic Hormone The osmolarity of the urine is regulated by nervous and hormonal control Antidiuretic hormone ADH makes the collecting duct epithelium more permeable to water An increase in osmolarity triggers the release of ADH which helps to conserve water Binding of ADH to receptor molecules leads to a temporary increase in the number of aquaporin proteins in the membrane of collecting duct cells Mutation in ADH production causes severe dehydration and results in diabetes insipidus Alcohol is a diuretic as it inhibits the release of ADH The Renin Angiotensin Aldosterone System The renin angiotensin aldosterone system RAAS is part of a complex feedback circuit that functions in homeostasis A drop in blood pressure near the glomerulus causes the juxtaglomerular apparatus JGA to release the enzyme renin Renin triggers the formation of the peptide angiotensin II Angiotensin II Raises blood pressure and decreases blood flow to the kidneys Stimulates the release of the hormone aldosterone which increases blood volume and pressure Homeostatic Regulation of the Kidney ADH and RAAS both