e. Kidney Function (1) Glomerulus: filtration (2) PCT: tubular reabsorption (3) Loop of Henle

1. 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

2. DCT GLOM PCT CD LOOP

3. NaCl H2O 300

4. 300 100 NaCl 600 400 NaCl 900 700 NaCl H2O H2O H2O H2O 1200 1000 1400 300

5. 300 100 NaCl 600 600 400 NaCl 900 900 700 NaCl 1200 1200 1000 1400 1400 300 300

6. (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

7. 100 NaCl 400 700 H2O 1000 300 300 300 600 900 1200 1400

8. (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

9. (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

10. 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

11. Decreased Blood Pressure Increased Blood Osmolarity + + Aortic Baroreceptors CNS Chemoreceptors + + AVP RELEASE

12. 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

13. 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)

14. 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

15. 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

16. 4. Salt Glands • marine elasmobranchs: rectal gland • marine reptiles and birds: facial

17. 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

18. 5. Gut • primary location for water and salt uptake • ion pumping into animal • water entry by osmosis

19. 6. Gills • pump ions in or out • water follows by osmosis

20. 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

21. 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

22. 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

23. 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

24. 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”

25. Additional osmoregulatory problem • disposal of nitrogenous wastes • protein catabolism makes ammonia • increases osmotic pressure of blood • toxic

26. All fish • ammonia highly soluble in water • diffuses out of blood at gills • Fish are “ammonotelic” • excrete ammonia as nitrogenous waste

27. 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

28. 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

29. (4) Respiratory system • nasal labyrinth • inhale: air warmed and humidified • exhale: H2O condensed by cooling • Respiration still primary H2O loss in xeric environments

30. (5) Nitrogenous wastes • terrestrial: retain and detoxify ammonia • Mesic environments: • ammonia converted to urea in liver • less toxic • concentrated in urine • “ureotelic”

31. 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

32. Ammonia • fish • aquatic amphibian larvae • crocodilians • Urea • mesic reptiles and amphibians • mammals • Uric acid • birds • xeric reptiles and amphibians

33. 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