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Ch. 14 Part 5 Loop of Henle, Distal Convoluted Tubule, Collecting Duct, Osmoregulation Loop of Henle Review • Found in 1/3 of all nephrons in kideny • Begins in Cortex of Kidney and Dips into Medulla of Kidney • Two Parts: • Descending LOH • Ascending LOH • Function: • To create a very high concentration of Na+ and Cl- ions in tissue fluid of medulla, enabling water to be reabsorbed from fluid in collecting duct as it flows through medulla • Enables concentrated urine production • Ensures water is conserved in body and not released in urine • Prevents dehydration Ascending Loop of Henle Loop of Henle Reabsorption Descending Loop of Henle • • • • Permeable to Water Permeable to Na+ and Cl- ions Filtrate flows down DLOH Water moves OUT of LOH into tissue fluid via OSMOSIS • Na+ and Cl- in medulla tissue fluid diffuse INTO DLOH (down their conc.grad.) • Bottom of LOH • Filtrate contains LESS water and LOTS of Na+ and Cl- ions (lower water potential) • Longer loops = more concentrated filtrate becomes in LOH • • • • Impermeable to water Permeable to Na+ and Cl- ion Permeable to Urea Cells ACTIVELY TRANSPORT Na+ and Cl- ions OUT of Filtrate in LOH and INTO tissue fluid in medulla • Decreases water potential in tissue fluid • Increases water potential of filtrate fluid inside of ascending ALOH • High concentrations of Na+ and Cl- build up near bottom of LOH (between DLOH and ALOH) • As filtrate moves UP ALOH, filtrate becomes less concentrated with Na+ and Cl- (it has been losing them the whole way up) • Still removed b/c higher portions of ALOH are found in the medulla that has less concentrated tissue fluid (near top of medulla, close to cortex) • Concentration of Na+ and Cl- inside ALOH is never drastically different from concertation of ion sin tissue fluid Counter-Current Multiplier • Mechanism used in the Loop of Henle • Created by have the DLOH and ALOH run side by side • Fluid flows down in DLOH • Current goes down • Fluid flows up in ALOH • Current goes up • Enables maximum concentration of solutes to build up inside and outside the tube at the bottom of the LOH Urea in Loop of Henle and Collecting Duct • Ascending LOH and Collecting Ducts PERMEABLE to UREA • Urea has diffused into tissue fluids • High concentrations on UREA in Medulla tissue fluid • Collecting Duct dips into medulla • Some Urea diffuses back out of collecting duct and INTO medulla tissue fluid (increasing solutes, lowering water potential) • Water flows OUT of collecting duct by OSMOSIS • Water is reabsorbed into tissue fluid • Happens until water potential of medulla tissue fluid is equal to water potential of urine(may be greater than water potential in blood) • Controlled by ADH • Anti-diuretic hormone (more on this later) • Some urea remains in collecting duct and is excreted in urine Reabsorption in the Distal Convoluted Tubule (DCT) and Collecting Duct • First part of DCT • Functions same as ALOH • Na+ and Cl- actively pumped out • Second Part of DCT • Functions same as collecting duct • Na+ ions are actively pumped from filtrate fluid IN tubule TO tissue fluid INTO peritubular capillaries • K+ ions ACTIVELY TRANSPORTED INTO tubule • Rate of ions pumping in and out varies • Regulates concentration of ions in blood Animal Adaptations • Mammals can produce highly concentrated urine to conserve water • Increase thickness of medulla of kidneys = more concentrated urine (more water reabsorbed by tissues) • Deep LOH with ascending LOH that have lots of Na+-K+ pumps • Cytoplasm has lots of mitochondria (lots of ATP to pum Na+) • Human concertation of urine = 4x that of our blood plasma • Desert rodents concertation of urine = 20 x that of their blood plasma Osmosregulation • Control of water potential of body fluids • Important part of homeostasis • Involves: • Hypothalamus • Osmoreceptors sensory neurons • Monitor water potential of blood • Posterior pituitary gland • Kidney ADH • Anti-diuretic hormone • “Diuresis” = production of dilute urine (release of water) • ADH stops urine from being dilute by stimulating reabsorption of water • Peptide hormone made of 9 amino acids • Hypothalamus detects changes in water potential of blood • Using OSMORECPTOR sensory neurons • Detect decrease in water potential and send nerve impulse to stimulate Posterior Pituitary Gland • Secreted by posterior pituitary gland • Target cells: cells of the collecting duct of nephron in kidney • Function of ADH: REDUCE water loss in urine by making kidney reabsorb as much water as possible • Stops dilute urine from being made • Creates MORE aquaporins in Collecting duct so more water can leave collecting duct, go into tissue fluid and be reabsorbed by peritubular capillaries Decrease in Water Potential in Blood • Decrease in water potential below SET POINT is detected by osmoreceptors in hypothalamus • Nerve impulses sent along neurons that end at posterior pituitary gland • Impulses stimulate posterior pituitary gland to release ADH into blood capillaries and travel all over body Effects of ADH on Kidney • Collecting ducts allow reabsorption of water (OUT of nephron and into medulla/tissuecapillaries) • ADH molecules BIND to receptor proteins on cell surface membranes of collecting duct cells • Makes collecting duct more permeable to water by INCREASING number of aquaporins • Aquaporins -water permeable channels in cell surface membrane of collecting duct cells • Binding of ADH to receptor proteins on cell membrane activate ENZYMES inside of cell • Activates series of enzyme controlled reactions that end in final product: ACTIVE PHOSPHORYLASE ENZYME • PHOSPHORYLASE causes vesicles in cytoplasm of cell (vesicles contain aquaporins in their membrane) to begin moving towards cell membrane (review Golgi body modifications to proteins) • Vesicles fuse with cell membrane, thus increasing the number of aquaporins • Water can now freely move through the membrane DOWN water potential gradient, into concentrated tissue fluid peritubular capillaries blood plasma • Fluid flows through collecting duct • Fluid in collecting duct = HIGH water potential • Tissue fluid in medulla = LOW water potential • Water leaves collecting duct cells • Fluid in collecting duct becomes more concentrated • Increase of water in blood • This is due to reabsorption of water due to release of ADH • Volume of fluid flowing into bladder will be less = more concentrated urine (less dilute) Effects of Lots of Water in Body • Water potential in blood INCREASES • Osmoreceptors in hypothalamus detect increase • Osmoreceptors are no longer stimulated • Neurons in posterior pituitary gland no longer stimulated to release ADH • No ADH being secreted • Aquaporins in cell membrane of collecting duct cells move OUT of membrane and become vesicles in the cytoplasm again • Takes 10-15 minutes for this to happen • Not immediate • Collecting duct becomes impermeable to water • Lots of water flowing in fluid in collecting duct = increased volume into bladder = DILUTE urine • Ensures maintenance of water potential Response to decrease in ADH • Collecting duct cells do NOT respond immediately to decrease in ADH secretion by posterior pituitary gland • ADH already in blood needs to be broken down • Half of ADH is blood is destroyed every 15-20 minutes • Eventually ADH stops arriving at collecting duct cells • No attachment to receptor proteins • Aquaporins removed and become vesicles in cytoplasm again • 10-15 minutes Alcohol and ADH • Alcohol affects the pituitary gland • Reduces secretion of ADH • No ADH attaching to receptors on collecting duct cells = no aquaporins = impermeable to water = water stays in collecting duct with urine (NO water reabsorption as filtrate moves through collecting duct) • Dilute urine • Dehydration