BIO 311D 1st Edition Lecture 22 Outline of Last Lecture I. Forms of Nitrogenous wastesII. AmmoniaIII. UreaIV. Uric acidV. Excretory ProcessesVI. KidneysVII. Loop of HenleOutline of Current Lecture I. NephronII. Proximal TubuleIII. Descending LimbIV. Ascending LimbV. Distal TubuleVI. Collecting DuctVII. Solute Gradients and Water ConservationVIII. Birds, Reptiles, and Marine AnimalsCurrent LectureThe 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 moleculesThe filtrate formed by the nephrons in the kidney is not the same as urine. The filtrate is first refined andconcentrated by the processes of ___, forming the urine that leaves the bodyA. Filtration and secretionB. Reabsorption and secretionC. Reabsorption and exertionD. Filtration and reabsorptionE. Secretion and excretion From Blood Filtrate to Urine: A Closer LookProximal 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 concentratedDescending 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 regulationThe two-solute model explaining urine production in the nephron states thatA. NaCl moves out of the nephron and into the interstitial fluid in the descending loop of henleB. The fluid entering the distal convoluted tubule is more concentrated in NaCl than is the fluid entering the proximal convoluted tubuleC. The transport epithelium in the ascending loop of henle is relatively impermeable to waterD. All urea movements along the nephron are from the interstitial fluid into the tubule fluidE. 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 transportsolutes against concentration gradients• The two primary solutes affecting osmolarity are NaCl and ureaWhy 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 increaseBy 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 henleWhat is the advantage of being able to excrete a concentrated waste?- To conserve waterMammals• 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 loopsBirds 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 acidThe blood diet of vampire bats requires the urinary excretion ofA. Dilute urine immediately after drinking blood and then excretion of concentrated urine after digesting the abundant proteins in the bloodB. Many more sodium ions than are in the urine of other small predatory mammalsC. Many fewer potassium ions than are in the urine of other small predatory mammalsD. Many fewer bicarbonate ions and many more hydrogen ions than are in the urine of other small predatory mammalsFreshwater 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 bladderMarine 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 excretedHormonal 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 waterAntidiuretic 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 ADHThe 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