Topic11:HumanPhysiology11.3TheKidneyandOsmoregulation

EssentialUnderstanding:11.3Allanimalsexcretenitrogenouswasteproductsand

someanimalsalsobalancewaterandsoluteconcentrations.

Excretion•  Releaseofmetabolicwastesandexcesswater

MetabolicWastes•  Producedbychemicalreactionslikerespiration,hydrolysis,synthesis,andneutralization– Water–  Carbondioxide–  Salts,–  Urea

NitrogenousWasteProducts

•  Wasteproductfrommetabolismofaminoacids•  Wecannotstoreexcessaminoacidsthusundergodeamination(removaloftheaminegroupNH2)–  Aminegroupisincorporatedintowastemolecules:

•  Urea(Mammals)•  UricAcid(Birds)•  Ammonia(Fish)

•  Organs/structuresusedtoremovemoleculeslikethoseabovevarybasedonspecies: eg.Humans –kidneys Insects –Malpighiantubules

ExcretioninInsects•  Insectshaveanopencirculatorysystem(bloodoutside

ofbloodvessels)•  ThereareMalpighiantubulesinthecavitieswhereblood

pools–  Closedatthedistalend,openstotheinsectsgutattheproximalend

–  Selectivereabsorptionoccurshere,wasteproductsremaininthetubules(eg.Saltions,nitrogenouswastes(uricacid),excesswateretc.)whereitmeetsupwiththegutandremovedwiththedigestivewastes(feces)

Fromboundless.com

MalpighianTubules

Fromhttp://www.pleasanton.k12.ca.us/avhsweb/thiel/apbio/review/excretory.html

ExcretioninMammals•  Mammalshaveaclosedcirculatorysystem(bloodinsideofbloodvessels)

•  Thekidneysfunctiontofilterwasteproductsfromtheblood–  Renalartery–bloodenterskidney–  Renalvein–bloodexitskidney

Nephron•  Functionalunitofthekidney

Youneedtobeabletodrawandlabeladiagramofthe

nephron

Nephrons•  Filteringunitofthekidney(1.25millionunits)•  Consistsof

1.  Capillarybed(glomerulus)2.  Bowman’scapsule(surroundsglomerulus)3.  ProximalConvolutedTubule(PCT)4.  LoopofHenle5.  DistalConvolutedTubule(DCT)6.  Peritubularbed(secondcapillarybedsurroundingthe

tubules)7.  Collectingduct

Cortex

Medulla

•  Unfiltered blood flows into nephron via afferent arteriole & then into glomerulus (fenestrated)

•  Pores/slits opens when blood pressure increases from efferent arteriole

•  Site of glomerular ultrafiltration

Afferentarteriole

•  The filtrate is pushed out into the Bowman’s capsule through the basement membrane preventing large molecules from entering •  These are small particles (water, urea, ions (H+,

HCO3-, Na+), glucose, amino acids, vitamins… etc.

•  Larger molecules (RBC, WBC, proteins) carry thru the efferent arteriole

•  Leads to peritubular capillaries eventually to the renal vein

efferentarteriole

•  Filtrate enters the proximal convoluted tubule (PCT) •  Site of Selective reabsorption •  PCT is one cell thick; has mitochondria for active

transport, and microvilli to increase absorption

Reabsorptionof:

- mostglucose

- mostA.A.

- ~65%ofNa+

- ~65%water

- smallproteins

- HCO3-

- vitamins

- Otherions

Glucose,A.A.,water

•  Salt(Na+,Cl-,K+)– Majorityofsaltsareactivelytransportedintotubuleandthentakenoutofperitubularcapillarybed

•  Water–  Followssaltviaosmosisthroughsameroute–  Fromhypotonictohypertonicregionfollowingsolutes

•  Glucose– Allglucoseisreabsorbedintobloodstreamviaactivetransport

•  Concentrated filtrate moves into the loop of Henle •  Descending loop - 10-20% water reabsorbed (relatively

impermeable to salts)

•  Ascending loop - 20% Na+ (relatively impermeable to water)

Na+

H2O

LoopofHenlecreatesahypertonic(highsolute

concentration)environmentinthemedullaofthekidney

•  Concentrated filtrate moves into the distal convoluted tubule (DCT)… from efferent arteriole… •  Site of Tubular Secretion (some in the PCT too!) •  Hormone controlled secretion (aldosterone, ADH)

Na+

•  Concentrated filtrate moves into the Collecting Duct •  and now the URINE passes into the:

renal pelvis → ureter → urinary bladder → urethra → out of body

H2O

Urine

UrineComposition •  Dissolved salts (NaCl) •  Metabolic waste (ammonia, urea)

•  Note: ammonia is toxic so it is quickly converted to urea in the liver

•  Small molecules from the breakdown of hormones •  Sometimes there is protein, glucose, blood

•  These are NOT normal and usually signal something is nor functioning properly

WaterHomeostasisHormones: 1.  Antidiuretic Hormone (ADH)

·  Aka. Vassopressin Source: •  Release controlled by Hypothalamus

•  produced in the posterior pituitary gland

2.  Aldosterone Source: •  Produced by Adrenal Cortex

ADH 1.  Hypothalamus detects variations in osmolarity of blood

·  As [water] in blood changes, the amount of water diffusing in or out of hypothalamus cells changes

2.  Change stimulates it to vary the amount of releasing hormone sent to posterior pituitary

3.  Target cells for ADH = collecting duct (CD) and Distal Convoluted Tubule (DCT) ·  Cells of CD and DCT become more permeable to water in the

presence of ADH

4.  Peri-tubular environment is salty (NaCl) ·  Thus, any increase in water permeability causes water to leak

through membrane into peri-tubular capillary network

ADH-NegativeFeedback

•  When [water] in blood falls, the body becomes dehydrated

•  Posterior Pituitary signaled to release more ADH •  ADH acts on CD and DCT •  As More water is reabsorbed; [water] in blood ↑’s

•  = reduced stimulus to hypothalamus •  Hypothalamus slows secretion of releasing hormone

•  Balance is restored!

This is NEGATIVE FEEDBACK b/c a decrease in

[water] in blood stimulates a series of events that reverse the imbalance

ADH Do you know why you often produce a darker urine in the morning?

Because: ADH is primarily released at night!

Thus, we reabsorb much of our water while we sleep!

Aldosterone

•  Affects both WATER and SALT reabsorption •  Asaresult,itspresencecausesan↑inbothbloodpressure&

volumewithoutchangingtonicity(concentration)

•  Secretions of RENIN from juxtaglomerular apparatus promote release of aldosterone

JuxtaglomerularApparatus

•  prefix"juxta-"fromtheLatinprepositionmeaningnear,nearby,close.

•  acollectivetermreferringtothecellsneartheglomerulusinthekidney

•  Thejuxtaglomerularcellsarespecializedcellsthatstimulatethesecretionoftheadrenalhormonealdosterone

•  Playsamajorroleinrenalautoregulation

•  JGA cells are sensitive to changes in blood pressure •  When pressure DROPS, glomerular filtration slows

•  Stimulates secretion of RENIN

•  Renin is circulated (via blood) to adrenal cortex •  Stimulates release of aldosterone

•  Aldosterone affects DCT •  DCT excretes MORE K+ and reabsorbs more Na+

•  As more Na+ is reabsorbed, more H2O is absorbed •  ↑ in water & salt causes an ↑ in blood volume and ↑ in blood

pressure

ADHandAldosteroneBoth increase water reabsorption…. but….

1.  ADH •  Response to dehydration (lack of water) •  Reduced blood volume WITH an increase in

osmolarity

2.  Aldosterone •  Response to fluid and salt loss

•  Eg. Loss of blood (wound) or diarrhea •  Reduced blood volume WITHOUT an increase in

osmolarity

OverallfunctionofKidneys

•  Loweramountofbloodurea•  Loweramountofsaltionsinblood•  Loweredamountofwaterinblood•  Identicalglucoseandproteinlevels(innormalfunctioningkidney)

OsmoregulatorsvsOsmoconformersOsmoregulators

–  Internaltissueshavedifferentconcentrationsofsoluteconcentrationscomparedtoenvironment

–  Mechanismsadaptedtoregulatewaterbalance(highenergyrequired)

–  Mostanimals–  Freshwateranimalstendtobeosmoregulators

Osmoconformers–  Internaltissuessimilartoexternalenvironment–  Nomechanismsrequiredastheyareiso-osmotictowatersurroundings

–  Environmenttheyliveinarelimited–  Eg.Clams,scallops(molluscs),seastars(echinoderms),cnidarians,seacrabs,

–  SaltwateranimalstendtobeOsmoconformers