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

Loop of Henle Reabsorption Ascending Loop of Henle Loop of Henle Reabsorption Descending Loop of Henle 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 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

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/tissuecapillaries) 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