e. Kidney Function • (1) Glomerulus: filtration • (2) PCT: tubular reabsorption • (3) Loop of Henle • (a) descending loop: filtrate concentrates • (b) ascending loop: filtrate dilutes • Constant recycling of salt creates • “standing salt gradient” in kidney medulla
DCT GLOM PCT CD LOOP
NaCl H2O 300
300 100 NaCl 600 400 NaCl 900 700 NaCl H2O H2O H2O H2O 1200 1000 1400 300
300 100 NaCl 600 600 400 NaCl 900 900 700 NaCl 1200 1200 1000 1400 1400 300 300
(4) Distal convoluted tubule • both active absorption and secretion • (a) control ion concentrations in filtrate • e.g., Na+, Cl-, K+, HCO3-, H+ • regulates blood pH and ion composition • (b) removes wastes from blood by secretion • At end of DCT, filtrate is back to 300 mosm/L
100 NaCl 400 700 H2O 1000 300 300 300 600 900 1200 1400
(5) Collecting Duct • Filtrate in CD passes down through standing salt gradient in medulla ECF • Water will leave filtrate by osmosis • Water picked up by blood and returned to general circulation • Salts and wastes remain behind to form a concentrated urine
(6) Summary • All kidneys: • elimination of wastes • conservation of needed salts and nutrients • Looped kidneys (birds and mammals only): • self generating osmotic gradient gives ability to concentrate urine • massive savings of water in animals with high waste production and water loss
f. Control of kidney function • Control water reabsorption in collecting duct • Neurohypophysis/neural lobe • arginine vasopressin (AVP) in mammals • arginine vasotocin (AVT) in all others • 9 amino acid peptide • Control: endocrine reflex arc
Decreased Blood Pressure Increased Blood Osmolarity + + Aortic Baroreceptors CNS Chemoreceptors + + AVP RELEASE
AVP action • increase permeability of cells to water • via cAMP, induces production of proteins • “water channels” • works in kidney, bladder, skin • kidney • increases permeability of cells of collecting duct to water
ECF 300 300 mosm 800 1200 300 mosm H2O to circulation NO AVP low water permeability HIGH AVP high water permeability Large amounts of dilute urine Small amounts of concentrated urine DIURESIS ANTIDIURESIS 1200 mosm 300 mosm AVP = AntiDiuretic Hormone (ADH)
2. Bladder • homeotherms • storage organ for hyperosmotic urine • poikilotherms • epithelium is thin, contains ion pumps • Na+, Cl-, pumped to blood, H2O follows • H2O permeability controlled by AVT • AVT increases, water uptake increases
3. Integument (skin) • barrier to environment • most vertebrates: impermeable to salts, H2O • waxy coating, dead cells, scales, mucus • amphibians: no barrier to H2O • lose H2O rapidly • can take up H2O along with ions • AVT: increases skin permeability
4. Salt Glands • marine elasmobranchs: rectal gland • marine reptiles and birds: facial
All drink sea water to gain water • no freshwater access • high salt load • no looped kidney: dilute urine • Excrete salt load from salt glands • concentrated saline solution excreted • active transport of Na+/Cl- to outside • 2-3 X osmolarity of blood plasma
5. Gut • primary location for water and salt uptake • ion pumping into animal • water entry by osmosis
6. Gills • pump ions in or out • water follows by osmosis
C. Osmoregulatory Environments • 1. Sea water (1000 mosm/L) • Problem: if blood pOs does not equal water pOs • then water and solutes will diffuse across gill • 2 strategies: • a. conform • let blood osmolarity = environment • hagfish: plasma = 1000 mosm salt • sharks: plasma = 500 mosm salt, 500 mosm urea • kidney must still regulate blood composition
b. Regulate at 300-350 mosm/L • gill in salt water: • osmotic H2O loss from blood • passive salt gain from environment • response of marine fish: • (1) drink sea water • gain H2O and salt in gut • (2) excrete Na+, Cl- by pumping out at gills • (3) divalent ions, wastes excreted in urine
Marine fish kidney • adapted to minimize water loss in urine • very low glomerular filtration, down to 0 • Overall strategy: • gain water and salt by drinking • excrete salt gained • conserve water at all locations
2. Fresh water (<100 mosm/L) • All animals regulate at 300-350 mosm/L • Problem • diluting: lose salt, gain H2O at gills • Solution • don’t drink • gain salt through diet • conserve salt • pump in at gills • reabsorb from bladder
Freshwater fish excrete excess water at kidney • adapted to maximize water loss in urine • very high glomerular filtration, no loop • “copious amounts of dilute urine”
Additional osmoregulatory problem • disposal of nitrogenous wastes • protein catabolism makes ammonia • increases osmotic pressure of blood • toxic
All fish • ammonia highly soluble in water • diffuses out of blood at gills • Fish are “ammonotelic” • excrete ammonia as nitrogenous waste
3. Terrestrial Environments • a. Take up as much water as possible • (1) Drink • (2) Eat • H2O trapped in food as humidity • H2O generated by biochemical breakdown of complex nutrients • “metabolic water” • 60 ml H2O/100 g dry barley • (3) Absorb: amphibian skin, bladder
b. Reduce water loss • (1) Avoid hot environments (nocturnal) • (2) Impermeable skin • (3) Kidney • Produce a concentrated urine: loop • Kidney water conservation ability: • U/P ratio = urine osmolarity • plasma osmolarity • reptiles: no loop, U/P up to 1 • birds: small loop, U/P up to 6 • mammals: great loop, U/P up to 25
(4) Respiratory system • nasal labyrinth • inhale: air warmed and humidified • exhale: H2O condensed by cooling • Respiration still primary H2O loss in xeric environments
(5) Nitrogenous wastes • terrestrial: retain and detoxify ammonia • Mesic environments: • ammonia converted to urea in liver • less toxic • concentrated in urine • “ureotelic”
Xeric environments • urea still requires too much water • ammonia is converted in liver to uric acid • precipitates as insoluble salt after kidney • “uricotelic” • Huge water savings • Ammonia: 500 mls H2O to excrete 1 g N • Urea: 50 mls H2O to excrete 1 g N • Uric Acid: 10 mls H2O to excrete 1 g N
Ammonia • fish • aquatic amphibian larvae • crocodilians • Urea • mesic reptiles and amphibians • mammals • Uric acid • birds • xeric reptiles and amphibians
d. Store H2O: amphibians • store H2O in bladder and lymph • tolerate dehydration • draw water out of lymph • draw water out of urine from bladder • allow blood osmolarity to rise to 600
