Lecture Osmoregulation and Excretion. Osmoregulation balancing the uptake and loss of water and solutes based on the controlled movement of solutes between.

Lecture

Osmoregulation and Excretion

Osmoregulation

• balancing the uptake and loss of water and solutes

• based on the controlled movement of solutes between internal fluids and the external environment

Osmosis and Osmolarity• cells require a balance

between uptake and loss of water

• water enters and leaves a cell through osmosis

• movement of water is determined by osmolarity or osmotic pressure

• Osmolarity = total solute concentration (in moles/liter)– solute concentration of a

solution determines the movement of water across a selectively permeable membrane

Selectively permeablemembrane

Solutes Water

Net water flow

Hyperosmotic side: Hypoosmotic side:

Lower free H2Oconcentration

•

Higher soluteconcentration

•

Higher free H2Oconcentration

•

Lower soluteconcentration

•

- If two solutions are isoosmotic, the movement of water is equal in both directions- If two solutions differ in osmolarity, the net flow of water is from the hypoosmotic (low solute) to the hyperosmotic (high solute) solution

water moves from low solute to high solute concentration

Osmotic pressure

-Osmosis is controlled by tonicity = degree to which a the concentration of a specific solute surrounding a cell causes water to enter or leave the cell-SO it is the concentration of solutes that causes the water to move

-experiment – U shaped tube divided by a membrane permeable to water only-increase the solute concentration in the right half of the tube-this causes water to flow toward the solutes-pressure needed to counteract this rise in water= osmotic pressure-therefore increasing solute concentration increases osmotic pressure-water will move in to decrease this OP

-Osmotic pressure is important in determining how much fluid remains in your blood plasma and how much leaves to surround the cells in your tissues

Osmotic Challenges to Aquatic Animals

• Osmoconformers – marine animals– are isoosmotic with their surroundings and do not

regulate their osmolarity• Osmoregulators – aquatic animals that expend energy

to control water uptake and loss – exist in a hyperosmotic or hypoosmotic environment– can be freshwater, marine or animals that live in

temporary waters– can also be terrestrial animals too!

• aquatic animals can be classified on how well they tolerate changes in external osmolarity

• stenohaline - cannot tolerate substantial changes in external osmolarity

• euryhaline - can survive large fluctuations in external osmolarity

Marine Animals

• all osmoconformers are marine• internal osmolarity of its tissue fluids is the same as its

environment• but they can differ significantly in the concentrations of specific

solutes• must actively transport these solutes to maintain homeostasis• but not all marine animals are osmoconformers!!!

– most marine animals are osmoregulators

Marine Animals• most marine vertebrates and invertebrates are osmoregulators– marine bony fishes are hypoosmotic to

seawater– they constantly lose water by osmosis– they balance water loss by drinking

seawater– this seawater brings in salt ions– excess salt is eliminated through their

gills and kidneys– gills – chloride cells actively transport

chloride ions out and allow sodium ions to follow passively

– kidneys – excess Ca, Mg and sulfate ions are excreted with minimal loss of water

(a) Osmoregulation in a marine fish

Gain of waterand salt ionsfrom food

Excretionof salt ionsfrom gills

Osmotic waterloss through gillsand other partsof body surface

Gain of waterand salt ionsfrom drinkingseawater

Excretion of salt ions andsmall amounts of water inscanty urine from kidneys

Key

Water

Salt

Marine sharks

• considered osmoconformers• sharks and other chondroichthyes (cartilaginous fishes) use a unique method of

osmoregulation• sharks do NOT drink seawater – WHY??• unlike bony fish – sharks are not hypo-osmotic to seawater

– shark tissues contain a high level of urea to maintain osmotic pressure close to that of seawater – they are almost isoosmotic

– so no loss of water and no need to drink seawater– to protect their proteins from urea damage - body fluids contain the organic molecule

= trimethylamine oxide (TMAO) • like marine bony fish – sharks have an internal salt concentration that is lower

than seawater– salt tends to move in - especially across the gills– rectal glands remove excess NaCl via the cloaca (done by chloride cells in marine bony

fishes)• but they are not exactly isoosmotic – water will slowly move into the shark over

time

Freshwater Fish

• body fluids of freshwater fish are hyper-osmotic

• constantly gain water by osmosis and lose salts by diffusion

• these fish do NOT drink water• excrete large amounts of very dilute

urine• salts are replenished by eating• some fish – e.g. salmon – travel between

salt and freshwater environments– these fish are euryhaline– when in rivers – osmoregulate like other

freshwater fish– when in the sea – osmoregulate like marine

fishes– increase their production of cortisol which

increases production of chloride cells in the gills – increased excretion of salt

(b) Osmoregulation in a freshwater fish

Gain of waterand some ionsin food

Uptake ofsalt ionsby gills

Osmotic watergain throughgills and otherparts of bodysurface

Excretion of salt ions andlarge amounts of water indilute urine from kidneys

Key

Water

Salt

(b) Osmoregulation in a freshwater fish

Gain of waterand some ionsin food

Uptake ofsalt ionsby gills

Osmotic watergain throughgills and otherparts of bodysurface

Excretion of salt ions andlarge amounts of water indilute urine from kidneys

Key

Water

Salt

(a) Osmoregulation in a marine fish

Gain of waterand salt ionsfrom food

Excretionof salt ionsfrom gills

Osmotic waterloss through gillsand other partsof body surface

Gain of waterand salt ionsfrom drinkingseawater

Excretion of salt ions andsmall amounts of water inscanty urine from kidneys

Key

Water

Salt

“Temporary” aquatic fish

• some aquatic animals can survive periods of dehydration or dessication– e.g. live in ponds or films of water

• enter a dormant state when their environments dry up = anhydrobiosis– e.g. tardigrades or water bears

• requires adaptations that keep their cell membranes intact– e.g. some nematodes – dessicated worms contain large amounts of sugars

• trehalose – sugar replaces the water normally associated with proteins and lipids

(a) Hydrated tardigrade (b) Dehydratedtardigrade

50 m

50 m

Terrestrial animals

• threat of dehydration is constant• routes for water loss

– urine & feces– across respiratory surfaces– across skin

• adaptations to reduce water loss are key to survival on land

• 1.) body coverings of most terrestrial animals help prevent dehydration

• 2.) behavioral modifications - desert animals & nocturnal lifestyle

• 3.) acquisition of water through eating moist food and producing water metabolically through cellular respiration

Water balance ina kangaroo rat(2 mL/day)

Water balance ina human(2,500 mL/day)

Ingestedin food (0.2)

Ingestedin food (750)

Ingestedin liquid(1,500)

Watergain(mL)

Waterloss(mL)

Derived frommetabolism (1.8)

Derived frommetabolism (250)

Feces (0.09)

Urine(0.45)

Feces (100)

Urine(1,500)

Evaporation (1.46) Evaporation (900)

• kangaroo rats lose so little water that they can recover 90% of the loss from metabolic water and gain the remaining 10% in their diet of seeds.

• these and many other desert animals do not drink.

Osmoregulation and Energy• Osmoregulators must expend energy to maintain

osmotic gradients• the amount of energy differs based on:

– 1. how different the animal’s osmolarity is from its surroundings

– 2. how easily water and solutes move across the animal’s surface

– 3. the work required to pump solutes across the membrane

• to minimize this energy cost – most animals have body fluids that are close to the salinity of their environment

Transport Epithelia in Osmoregulation• most animals osmoregulate by regulating the

solute content of body fluid that bathes their cells– in those animals with an open circulatory system –

tissues are bathed in hemolymph– in those animals with a closed system – cells are bathed

in interstitial fluid – derived from blood plasma• the composition of these fluids are maintained by

osmoregulatory structures– range from single cells to complex tissues/organs

Transport Epithelia in Osmoregulation

• most animals rely upon the presence of a transport epithelia– found in specific organs– one or more layers of epithelial cells that are specialized

for moving solutes in specific directions– they are typically arranged in complex tubular networks

Excretory Systems• variations on a tubular theme– filtration – collects a filtrate

from the blood into the tubule– reabsorption – transport

epithelium lining the tubule reclaim needed ions and water

– secretion – toxins and excess ions are extracted from body fluids and added to the filtrate

– excretion – the altered filtrate (urine) leaves the system

– absorption = entrance of new materials into the blood (e.g. via digestive absorption)

CapillaryFiltration

Excretorytubule

Reabsorption

Secretion

Excretion

Filtra

teU

rine

2

1

3

4

Excretory Systems

• animal systems to dispose of metabolic wastes and control body fluid composition

• 1. protonephridia• 2. metanephridia• 3. Malphigian tubules• 4. Nasal glands of marine birds• 5. Kidneys

Nephridia• protonephridia:– freshwater flatworms– network of dead-end tubules connected to external openings in the

worm– branch throughout the worm– tubules end as flame bulbs – contains a tuft of cilia– cilia beating draws in interstitial fluid and processes it– filtrate is released into the connected tubule and is expelled outside

as a dilute urine– osmoregulatory function – gets rid of excess water only

Tubules ofprotonephridia

Tubule

Flamebulb

Nucleusof cap cell

Cilia

Interstitialfluid flow

Opening inbody wall

Tubulecell

Nephridia• metanephridia:

– annelids– collect fluid directly from the coelom– pair found in each worm segment– fluid drawn in via a ciliated funnel –

opening is called the nephrostome– coiled tubule lined with a transport

epithelium – reabsorbs most solutes and moves them into the capillary network

– remaining filtrate (urine) moves outside via the nephridiopore

– osmoregulatory and excretory functions• removal of excess water and

nitrogenous wastes

Components of a metanephridium:

Collecting tubule

Internal opening

Bladder

External opening

Coelom Capillarynetwork

Malphigian tubules

• insects and terrestrial arthropods• osmoregulation and excretion

function • dead-end tips immersed in

hemolymph• open into the midgut• no filtration of fluids by this

system• transport epithelium secretes

wastes and certain solutes into the tubules

• water follows by osmosis• as fluid flows into the rectum –

water & solutes are reclaimed and waste are expelled out the anus

• main waste = uric acid

Digestive tract

Midgut(stomach)

Malpighiantubules

Rectum

IntestineHindgut

Salt, water, andnitrogenous

wastes

Feces and urine

Malpighiantubule

To anus

Rectum

Reabsorption

HEMOLYMPH

Nasal glands• marine birds, iguanas and some marine turtles• allows them to drink seawater – yet their urine has little NaCl in it• nasal glands within the beak produced a concentrated NaCl solution• remove excess sodium chloride from the blood – by counter-current

exchange• birds drink seawater – removal of NaCl from the water via these glands

Nasal saltgland

Ducts

Nostril with saltsecretions

(a) Location of nasal glandsin a marine bird

(b) Secretorytubules

(c) Countercurrentexchange

Key

Salt movementBlood flow

Nasal gland

CapillarySecretory tubuleTransportepithelium

Vein Artery

Central duct

Secretory cellof transportepithelium

Lumen ofsecretorytubule

Saltions

Blood flow Salt secretion

Kidney• Kidneys: formation of urine– contains the functional unit for

filtration = Nephron– production of urine,

absorption of water and salts

• Ureters: transfer of urine from kidneys to bladder

• Urethra: transfer of urine

from bladder to outside

-about one million nephrons-kidneys filter 180 L fluid per day!!!!-each nephron is a renal corpuscle + renal tubules-renal corpuscle: filtering unit consisting of a tangled cluster of capillaries -> glomerulus + glomerular capsule (Bowman’s capsule)-renal tubules: for reabsorption of water and ions leading to final urine volume and composition

Nephron

Nephron Types

Cortical nephron

Juxtamedullarynephron

Renalcortex

Renalmedulla

• path of filtrate through the nephron:

• Bowman’s capsule Proximal convoluted tubule Loop of Henle Distal convoluted tubule

• several nephrons dump into a common Collecting Duct

• water and salts are reclaimed from the filtrate flowing through the nephron

• water and salts are returned to the blood supply

• nephron is associated with capillary networks– 1. Peritubular capillary network –

“covers” the PCT and DCT– 2. Vasa recta – “covers” the loop of

Henle

Reabsorption through the Nephron

Nephron Organization

Afferent arteriolefrom renal artery Glomerulus

Bowman’scapsule

Proximaltubule

PeritubularcapillariesDistal

tubule

Efferentarteriolefrom glomerulus

Collectingduct

Branch ofrenal vein

Vasarecta

Descendinglimb

Ascendinglimb

Loopof

Henle

• renal corpuscle, PCT and DCT are located in the kidney’s cortex• the loop of Henle and Collecting duct are found in the medulla• two types of nephrons depending on where the renal corpuscle is located in the

cortex• 1. Cortical – closer to the periphery of the cortex• 2. Juxtamedullary – deeper in the cortex; closer to the medulla

Nephron

Nephron Types

Cortical nephron

Juxtamedullarynephron

Renalcortex

Renalmedulla

Nephron

Path of filtrate/urine:

• 80-85% of all nephrons• shorter loop of Henle• loop is covered with an

extensive capillary network (vasa recta)

• cortical nephrons make “normal” urine

Juxtamedullary Nephron

• 15-20% of nephrons are juxtamedullary nephrons• Renal corpuscles close to medulla and long loops of Henle extend into deepest medulla enabling

excretion of dilute or concentrated urine• Loop of Henle’s ascending limb divided into thick and thin regions – different functions in making urine

Filtration – The Glomerulus• glomerulus: capillary tangle derived from afferent arterioles (into) and lead into

efferent arterioles (out) • surrounded by a glomerular capsule (Bowman’s capsule) • glomerular capsule: site of initial filtration and the first step in the formation of

urine– consists of visceral and parietal layers– space between the visceral and parietal layers = glomerular/Bowman’s capsule– visceral layer consists of a layer of epithelial cells (podocytes) that cover the capillaries – endothelial cells of the capillary have gaps between them– spaces between the podocytes + spaces between the endothelial cells forms the

filtration membrane

Transport Epithelium of the Renal Tubule

• following filtration at the Bowman’s capsule – the filtrate moves through the rest of the nephron for the reabsorption of salts and water

• the transport epithelium of the PCT is a single layer of cuboidal epithelial cells with microvilli

• the cells change their shapes and structures in the loop of Henle (ascending limb= squamous; descending limb= cuboidal) and DCT (cuboidal)

• each cell type has a unique function in reabsorbing salts and water

Two Reabsorption Routes• one of two routes before re-

entering the blood• Paracellular reabsorption

– 50% of reabsorbed materialmoves between cells bydiffusion

– cells are linked by “leaky” tight junctions

• Transcellular reabsorption– material moves through

the cell by active transport

Proximal tubule Distal tubule

Filtrate

CORTEX

Loop ofHenle

OUTERMEDULLA

INNERMEDULLA

Key

Active transport

Passive transport

Collectingduct

NutrientsNaCl

NH3

HCO3 H2O K

H

NaClH2O

HCO3

K H

H2ONaCl

NaCl

NaCl H2O

Urea

– the transport epithelium lining the nephron reabsorbs about 99% of the filtered water and many of the solutes and returns them to the blood plasma

– principal materials reabsorbed – glucose, amino acids, urea, Na+, K+, Ca+, Cl-, HCO3- and HPO4-

– return to the blood through reabsorption into the peritubular capillary network or vasa recta

Tubular Reabsorption = The Tubules

The Proximal Convoluted Tubule• PCT is the site of water reabsorption

(PASSIVE) - associated with the ACTIVE reabsorption of sodium ions– active Na+ uptake from the PCT is by

sodium pumps – sodium pumped out of the transport

epithelium into the interstitial fluid between the nephron and the capillary

– creates a Na+ gradient of higher Na+ in the filtrate and lower Na+ in the PCT cell

– causes Na+ to diffuse out of the urine to replace it (carries a glucose with it)

– chloride, bicarbonate and phosphate ions follow it - salt reabsorption

– the active transport of ions into the blood plasma increases osmotic pressure within the blood

– therefore water moves out of the PCT into the capillaries PASSIVELY!

Loop of Henle• active transport of Na+ continues

through the loop of Henle• descending loop of Henle is quite

permeable to water but impermeable to solute movement– salt reabsorption in the ascending limb

determines how much water is reabsorbed from the descending limb

– known as a counter current multiplier system

• ascending loop is the opposite – permeable to salt – Na+ is pumped out of the cell into the

blood– causes more Na+ to diffuse into the cell –

carries with it Cl- and K+ ions (K+ ions return to the filtrate)

– positively charged cations diffuse into the blood (more of a –ve charge in the interstitial fluid)

Reabsorption within Loop of Henle• counter current multiplier system• creates a Na gradient within the

interstitial fluid around the nephron• determines the movement of water out of

the filtrate and eventually into the blood

1. Na+ pumps in the ascending limb pump Na+ into the interstitial fluid between the two limbs of the loop of Henle

- causes a temporary increase in osmotic pressure within the interstitial fluid

2. increase in OP causes water to be “sucked out” of filtrate in descending limb into the interstitial fluid

3. water then moves into the capillaries4. NaCl is returned to the ascending limb to

start over again

DCT and Collecting Duct

• DCT and collecting duct are normally impermeable to water !!!!

• the DCT and CD become permeable upon action of hormones

• two types of cells found in the DCT and CD– principal cells – contain receptors for

the hormones aldosterone and ADH• increases the synthesis of:• 1. Na/K pumps – for more NaCl

reabsorption• 2. aquaporins - form water “pores” in the

principal cell – more water reabsorption

DCT and Collecting Duct

• intercalated cells – play a role in maintaining blood pH

• pump H+ ions into the urine• the H+ ions combine with ammonia to form

ammonium• urine is slightly acidic and smells like

ammonia

Reabsorption & Secretion in the Collecting Duct

• By end of DCT, 95% of solutes & water have been reabsorbed and returned to the bloodstream

• Cells in the collecting duct make the final adjustments– principal cells reabsorb more Na+ and secrete K+– intercalated cells reabsorb more K+ & bicarbonate

ions and secrete H+

Nitrogenous Wastes

• function of the kidney is two-fold– osmoregulation– excretion of metabolic wastes

• metabolic wastes must be dissolved in water • major metabolic wastes are due to breakdown of

proteins and nucleic acids• 1. Ammonia – very toxic, requires a lot of water to

eliminate• 2. Urea• 3. Uric acid – less toxic, requires little water to

eliminate

Ammonia• product of amino acid breakdown• can only be tolerated in very low concentrations• to decrease toxicity requires large amounts of water

– no problem for aquatic animals• ammonia diffuses easily across cell membranes

– in aquatic invertebrates – diffuses across the body wall• fish – ammonia is lost as ammonium across the

surface of the gills• problem for terrestrial animals• also a problem for marine animals

– since they lose large amounts of water via osmosis

Urea

• mammals, adult amphibians, sharks and some marine fishes and turtles excrete urea

• produced in the liver through the urea cycle– combination of NH3 with CO2 and H2O– requires energy

• can be transported safely and stored in tissues without toxicity

• urea is often retained in body fluids – contributes to osmotic pressure

Uric acid

• in humans - formed from the breakdown of nucleic acids– breakdown of purines – A and G

• also the main excretory product of insects, terrestrial snails, many reptiles and birds

• does not dissolve well in water• excreted as a semi-solid with little water loss• even more energetically expensive to synthesis then urea• guano – droppings of birds

– mixture of uric acid and brown feces– excellent source of nitrogen for soils

Antidiuretic Hormone

• also known as vasopressin• release is controlled by the

hypothalamus control over the posterior-pituitary’s release of ADH

• Antidiuretic hormone (ADH) makes the collecting duct epithelium more permeable to water

• an increase in osmolarity of blood triggers the release of ADH, which helps to conserve water

• binding of ADH to receptor molecules leads to a temporary increase in the number of aquaporin proteins in the membrane of collecting duct cells

ThirstHypothalamus

ADH

Pituitarygland

Osmoreceptors inhypothalamus trigger

release of ADH.

STIMULUS:Increase in blood

osmolarity (forinstance, after

sweating profusely)

Homeostasis:Blood osmolarity

(300 mOsm/L)

Drinking reducesblood osmolarity

to set point.

H2O reab-sorption helpsprevent further

osmolarityincrease.

Increasedpermeability

Distaltubule

Collecting duct

ADH• ADH –regulates water reabsorption by

increasing the permeability of the principal cells in the collecting duct to water– in the absence of ADH the principal

cells of the CT have low permeability to water

– ADH stimulates the insertion of aquaporins into the apical membrane• water permeability increases• when the OP of the blood plasma

increases due to dehydration – osmoreceptors in the hypothalamus detect this drop and stimulate the release of ADH

• increased OP of blood plasma also causes increased reabsorption of water from the urine through the aquaporin channels

ADHreceptor

COLLECTINGDUCT CELL

LUMEN

Second-messengersignaling molecule

Storagevesicle

Aquaporinwater channel

Exocytosis

H2O

H2O

ADH

cAMP

Renin-Angiotensin-Aldosterone

• when blood volume and BP drop – kidney secretes renin into the blood

• in the blood renin cleaves angiotensinogen (made by liver hepatocytes) to form the active enzyme angiotensin I

• the enzyme ACE (in the lung) – cleaves angiotensin I to form angiotensin II

• angiotensin II– 1. enhances reabsorption of Na+, Cl+ and

water in the PCT – 2. stimulates the release of

aldosterone by the adrenal cortex • aldosterone: stimulates the principal cells of

the DCT & collecting ducts to reabsorb more Na and Cl- and secrete more K into the blood

• osmotic consequence of this causes an increased reabsorption of water

• increase in blood volume and pressure results

Adaptations by the Vertebrate Kidney: Mammals

• The juxtamedullary nephron is key to water conservation in terrestrial animals– juxtamedullary neurons with their

long loops of Henle are responsible for the production of concentrated urine by allowing greater retention of water

– also allows mammals to get rid of nitrogenous wastes and salts without losing too much water

• Na+ gets pumped into the interstitial fluid between the loop and DCT and Collecting Duct

• causes water to move out of DCT and Collecting Duct (but ADH must be produced for this to happen)

so mammals that inhabit dry environments have long loops of Henle, while those in fresh water have relatively short loops

Birds and Other Reptiles

• Birds have the same kinds of nephrons as mammals BUT – conserve water by excreting uric acid instead of urea

• some reptiles have only cortical nephrons but also excrete nitrogenous waste as uric acid– cortical nephrons produce iso-osmotic or hypo-

osmotic urine – the cloaca is used to reabsorb water

Freshwater Fishes and Amphibians

• Freshwater fishes conserve salt in their DCTs and excrete large volumes of dilute urine– freshwater fish are hyper-osmotic to their environment– must excrete water continuously– kidneys have many nephrons producing filtrate at a high

rate

• Kidney function in amphibians is similar to freshwater fishes– amphibians conserve water on land by reabsorbing water

from the urinary bladder

Marine Bony Fishes

• Marine bony fishes are hypo-osmotic compared with their environment– lose water– so they gain water and salts from the environment

• their kidneys have fewer and smaller nephrons• their nephron have small glomeruli and some lack

glomeruli entirely• filtration rates are low, and very little urine is excreted• main function is to get rid of divalent ions like calcium,

magnesium and sulfate ions– secrete these into the PCT and excrete them in their urine