Excretion and Excretion and OsmoregulationOsmoregulation

OutlineOutline

• IntroductionIntroduction

• Comparative physiology of osmotic Comparative physiology of osmotic regulationregulation

• Mammalian kidneyMammalian kidney

• Evolution of the vertebrate kidneyEvolution of the vertebrate kidney

All organisms have an All organisms have an excretory system of some type excretory system of some type in order to in order to • Manage solutes in body fluidsManage solutes in body fluids

• Manage water content of the bodyManage water content of the body

• Remove metabolic end-productsRemove metabolic end-products

• Remove foreign substancesRemove foreign substances

Two basic modes of Two basic modes of excretionexcretion• UltrafiltrationUltrafiltration- pressure filtrate of - pressure filtrate of

blood, withholds protein and large blood, withholds protein and large solutes but water and small solutes solutes but water and small solutes passpass

• Active transport-Active transport- against a against a concentration gradient. If directed concentration gradient. If directed away from the organism, away from the organism, secretionsecretion. . If directed toward the organism, If directed toward the organism, reabsorptionreabsorption..

Three functions of excretory Three functions of excretory systemssystems

• UltrafiltrationUltrafiltration

• SecretionSecretion

• ReabsorptionReabsorption

Osmotic EnvironmentsOsmotic Environments

• Aquatic environments (71% of Aquatic environments (71% of Earth’s surface)Earth’s surface)– Sea waters- 3.5% saltsSea waters- 3.5% salts– Mediterranean – 4% saltsMediterranean – 4% salts– Saturated with salts- Saturated with salts-

•Great salt lake (NaCl)- no fish, some Great salt lake (NaCl)- no fish, some shrimpshrimp

•Dead Sea (MgClDead Sea (MgCl22))

Osmoconformers and Osmoconformers and osmoregulatorsosmoregulators

• OsmoconformerOsmoconformer- change osmotic - change osmotic concentration of body fluids to match concentration of body fluids to match environmentenvironment

• OsmoregulatorOsmoregulator- regulate composition - regulate composition of body fluids regardless of of body fluids regardless of environmentenvironment

• Seawater-1000 mOsM- animals risk Seawater-1000 mOsM- animals risk losing water to the environmentlosing water to the environment

• Fresh water-animals risk taking on Fresh water-animals risk taking on too much water and losing salttoo much water and losing salt

Two major categories of Two major categories of resistance to changes in resistance to changes in osmotic environmentosmotic environment• StenohalineStenohaline –limited tolerance to –limited tolerance to

changes in osmotic composition of changes in osmotic composition of environment-most fish are environment-most fish are stenohalinestenohaline

• EuryhalineEuryhaline- can tolerate wide - can tolerate wide fluctuations in environmental fluctuations in environmental osmotic concentrationsosmotic concentrations

Marine vertebratesMarine vertebrates

• Elasmobranchs, hagfish, crab-Elasmobranchs, hagfish, crab-eating frogseating frogs– Osmolarity of body fluids equals Osmolarity of body fluids equals

that of sea-water, i.e. 1000 mOsMthat of sea-water, i.e. 1000 mOsM

• Lampreys, teleost fishesLampreys, teleost fishes– Osmolarity of body fluids is one-Osmolarity of body fluids is one-

third that of seawater, approx. 300 third that of seawater, approx. 300 mOsM.mOsM.

Marine ElasmobranchsMarine Elasmobranchs• Maintain salt at one-third that of Maintain salt at one-third that of

seawater, but add organic urea to body seawater, but add organic urea to body fluids-100 times greater than mammalsfluids-100 times greater than mammals

• Total body fluid osmolarity is close to Total body fluid osmolarity is close to seawaterseawater

• Produce trimethylamine oxide (TMAO)- Produce trimethylamine oxide (TMAO)- neutralizes toxic effects of ureaneutralizes toxic effects of urea

• Shark proteins including enzymes are Shark proteins including enzymes are dependent upon ureadependent upon urea

• Excess Na+ excreted by ‘rectal glandExcess Na+ excreted by ‘rectal gland’’

TMAOurea

TeleostsTeleosts

• All teleosts maintain body salt All teleosts maintain body salt concentration at 1/3 that of seawaterconcentration at 1/3 that of seawater

• Marine teleosts must drink water and Marine teleosts must drink water and get rid of excess ions from waterget rid of excess ions from water

• Ion excretion (Na, Cl) is performed by Ion excretion (Na, Cl) is performed by gillsgills

• Mg, SO4, and other divalent ions Mg, SO4, and other divalent ions secreted by kidneyssecreted by kidneys

Saltwaterhttp://www.itresourcing.com.au/aquaculture/species/images/

Freshwater http://www.itresourcing.com.au/aquaculture/species/images/

AmphibiansAmphibians

• Risk losing ions to freshwaterRisk losing ions to freshwater

• Actively reabsorb Na and Cl across Actively reabsorb Na and Cl across skinskin

Terrestrial vertebratesTerrestrial vertebrates

• Face dehydration from any body Face dehydration from any body surfacesurface

• Birds and reptiles produce uric acid Birds and reptiles produce uric acid to conserve waterto conserve water

• Develop skin which prevents water Develop skin which prevents water lossloss

Dipodomys microps, the Kangaroo rat

Kangaroo ratsKangaroo rats

• Abundant in desertsAbundant in deserts

• Do not drink waterDo not drink water

• If given a diet of barley or oats, body If given a diet of barley or oats, body weight remains the same for monthsweight remains the same for months

• Water (g) formed per gram of Water (g) formed per gram of substrate:substrate:– Starch = 0.56Starch = 0.56– Fat=1.07Fat=1.07– Protein=0.45Protein=0.45

Nasal turbinatesNasal turbinates

• Increase water reabsorption from Increase water reabsorption from exhaled airexhaled air

• Nasal turbinates reduce temperature Nasal turbinates reduce temperature of exhaled airof exhaled air

• More water vapor condenses on More water vapor condenses on nasal epithelium during exhalationnasal epithelium during exhalation

Phylogeny of Phylogeny of osmoregulatory and osmoregulatory and

excretory organsexcretory organs

Contractile vacuole in Contractile vacuole in ParameciumParamecium

• Found only in freshwater organismsFound only in freshwater organisms

• Not a true excretory organ because it Not a true excretory organ because it does not produce an ultrafiltratedoes not produce an ultrafiltrate

• Excess water is channeled to Excess water is channeled to vacuole, vacuole swellsvacuole, vacuole swells

• Water expelled through poreWater expelled through pore

Paramecium- contractile vacuole

ProtonephridiaProtonephridia• Blind ended excretory organBlind ended excretory organ

• Solenocyte (1 flagella), flame cell (many Solenocyte (1 flagella), flame cell (many flagella)flagella)

• Found in flatworms, rotifersFound in flatworms, rotifers

• May or may not produce an ultrafiltrateMay or may not produce an ultrafiltrate

• Since there is no circulatory system in these Since there is no circulatory system in these animals, flame cells must be located animals, flame cells must be located throughout the bodythroughout the body

http://www.cartage.org.lb/en/themes/Sciences/Lifescience/GeneralBiology/Physiology/ExcretorySystem/Invertebrate/flatwormexcret.gif

MetanephridiaMetanephridia

• Only in eucoelomate animalsOnly in eucoelomate animals

• Filters fluid from coelomFilters fluid from coelom

• Produces an ultrafiltrateProduces an ultrafiltrate

Earthworm

Malpighian tubulesMalpighian tubules

• Found in insectsFound in insects

• KCl and NaCl transported from KCl and NaCl transported from coelomic fluid into Malpighian tubulescoelomic fluid into Malpighian tubules

• Initial urine formed in tubulesInitial urine formed in tubules

• Final urine formed in rectumFinal urine formed in rectum

• Produces a concentrated urine but not Produces a concentrated urine but not an ultrafiltratean ultrafiltrate

How it worksHow it works

• KK++ actively transported into M. actively transported into M. tubule, conc. 30 times body fluidtubule, conc. 30 times body fluid

• Passive Cl diffusion into M. tubulePassive Cl diffusion into M. tubule

• Water follows ionsWater follows ions

• In hind gut, water is reabsorbed, uric In hind gut, water is reabsorbed, uric acid precipitatesacid precipitates

Direction of blood flowDirection of blood flow

• Afferent arterioleAfferent arteriole

• GlomerulusGlomerulus

• Efferent arterioleEfferent arteriole

• Vasa rectaVasa recta

• VeinVein

Function of nephronFunction of nephron• Glomerular filtration (urea, glucose, Na, Glomerular filtration (urea, glucose, Na,

K, Cl)-no proteinsK, Cl)-no proteins– Same proportion of ions, solutes and Same proportion of ions, solutes and

water in glomerulus as in blood plasmawater in glomerulus as in blood plasma– No blood cells filteredNo blood cells filtered– Filtration based upon molecular sizeFiltration based upon molecular size

Function of nephronFunction of nephron

• Tubular reabsorptionTubular reabsorption– 99.9% of water99.9% of water– Most saltsMost salts– Most reabsorption by the proximal Most reabsorption by the proximal

tubulestubules

Function of the nephronFunction of the nephron

• Tubular synthesisTubular synthesis– Some amino acids are deaminatedSome amino acids are deaminated

• Tubular secretionTubular secretion– Regulates blood levels of K, HRegulates blood levels of K, H++ and HCO and HCO33

Brush border in proximal convoluted tubule

Glomerular filtrationGlomerular filtration• 125 mL/min, about 200 L/day125 mL/min, about 200 L/day• Affected byAffected by

– Net hydrostatic pressure difference between Net hydrostatic pressure difference between capillary and Bowman’s capsule-favors filtrationcapillary and Bowman’s capsule-favors filtration

– Colloid osmotic pressure of blood, opposes Colloid osmotic pressure of blood, opposes filtrationfiltration

– Hydraulic permeability-sieve like properties of the Hydraulic permeability-sieve like properties of the filtration barrierfiltration barrier•Capillary endotheliumCapillary endothelium•Basement membraneBasement membrane•Bowman’s capsuleBowman’s capsule

1

3

2

Calculating GFRCalculating GFR

• Patient injected with inulinPatient injected with inulin

• After a while, plasma sample drawn and After a while, plasma sample drawn and inulin concentration measured in plasmainulin concentration measured in plasma

• Concentration of inulin measured in Concentration of inulin measured in urineurine

• Urine volume determinedUrine volume determined

• Since all inulin removed, GFR equals Since all inulin removed, GFR equals inulin clearanceinulin clearance

Glomerular Filtration rateGlomerular Filtration rate

• GFR = VU/PGFR = VU/P

• V = urine volume, mL/minV = urine volume, mL/min

• U = urine inulin, g/mLU = urine inulin, g/mL

• P = plasma inulin, g/mLP = plasma inulin, g/mL

• GFR, mL/minGFR, mL/min

• Inulin is freely filtered, not secreted, Inulin is freely filtered, not secreted, not reabsorbednot reabsorbed

Tubular reabsorptionTubular reabsorption

• 200 L (50 gallons) filtered, 1 L urine 200 L (50 gallons) filtered, 1 L urine produced per day, 99% of water produced per day, 99% of water reabsorbedreabsorbed

• 99% of sodium reabsorbed99% of sodium reabsorbed

• Plasma glucose clearance = 0Plasma glucose clearance = 0

• Transport maximum for glucose = Transport maximum for glucose = 365 mg/min365 mg/min

Proximal tubuleProximal tubule

• Reabsorbs 67% of Na in lumenReabsorbs 67% of Na in lumen• All glucose reabsorbed All glucose reabsorbed • Water follows passivelyWater follows passively• 66-75% of filtrate reabsorbed before 66-75% of filtrate reabsorbed before

loop of Henleloop of Henle• At the end of the proximal tubule, At the end of the proximal tubule,

fluid is isosmotic even though Na and fluid is isosmotic even though Na and water reabsorbedwater reabsorbed

Concentrating mechanismConcentrating mechanism

• As fluid moves from proximal tubule As fluid moves from proximal tubule to descending loop of Henle it is to descending loop of Henle it is isosmotic with ECFisosmotic with ECF

Descending loop of HenleDescending loop of Henle

• Not permebale to NaCl, ureaNot permebale to NaCl, urea

• Permeable to waterPermeable to water

Concentrating mechanismConcentrating mechanism

• In descending loop, water moves out In descending loop, water moves out of tubule because descending loop is of tubule because descending loop is permeable to waterpermeable to water

Ascending limb of the loop of Ascending limb of the loop of HenleHenle

• No transport of NaCl in thin limbNo transport of NaCl in thin limb

• Permeable to NaCl, passive diffusionPermeable to NaCl, passive diffusion

• Impermeable to water, ureaImpermeable to water, urea

Concentrating mechanismConcentrating mechanism

• As fluid moves up ascending limb, As fluid moves up ascending limb, – NaCl moves out from fluid NaCl moves out from fluid passivelypassively in in

thin segmentthin segment– NaCl is transported out in thick segmentNaCl is transported out in thick segment– No movement of water here as No movement of water here as

impermeable to waterimpermeable to water

Medullary thick limbMedullary thick limb

• Active transport of NaCl from lumen Active transport of NaCl from lumen to ECFto ECF

• Impermeable to waterImpermeable to water

• Because of NaCl transport, urine is Because of NaCl transport, urine is slightly hypo-osmotic hereslightly hypo-osmotic here

Distal tubuleDistal tubule

• Transport of KTransport of K++, H, H++, NH, NH33++ into lumen into lumen

• Transports Na, Cl and HCOTransports Na, Cl and HCO33-- out of out of

lumenlumen

• Permeable to water, water follows Permeable to water, water follows NaClNaCl

Collecting ductCollecting duct

• Permeable to water. Water leaves Permeable to water. Water leaves urine for higher osmotic gradient in urine for higher osmotic gradient in extracellular fluid.extracellular fluid.

• Permeable to urea at the distal endPermeable to urea at the distal end

• Permeability to water is under Permeability to water is under hormonal control be ADHhormonal control be ADH

Concentrating mechanismConcentrating mechanism

• Only birds and mammals are able to Only birds and mammals are able to produce a concentrated urineproduce a concentrated urine

• Only birds and mammals have a loop Only birds and mammals have a loop of Henleof Henle

• Desert mammals have longer loops Desert mammals have longer loops of Henleof Henle

1. Active transport of NaCl out of ascending thick limb and distal tubule

2. Water follows osmotic gradient and leaves distal tubules and descending limb

3. Urea becomes more concentrated- the only part of collecting duct that is permeable to urea is medullary portion

4. Because of higher osmolarity at bottom of loop, water tends to leave descending limb of loop, therefore higher osmotic conc of tubular fluid at bottom of loop

5. Because of high osmotic conc of tubular fluid, NaCl follows passively out of ascending limb

End result-high osmotic concentration of urea in inner medulla interstitium is due to urea leaving collecting duct.