Excretion and Osmoregulation

Outline Introduction Comparative physiology of osmotic regulation Mammalian kidney Evolution of the vertebrate kidney

All organisms have an excretory system of some type in order to Manage solutes in body fluids Manage water content of the body Remove metabolic end-products Remove foreign substances

Two basic modes of excretion Ultrafiltration- pressure filtrate of blood, withholds protein and large solutes but water and small solutes pass Active transport- against a concentration gradient. If directed away from the organism, secretion. If directed toward the organism, reabsorption.

Three functions of excretory systems Ultrafiltration Secretion Reabsorption

Osmotic Environments Aquatic environments (71% of Earth’s surface) Sea waters- 3.5% salts Mediterranean – 4% salts Saturated with salts- Great salt lake (NaCl)- no fish, some shrimp Dead Sea (MgCl2)

Osmoconformers and osmoregulators Osmoconformer- change osmotic concentration of body fluids to match environment Osmoregulator- regulate composition of body fluids regardless of environment Seawater-1000 mOsM- animals risk losing water to the environment Fresh water-animals risk taking on too much water and losing salt

Two major categories of resistance to changes in osmotic environment Stenohaline –limited tolerance to changes in osmotic composition of environment-most fish are stenohaline Euryhaline- can tolerate wide fluctuations in environmental osmotic concentrations

Marine vertebrates Elasmobranchs, hagfish, crab-eating frogs Osmolarity of body fluids equals that of sea-water, i.e. 1000 mOsM Lampreys, teleost fishes Osmolarity of body fluids is one-third that of seawater, approx. 300 mOsM.

Marine Elasmobranchs Maintain salt at one-third that of seawater, but add organic urea to body fluids-100 times greater than mammals Total body fluid osmolarity is close to seawater Produce trimethylamine oxide (TMAO)- neutralizes toxic effects of urea Shark proteins including enzymes are dependent upon urea Excess Na+ excreted by ‘rectal gland’

TMAO urea

Teleosts All teleosts maintain body salt concentration at 1/3 that of seawater Marine teleosts must drink water and get rid of excess ions from water Ion excretion (Na, Cl) is performed by gills Mg, SO4, and other divalent ions secreted by kidneys

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

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

Amphibians Risk losing ions to freshwater Actively reabsorb Na and Cl across skin

Terrestrial vertebrates Face dehydration from any body surface Birds and reptiles produce uric acid to conserve water Develop skin which prevents water loss

Dipodomys microps, the Kangaroo rat

Kangaroo rats Abundant in deserts Do not drink water If given a diet of barley or oats, body weight remains the same for months Water (g) formed per gram of substrate: Starch = 0.56 Fat=1.07 Protein=0.45

Nasal turbinates Increase water reabsorption from exhaled air Nasal turbinates reduce temperature of exhaled air More water vapor condenses on nasal epithelium during exhalation

Phylogeny of osmoregulatory and excretory organs

Contractile vacuole in Paramecium Found only in freshwater organisms Not a true excretory organ because it does not produce an ultrafiltrate Excess water is channeled to vacuole, vacuole swells Water expelled through pore

Paramecium- contractile vacuole

Protonephridia Blind ended excretory organ Solenocyte (1 flagella), flame cell (many flagella) Found in flatworms, rotifers May or may not produce an ultrafiltrate Since there is no circulatory system in these animals, flame cells must be located throughout the body

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

Metanephridia Only in eucoelomate animals Filters fluid from coelom Produces an ultrafiltrate

Earthworm

Malpighian tubules Found in insects KCl and NaCl transported from coelomic fluid into Malpighian tubules Initial urine formed in tubules Final urine formed in rectum Produces a concentrated urine but not an ultrafiltrate

How it works K+ actively transported into M. tubule, conc. 30 times body fluid Passive Cl diffusion into M. tubule Water follows ions In hind gut, water is reabsorbed, uric acid precipitates

Direction of blood flow Afferent arteriole Glomerulus Efferent arteriole Vasa recta Vein

Function of nephron Glomerular filtration (urea, glucose, Na, K, Cl)-no proteins Same proportion of ions, solutes and water in glomerulus as in blood plasma No blood cells filtered Filtration based upon molecular size

Function of nephron Tubular reabsorption 99.9% of water Most salts Most reabsorption by the proximal tubules

Function of the nephron Tubular synthesis Some amino acids are deaminated Tubular secretion Regulates blood levels of K, H+ and HCO3

Brush border in proximal convoluted tubule

Glomerular filtration 125 mL/min, about 200 L/day Affected by Net hydrostatic pressure difference between capillary and Bowman’s capsule-favors filtration Colloid osmotic pressure of blood, opposes filtration Hydraulic permeability-sieve like properties of the filtration barrier Capillary endothelium Basement membrane Bowman’s capsule

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Calculating GFR Patient injected with inulin After a while, plasma sample drawn and inulin concentration measured in plasma Concentration of inulin measured in urine Urine volume determined Since all inulin removed, GFR equals inulin clearance

Glomerular Filtration rate GFR = VU/P V = urine volume, mL/min U = urine inulin, g/mL P = plasma inulin, g/mL GFR, mL/min Inulin is freely filtered, not secreted, not reabsorbed

Tubular reabsorption 200 L (50 gallons) filtered, 1 L urine produced per day, 99% of water reabsorbed 99% of sodium reabsorbed Plasma glucose clearance = 0 Transport maximum for glucose = 365 mg/min

Proximal tubule Reabsorbs 67% of Na in lumen All glucose reabsorbed Water follows passively 66-75% of filtrate reabsorbed before loop of Henle At the end of the proximal tubule, fluid is isosmotic even though Na and water reabsorbed

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

Descending loop of Henle Not permebale to NaCl, urea Permeable to water

Concentrating mechanism In descending loop, water moves out of tubule because descending loop is permeable to water

Ascending limb of the loop of Henle No transport of NaCl in thin limb Permeable to NaCl, passive diffusion Impermeable to water, urea

Concentrating mechanism As fluid moves up ascending limb, NaCl moves out from fluid passively in thin segment NaCl is transported out in thick segment No movement of water here as impermeable to water

Medullary thick limb Active transport of NaCl from lumen to ECF Impermeable to water Because of NaCl transport, urine is slightly hypo-osmotic here

Distal tubule Transport of K+, H+, NH3+ into lumen Transports Na, Cl and HCO3- out of lumen Permeable to water, water follows NaCl

Collecting duct Permeable to water. Water leaves urine for higher osmotic gradient in extracellular fluid. Permeable to urea at the distal end Permeability to water is under hormonal control be ADH

Concentrating mechanism Only birds and mammals are able to produce a concentrated urine Only birds and mammals have a loop of Henle Desert mammals have longer loops of Henle

Active transport of NaCl out of ascending thick limb and distal tubule Water follows osmotic gradient and leaves distal tubules and descending limb Urea becomes more concentrated- the only part of collecting duct that is permeable to urea is medullary portion 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

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.