Osmoregulationand

Excretion

Osmoregulation: Regulation of solute concentrations and water balance by a cell or organism.

Osmoregulation balances the uptake and loss of water and solutes.

If cells uptake too much water they will burst If cells uptake too little water, or lose too

much water they will shrivel and die. Osmoregulation is impacted by water

intake/loss and also discharge of metabolic wastes.

Osmosis: movement of water across a selectively permeable membrane when two solutions separated by the membrane differ in osmotic pressure.

Initial flaccid cell

0.4 M sucrose solution Distilled water

Osmolarity(osmotic pressure): Total solute concentration expressed as molarity (moles of solute per liter of solution).

Unit of osmolarity: milliOsmoles per liter (mOsm/L); equivalent to solute concentration of 10-3M.

Human blood: 300 mOsm/L Seawater: 1000 mOsm/L

Isotonic: two solutions on either side of the membrane have same osmolarity; no net movement of water between two sides.

Hyperosmotic/ hypertonic: the solution with greater solute concentration (net movement of water towards this side)

Hypoosmotic/ hypotonic: the more dilute solution (net movement of water away from this side)

An animal has to have a way of maintaining water balance with its environment.

Osmoconformer: Isoosmotic with the surroundings Internal osmolarity same as the environment No net gain or loss of water All marine

Osmoregulator: Maintains its osmolarity independent of the environment. Because of the regulatory capacity can live in freshwater, dry

land, also marine habitats In hypoosmotic environment they discharge excess water In hyperosmotic environment they take up and retain more

water.

Based on what you know so far about thermoregulation can you make any guesses about osmoregulation in these two groups? Balancing act between control and cost.

Stenohaline: Cannot tolerate substantial changes in external osmolarity

Euryhaline: Can tolerate large fluctuations in osmolarity (organisms in intertidal zones, fishes that migrate between sea water and fresh water)

Osmoregulators and osmoconformers can be found in both these groups.

Some organisms in a tidal pool

Marine invertebrates: osmoconformes, need to transport some solutes to maintain homeostatis.

Marine bony fishes: lose water by osmosis drink large amounts of water actively transport out Cl- (chloride cells) passively transport out Na+

concentrated urine gets rid of other salts like Ca++, Na+, Mg++

Gain of water andsalt ions from foodand by drinkingseawater

Osmotic water lossthrough gills and other partsof body surface

Excretion ofsalt ionsfrom gills

Osmoregulation in a saltwater fish

Excretion of salt ions and small amountsof water in scantyurine from kidneys

Marine cartilaginous fishes: Shark tissue contains a high concentration of urea To prevent urea from damaging other organic molecules in

the tissues they have trimethyl amine oxide (TMAO) Because of high solute concentration in tissue water enters

the cells (sharks don’t drink) Produce concentrated urine.

Fresh water organisms: (opposite problem of marine organisms) Internal osmolarity is higher than surroundings;

problem of gaining water. Fishes don’t drink water , large volumes of urine Salt intake through food Chloride cells in gills actively transport in Cl-

Na+ follows

Excretion oflarge amounts ofwater in diluteurine from kidneys

Osmotic water gainthrough gills and other partsof body surface

Osmoregulation in a freshwater fish

Uptake ofsalt ionsby gills

Uptake ofwater and someions in food

Euryhaline organisms like salmon: In sea they drink sea

water and discharge salt through their gills

In freshwater they stop drinking and produce large volumes of dilute urine, gills take up salt

Life Cycle of Atlantic Salmon http://www.nefsc.noaa.gov/sos/spsyn/af/salmon/images/fig41_2.gif

Anhydrobiosis: dormant state when habitat dries up. 85% to 2% water in water bears. Cell membrane adaptations are poorly understood.

Hydrated tardigrade Dehydrated tardigrade

100 µm

100 µm

Land animals: Adaptations of body surface (thick cuticle) and

behavior (nocturnal) help reduce water loss. Some desert animals can metabolically generate

water (kangaroo rats)

Waterbalance in a kangaroo rat(2 mL/day)

Waterbalance ina human

(2,500 mL/day)

Watergain

Waterloss

Derived frommetabolism (1.8 mL)

Ingestedin food (0.2 mL)

Derived frommetabolism (250 mL)

Ingestedin food (750 mL)

Ingestedin liquid (1,500 mL)

Evaporation (900 mL)

Feces (100 mL)Urine(1,500 mL)

Evaporation (1.46 mL)

Feces (0.09 mL)Urine(0.45 mL)

Transport epithelia: Animals that live on sea water can also eliminate salt through specialized epithelial cells that can regulate the salt concentration.

Nostrilwith saltsecretions

Nasal salt gland

Nitrogenous wastes: As a result of metabolism proteins and amino

acids produce ammonia (NH3). Ammonium ion (NH4+) is highly toxic Animals either get rid of ammonia promptly or

expend energy and convert it to less toxic forms.

Forms of nitrogenous wastes:

ammonia urea uric acid

(differ in cost to convert and toxicity)

Ammonia: Most fishes, animals that produce shell-less eggs.

Excrete bulk of ammonia through gills, minor

amounts through kidneys. Very toxic, has to be transported in very dilute

solutions

Urea: Mammals, adult amphibians, some marine , bony

fishes, sharks, turtles. Advantage: Lower toxicity. Can go through

circulatory system, stored. Does not have to be so dilute, so less water loss during excretion.

Disadvantage: High energy cost.

Animals can switch mode of excretion at different stages of their life cycle. Tadpoles (ammonia), adult amphibians (urea).

Uric acid: reptiles, birds, land insects, animals that produce

shelled eggs. Advantage: less toxic than urea, needs less water

to be excreted (semisolid paste). Disadvantage: more expensive to produce than

urea.

Humans produce small amounts of uric acid. Gout: condition caused by inability to eliminate uric acid.

Nitrogenous bases

Nucleic acids

Amino acids

Proteins

—NH2

Amino groups

Most aquatic animals, including most bony fishes

Mammals, most amphibians, sharks, some bony fishes

Many reptiles (including birds), insects, land snails

Ammonia Urea Uric acid

Amount of nitrogenous waste: linked to energy budget (higher in endotherms than ectotherms).

Steps in urine formation: Filtration Reabsorption Secretion

Filtration

Reabsorption

Secretion

Excretion

Excretorytubule

Capillary

Filtrate

Urin

e

Filtration: Cells, large molecules (proteins) stay in the body

fluid Small molecules, (salts, sugars, amino acids,

nitrogenous wastes) and water pass through and form filtrate

Reabsorption : Selective process. Recovery of useful molecules Active transport – reabsorption of certain salts,

vitamins, hormones, amino acids Wastes, nonessential molecules are left behind

Secretion: Selective pumping of various solutes to adjust

osmotic movement of water into and out of the filtrate

Final step – removal of this filtrate from the body – release of urine

Diverse excretory systems:

Excretory system plays a very important role in water balance and homeostasis.

Systems show a lot of variation in different groups

Basic structure – network of tubules that provide a large surface area

Protonephridia(tubules)

Tubule

Nephridioporein body wall

Flamebulb

Interstitial fluidfilters throughmembrane wherecap cell and tubulecell interdigitate(interlock)

Tubule cell

Cilia

Nucleusof cap cell

Protonephridia - excrete low concentration of solute; flatworms, some rotifers, some annelids and molluscs

Metanephridia - found in most annelids (e.g. earthworms); excretory organs open internally to the coelom

Collectingtubule

Nephridio-pore

Capillarynetwork

Coelom

Bladder

MetanephridiumNephrostome

Malpighian tubules – extend from hemolymph to digestive tract; cells secrete nitrogenous wastes and other solutes into

hemolymph, these molecules and water move into malpighian tubules, excess water is reabsorbed in the rectum; other essential solutes are reabsorbed; lets the animal conserve water; found in insects, capability to conserve water helps in the

success of this group

Salt, water, andnitrogenous

wastes

Digestive tract

Midgut(stomach)

Malpighiantubules

RectumIntestine Hindgut

Reabsorption of H2O,ions, and valuableorganic molecules

Malpighiantubule

HEMOLYMPH

Anus

Rectum

Feces and urine

Kidneys: vertebrates and some other chordates, same basic plan as other systems – but highly organized and complex, closely associated with a network of capillaries.

Nephron: functional unit of a kidney

Excretory organs and major associated blood vessels

RenalmedullaRenalcortex

Renalpelvis

Section of kidney from a ratKidney structure

Ureter

Kidney

Glomerulus

Bowman’s capsule

Proximal tubule

Peritubular capillaries

Afferentarteriolefrom renalartery

Efferentarteriole from glomerulus

Distaltubule

Collectingduct

SEM20 µm

Branch ofrenal vein

Filtrate and blood flow

Vasarecta

DescendinglimbAscendinglimb

LoopofHenle

Renalmedulla

Nephron

Torenalpelvis

Renalcortex

Collectingduct

Juxta-medullarynephron

Corticalnephron

Posterior vena cava

Renal artery and vein

Aorta

Ureter

Urinary bladder

Urethra

Structure of mammalian excretory system

RenalmedullaRenalcortex

Renalpelvis

Section of kidney from a ratKidney structure

Ureter

Kidney

Renalmedulla

Nephron

Torenalpelvis

Renalcortex

Collectingduct

Juxta-medullarynephron

Corticalnephron

Glomerulus

Bowman’s capsule

Proximal tubule

Peritubular capillaries

Afferentarteriolefrom renalartery

Efferentarteriole from glomerulus

Distaltubule

Collectingduct

SEM20 µm

Branch ofrenal vein

Filtrate and blood flow

Vasarecta

DescendinglimbAscendinglimb

LoopofHenle

Detailed look at processing of blood in nephron:

Filtration Afferent arteriole has a bigger diameter than

efferent arteriole, thus pressure builds in the glomerulus

During filtration, blood in glomerulus is forced into Bowman’s capsule (cup shaped swelling at the blind end of the tubule)

Filtration is nonselective, only based on size, caused by the high blood pressure in the capillaries in the Bowman’s capsule.

Microscopic view of Bowman’s capsule

From filtrate to urine:

Filtrate contains water, salts (like NaCl), bicarbonate ions, hydrogen ions, urea, glucose, amino acids, drugs

Filtrate

H2O

Salts (NaCl and others)

HCO3–

H+

Urea

Glucose; amino acids

Some drugs

Key

Active transport

Passive transportINNERMEDULLA

OUTERMEDULLA

NaCl

H2O

CORTEX

Descending limbof loop ofHenle

Proximal tubule

NaCl Nutrients

HCO3–

H+

K+

NH3

H2O

Distal tubule

NaCl HCO3–

H+K+

H2O

Thick segmentof ascendinglimb

NaCl

NaCl

Thin segmentof ascendinglimb

Collectingduct

Urea

H2O

Proximal tubule: very critical recapture, to reabsorb nutrients and to maintain homeostasis in this case by maintaining pH and water balance

Na+, is reabsorbed by active transport, Cl- ions follow

Nutrients are absorbed actively H+ is actively secreted into the

tubule and NH3+ is transported passively to regulate the pH of the urine.

HCO3- is passively reabsorbed

Urine becomes more concentrated

CORTEX

Proximal tubule

NaCl Nutrients

HCO3–

H+

K+

NH3

H2O

Descending loop of Henle: Cells lining the tubule here

have special water transport proteins called aquaporins.

Cells lining the DLH are hyper osmotic, water moves out by osmosis

Reabsorption of other solutes – not significant

INNERMEDULLA

OUTERMEDULLA

H2O

CORTEX

Descending limbof loop ofHenle

Proximal tubule

NaCl Nutrients

HCO3–

H+

K+

NH3

H2O

Ascending loop of Henle:

Transport epithelium has ions channels (actively transports out NaCl)

Transport epithelium does not have water channels

Filtrate become more dilute as it travels up ALH.

INNERMEDULLA

OUTERMEDULLA

NaCl

H2O

CORTEX

Descending limbof loop ofHenle

Proximal tubule

NaCl Nutrients

HCO3–

H+

K+

NH3

H2O

Thick segmentof ascendinglimb

NaCl

Thin segmentof ascendinglimb

INNERMEDULLA

OUTERMEDULLA

NaCl

H2O

CORTEX

Descending limbof loop ofHenle

Proximal tubule

NaCl Nutrients

HCO3–

H+

K+

NH3

H2O

Distal tubule

NaCl HCO3–

H+K+

H2O

Thick segmentof ascendinglimb

NaCl

NaCl

Thin segmentof ascendinglimb

Collectingduct

Urea

H2O

Distal tubule: Regulates K+,

H+, NaCl and HCO3- and maintains ion concentration in the body

Collecting duct: Urea is absorbed

in the deeper parts of the collecting duct.

Water absorption is controlled by hormones

1600L of blood flows through the human kidney (300 times the total blood volume)

180L of initial filtrate is produced in the kidney 1.5L of urine is voided

Kidney’s role in water conservation: Human kidney can produce urine that is 4 to

5 times more concentrated than blood. Some desert animals can produce urine that

is 25 times more concentrated than blood Production of hyperosmotic urine is very

energy consuming Two solutes that play a key role in control of

urine osmolarity: NaCl and urea

Two solute model: Filtrate from Bowman’s capsule has same

osmolarity as blood. At the proximal tubule water and salts are

reabsorbed, so volume decreases but osmolarity stays the same.

While going through DLH osmolarity increases because water is absorbed epithelium is permeable to water not salts

Highest osmolarity occurs at the elbow of Loop of Henle

INNERMEDULLA

OUTERMEDULLA

CORTEX

Osmolarity ofinterstitial

fluid(mosm/L)

H2O

Activetransport

Passivetransport

300300

300 100

100

400 200H2O

H2O

H2O

H2O

H2O

H2O

600 400

900 700

1200

300

400

600

12001200

600

900

300

400

In the Ascending limb (permeable to salts and not water) osmolarity decreases

Loop of Henle creates concentration gradients, by expending energy: countercurrent multiplier system.

INNERMEDULLA

OUTERMEDULLA

CORTEX

Osmolarity ofinterstitial

fluid(mosm/L)

NaClH2O

Activetransport

Passivetransport

300300

300 100

100

400 200H2O

H2O

H2O

H2O

H2O

H2O

600 400

900 700

1200

300

400

600

12001200

600

900

300

400NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

Ascending and descending vessels of vasa recta – blood flows in the opposite direction as the kidney’s osmolarity gradient

As blood flows down towards the inner medulla, blood looses water and gains NaCl

As blood flows away from the medulla towards the cortex, water is gained and NaCl is lost

Urea enters the loop of Henle by diffusion but some leaks out of the collecting duct. This maintains the high osmolarity of the interstitial fluid, draws water out of the filtrate in the collecting duct and keeps urine hyperosmotic.

Urine is isotonic to the interstitial fluid of inner medulla but hyperosmotic to blood

INNERMEDULLA

OUTERMEDULLA

CORTEX

Osmolarity ofinterstitial

fluid(mosm/L)

NaCl

Urea

H2O

Activetransport

Passivetransport

300300

300 100

100

400 200H2O

H2O

H2O

H2O

H2O

H2O

600 400

900 700

1200

300

400

H2O

600

12001200

600

900

300

400NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

UreaH2O

UreaH2O

H2O

H2O

H2O

H2O

Like counter current exchange but this process costs a lot of energy expenditure.

This counter current-like system helps maintain the steep gradient, it is not lost due because the active transport maintains it

For its size energy consumes a lot of ATP

Adaptations of vertebrate kidney to a variety of environments

Mammalian kidney very well adapted to terrestrial life: juxtamedullary nephron is very well adapted to produce hyperosmotic urine.

Bannertail kangaroo rat(Dipodomys spectabilis)

Beaver (Castor canadensis)

Animals in drier environments need to produce more hyperosmotic urine, have longer Loop of Henle and deeper medulla

Animals in watery environments have shorter Loop of Henle and less ability to concenmtarte urine

Birds Kidneys do not have

nephrons that extend into the medulla

Chief water conservation adaptation is production of uric acid

Reptiles Produces uric acid Have cortical nephrons

which produce urine that is isotonic or even hypertonic to blood but water is reabsorbed in cloaca and urine discharged from the body is highly concentrated

Roadrunner(Geococcyx californianus)

Desert iguana(Dipsosaurus dorsalis)

Freshwater fishes Large volume of

urine, salts are reabsorbed in the distal tubules

Frogs When on land they

conserve water by reabsorption across the epithelium of the urinary bladder

Rainbow trout(Oncorrhynchus mykiss)

Frog (Rana temporaria)

Marine bony fishes Problem: gain salts from environment and tend to

lose water Lack distal tubule, smaller glomerulous, can adjust

amount of urine

Northern bluefin tuna (Thunnus thynnus)

Kidney function, water balance, blood pressure – hormonal control

ADH control & RAAS system

ADH control Osmoregulatory function of

kidney is controlled by nerves and hormones

Antidiuretic hormone (ADH)/vasopressin – key role in osmoregulation

ADH is produced by hypothalamus and is stored in the posterior pituitary

Hypothalamus monitors blood osmolarity using osmoreceptor cells

Blood osmolarity rises above 300mOsm/L: (eating salty food, sweating)

ADH is released into the bloodstream Changes epithelium of distal tubule and collecting duct and

makes them more permeable to water, increases reabsorption of water

Decreases blood osmolarity and increases urine osmolarity by decreasing urine volume

Blood osmolarity decreases below 300mOsm/L: (drinking large volumes of fluids)

ADH secretion goes down Permeability of distal tubule and collecting duct goes down, less

water is reabsorbed Volume of urine is high, osmolarity of blood goes up

Osmoreceptorsin hypothalamus

Hypothalamus

ADH

Pituitarygland

Increasedpermeability

Distaltubule

Thirst

Drinking reducesblood osmolarity

to set point

Collecting duct

H2O reab-sorption helpsprevent further

osmolarityincrease

Homeostasis:Blood osmolarity

STIMULUSThe release of ADH istriggered when osmo-receptor cells in the

hypothalamus detect anincrease in the osmolarity

of the blood

Osmolarity and ADH are linked by negative feedback loop

Genetic disorders that affect ADH production or ADH receptors can affect osmoregulatory function of kidney, cause severe dehydration by producing very dilute urine: diabetes insipidus

Alcohol inhibits ADH release, can cause excessive water loss, some dehydration and symptoms of hangover

Distaltubule

Aldosterone

Homeostasis:Blood pressure,

volume

STIMULUS:The juxtaglomerular

apparatus (JGA) respondsto low blood volume or

blood pressure (such as due to dehydration or

loss of blood)

Increased Na+

and H2O reab-sorption in

distal tubules

Reninproduction

Arterioleconstriction

Adrenal gland

Angiotensin II

Angiotensin I

JGA

Renin

ACE

An

gio

ten

sin

og

en

RAAS (Renin-angiotensin-aldosterone) system

When blood volume is low, blood pressure drops

Juxta glomerular apparatus (JGA) located near the afferent arteriole releases rennin

Renin converts angiotensinogen to angiotensin I

Angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II

Angiotensin II raises blood pressure by constricting arterioles

Angiotensin II also stimulates secretion of aldosterone by adrenal glands

Causes more reabsorption of Na+ and water and increases blood volume and pressure

High blood pressure drugs block ACE