Osmoregulation and Excretion

A.P. Biology

Ch. 44

Rick L. Knowles

Liberty Senior High School

Osmoregulation• Maintaining a balance of both water and

ions across a membrane/organism. Solute and water homeostasis.

• Osmolarity – moles of total solute per liter of water; usually in milliosmoles/L.

• Mechanism of homeostasis varies with the environment in which they’ve adapted (freshwater, saltwater, terrestrial).

Some Comparison

0

Milliosmoles/L (mosm/L)

Distilled,deionized Water

Freshwater 0.5 -15

300 Human Plasma

1,000 Seawater

5,000 Dead Sea

• Most animals are said to be stenohaline:– And cannot tolerate substantial changes in external

osmolarity; both osmoconformers and osmoregulators.

• Euryhaline animals:– Can survive large fluctuations in external osmolarity.

Figure 44.2

Tilapia, freshwater up to 2,000 mosm/L

Osmoregulation and Nitrogenous Wastes

• Other waste solutes must be removed from cells and organisms.

• A waste product of metabolizing amino acids and nucleic acids (deamination)- ammonia.

• Concept 44.2: An animal’s nitrogenous wastes reflect its phylogeny and habitat.

• The type and quantity of an animal’s waste products:– May have a large impact on its water balance.

Ammonia• Direct by-product of protein

and nucleic acids (deamination).

• Very toxic to cells.• Highly soluble in water.• Molecule of choice for

freshwater organisms; eliminated easily through kidneys, gill epithelia, etc.

• Downside: requires a lot of water.

Urea• Saltwater and terrestrial

mammals convert ammonia into urea.

• Less toxic; accumulate more in tissue.

• Less soluble in water than ammonia.

• Allows conservation of water.

Uric Acid• Birds and reptiles accumulate

waste in an egg.• Convert ammonia into uric acid.• Insoluble in water;

crystallizes.• Semisolid paste-guano.• Requires less water to

eliminate.

Proteins Nucleic acids

Amino acids Nitrogenous bases

–NH2

Amino groups

Most aquaticanimals, includingmost bony fishes

Mammals, mostamphibians, sharks,some bony fishes

Many reptiles(includingbirds), insects,land snails

Ammonia Urea Uric acid

NH3 NH2

NH2

O C

C

CN

CO N

H H

C O

NC

HN

O

H

• Among the most important wastes– Are the nitrogenous breakdown products of

proteins and nucleic acids

Figure 44.8

Osmoconformers• Most marine protists and invertebrates.• Are isoosmotic with marine environment.• Open channels and carriers for most ion

transport (Not all ions are in equilibrium).• Ex. Invertebrates like sea anemones, jellyfish,

and only vertebrate, Class Agnatha- hagfish.

Class Agnatha- Hagfish

Show me a real hagfish!

Video: Discovery- Blue Planet: Ocean World

Osmoregulators• Maintain constant osmotic

concentration in body fluids and cytoplasm despite external variations.

• Continuous regulation since environment and intake (diet) changes.

• Evolved special mechanisms for different environments.

• Ex. Most Vertebrates

The Problems• Freshwater Vertebrates- are

hyperosmotic, water enters body, tend to lose ions.

• Marine Vertebrates- are hypoosmotic, water leaves body, tend to gain ions.

• Terrestrial Vertebrates- are hypoosmotic, water leaves body through respiration, perspiration, skin.

Freshwater Protists• Problem: hyperosmotic; impossible to

become isoosmotic with dilute fresh water; tend to gain water; lose ions; no excretory organ.

• Solution: Contractile Vacuoles – active transport of water out of cell; less permeable to ions

• Downside: Active transport requires energy.

Freshwater Invertebrates• Water and wastes are passed into a

collecting vessel or primitive excretory organ.

• Membrane retains proteins and sugars and allows water and dissolved wastes to leave-selectively permeable.

• Ex. Freshwater jellyfish, etc,

• Concept 44.3: Diverse excretory systems are variations on a tubular theme.

• Excretory systems:–Regulate solute movement between

internal fluids and the external environment.

Excretory Processes• Most excretory systems

– Produce urine by refining a filtrate derived from body fluids

Figure 44.9

Filtration. The excretory tubule collects a filtrate from the blood.Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.

Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule.

Excretion. The filtrate leaves the system and the body.

Capillary

Excretorytubule

Filtrate

Urine

1

2

3

4

Nucleusof cap cell

Cilia

Interstitial fluidfilters throughmembrane wherecap cell and tubulecell interdigitate(interlock)

Tubule cell

Flamebulb

Nephridioporein body wall

TubuleProtonephridia(tubules)

Protonephridia: Flame-Bulb Systems• A protonephridium:

– Is a network of dead-end tubules lacking internal openings.

Figure 44.10

• The tubules branch throughout the body:

– And the smallest branches are capped by a cellular unit called a flame bulb.

• These tubules excrete a dilute fluid:

– And function in osmoregulation

Metanephridia• Each segment of an earthworm

– Has a pair of open-ended metanephridia

Figure 44.11Nephrostome Metanephridia

Nephridio-pore

Collectingtubule

Bladder

Capillarynetwork

Coelom

• Metanephridia consist of tubules:

– That collect coelomic fluid and produce dilute urine for excretion.

Terrestrial Insects• Problem: Must minimize water

loss.

• Solution: Use chitin as an exoskeleton.

Digestive tract

Midgut(stomach)

Malpighiantubules

RectumIntestine

Hindgut

Salt, water, and nitrogenous

wastes

Feces and urineAnus

Malpighiantubule

Rectum

Reabsorption of H2O,ions, and valuableorganic molecules

HEMOLYMPH

Malpighian Tubules• In insects and other terrestrial arthropods,

malpighian tubules– Remove nitrogenous wastes from hemolymph and

function in osmoregulation

Figure 44.12

Malpighian Tubules K+

K+

K+

Hemolymph

Water and waste

Hindgut

Water and K+

Na+/K+-ATPase

Conc. Waste

Malpighian Tubules• Use Malpighian tubules- blind end tubules that

extend into hemocoel (body cavity).• Cells waste and salts into hemolymphlumen

of tubule by diffusion and active transport.• K+ are actively transported into lumen; set up a

gradient.• Water and other ions leave the hemolymph and

follow into the lumen by passive diffusion.• Empty into hindgut; water reabsorbed; urine is

concentrated.• Na+/K+-ATPase moves ions from lumen of hindgut

into hemolymph.

Insects versus other Vertebrates

• Insects use a gradient to pull water through a membrane; open circulatory system = low blood pressure.

• Vertebrates- push water through a membrane; closed circulatory system = higher blood pressure.

More Complex Organisms Need Another

Solution

Introducing the Vertebrate Kidney!

Nephron (Tubule)

Gill Epithelia is Permeable

Hypertonic Cells

Hypotonic Env.

Water

Freshwater Bony Fishes• Problems: Water enters cells from

environment, solutes leave cells.• Solutions: Drink very little water;

excrete large amounts of dilute (hypoosmotic) urine with large kidneys; reabsorb ions in kidney tubules (active transport) back into blood; use chloride cells in gill epithelium (active transport).

• Freshwater animals maintain water balance:– By excreting large amounts of dilute urine.

• Salts lost by diffusion:– Are replaced by foods and uptake across the gills.

Figure 44.3b

Uptake ofwater and someions in food

Osmotic water gainthrough gills and other partsof body surface

Uptake ofsalt ions by gills

Excretion oflarge amounts ofwater in dilute urine from kidneys

(b) Osmoregulation in a freshwater fish

Hypotonic Cells

Hypertonic Env.

Water

Saltwater Bony Fishes• Problem: Tend to lose water, gain

ions, mostly at gills.• Solutions: Drink large amount of

water; kidney retains water and excretes ions (isoosmotic urine); use chloride cells in gills to actively transport some ions across gill epithelium.

• Marine bony fishes are hypoosmotic to sea water:

– Lose water by osmosis and gain salt by both diffusion and from food they eat.

• These fishes balance water loss:

– By drinking seawater.

Figure 44.3a

Gain of water andsalt ions from foodand by drinkingseawater

Osmotic water lossthrough gills and other partsof body surface

Excretion ofsalt ionsfrom gills

Excretion of salt ionsand small amountsof water in scantyurine from kidneys

(a) Osmoregulation in a saltwater fish

Cartilaginous Fishes• Problem: Same as marine bony fishes.• Solution: Reabsorb urea from nephron

tubule back into the blood; 100X blood [urea] than mammals (special protective solute,TMAO to protect proteins)blood is slightly hyperosmotic kidneys and gills do not have to remove ions; do not have to drink large volume of water.

Cartilaginous Fishes• Problem: Still must remove excess

Na+ and Cl- that diffuse across gills, diet, etc.

• Solution: Rectal Gland- uses Na+/K+-ATPase pumps to actively transport Na+ and Cl- out of blood by setting up a gradient.

How the Rectal Gland Works

Na+/K+-ATPase

Extracellular Fluid

Lumen of Rectal Gland

Na + K+

Na+ Cl-

Cotransporter

Na + Cl-

Chloride ChannelCl-

Cl-

Na+

Na+To Rectum

How could a marine shark enter freshwater?

By controlling the amount of solutes!

Video: National Geographic Presents: Attacks of the Mystery

Shark

Rectal Gland• Very common mechanism for

removing salt in marine animals.• Problem: Marine birds and reptiles

have freshwater kidneys designed to reabsorb salt from urine into blood.

• Use similar salt glands in nostrils to excrete salt.

• An example of transport epithelia is found in the salt glands of marine birds.

• Remove excess sodium chloride from the blood.

Figure 44.7a, b

Nasal salt gland

Nostrilwith saltsecretions

Lumen ofsecretory tubule

NaCl

Bloodflow

Secretory cellof transportepithelium

Centralduct

Directionof saltmovement

Transportepithelium

Secretorytubule

Capillary

Vein

Artery

(a) An albatross’s salt glands empty via a duct into thenostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist.

(b) One of several thousand secretory tubules in a salt-excreting gland. Each tubule is lined by a transportepithelium surrounded by capillaries, and drains intoa central duct.

(c) The secretory cells actively transport salt from theblood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentrationgradient of salt in the tubule (aqua), this countercurrentsystem enhances salt transfer from the blood to the lumen of the tubule.

Show me some marine reptiles! Salt glands in

action!

Video: Corwin Experience- Galapagos

Animals That Live in Temporary Waters

• Some aquatic invertebrates living in temporary ponds– Can lose almost all their body water and survive in a

dormant state

• This adaptation is called anhydrobiosis.

Figure 44.4a, b(a) Hydrated tardigrade (b) Dehydrated tardigrade

100 µm

100 µm

• The nephron, the functional unit of the vertebrate kidney– Consists of a single long tubule and a ball of capillaries

called the glomerulus

Figure 44.13c, d

Juxta-medullarynephron

Corticalnephron

Collectingduct

To renalpelvis

Renalcortex

Renalmedulla

20 µm

Afferentarteriolefrom renalartery

Glomerulus

Bowman’s capsuleProximal tubule

Peritubularcapillaries

SEM

Efferentarteriole fromglomerulus

Branch ofrenal vein

DescendinglimbAscendinglimb

Loopof

Henle

Distal tubule

Collectingduct

(c) Nephron

Vasarecta(d) Filtrate and

blood flow

Vertebrate Kidneys

• Four Functions:

1. Filtration

2. Reabsorption

3. Secretion

4. Excretion

1. Filtration• Glomerulus- tightly-woven ball of

capillaries embedded in a cup-shaped tubule- Bowman’s capsule.

• Slits/pores in capillaries and capsule allow liquid/solutes through but prevent cells and large proteins from entering the nephron.

• Produces isoosmotic filtrate with blood

Filtration of the Blood

• Filtration occurs as blood pressure:– Forces fluid from the blood in the

glomerulus into the lumen of Bowman’s capsule.

Pathway of the Filtrate

• From Bowman’s capsule, the filtrate passes through three regions of the nephron:

–The proximal tubule, the loop of Henle, and the distal tubule

• Fluid from several nephrons:

–Flows into a collecting duct

Blood Vessels Associated with the Nephrons

• Each nephron is supplied with blood by an afferent arteriole:– A branch of the renal artery that subdivides into the

capillaries• The capillaries converge as they leave the glomerulus

– Forming an efferent arteriole.• The vessels subdivide again:

– Forming the peritubular capillaries, which surround the proximal and distal tubules.

Proximal tubule

FiltrateH2OSalts (NaCl and others)HCO3

–

H+

UreaGlucose; amino acidsSome drugs

KeyActive transport

Passive transport

CORTEX

OUTERMEDULLA

INNERMEDULLA

Descending limbof loop ofHenle

Thick segmentof ascendinglimb

Thin segmentof ascendinglimb

Collectingduct

NaCl

NaCl

NaCl

Distal tubuleNaCl Nutrients

UreaH2O

NaClH2OH2OHCO3

K+

H+ NH3

HCO3

K+ H+

H2O

1 4

32

3 5

From Blood Filtrate to Urine: A Closer Look

• Filtrate becomes urine:– As it flows through the mammalian nephron and

collecting duct.

Figure 44.14

Transport Epithelium

2. Reabsorption• Must return most of the water and

solutes to the blood. (2000 l of blood 180 l water 1-2 l urine daily).

• Reabsorb glucose, amino acids, divalent cations in proximal tubule by active transport carriers.

• If not reabsorbed, lost in the urine.• Ex. Diabetes mellitus

3. Secretion• Foreign molecules and wastes

(ammonia, urea) are secreted into lower portions of tubule.

• Opposite direction as reabsorption (CapillaryTubule).

• Ex. Antibiotics and other drugs, bacterial debris

• Secretion and reabsorption in the proximal tubule:

– Substantially alter the volume and composition of filtrate

• Reabsorption of water continues:

– As the filtrate moves into the descending limb of the loop of Henle

4. Excretion• Urine is a solution of:

Harmful drugs, hormones, nitrogenous wastes, and excess K+, H+, water.

• Homeostasis of:

pH, electrolytes, blood volume and pressure.

• As filtrate travels through the ascending limb of the loop of Henle:– Salt diffuses out of the permeable tubule

into the interstitial fluid.• The distal tubule:

– Plays a key role in regulating the K+ and NaCl concentration of body fluids.

• The collecting duct:– Carries the filtrate through the medulla to

the renal pelvis and reabsorbs NaCl.

• Concept 44.5: The mammalian kidney’s ability to conserve water is a key terrestrial adaptation.

• The mammalian kidney:

– Can produce urine much more concentrated than body fluids, thus conserving water.

Solute Gradients and Water Conservation

• In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts:

– Are largely responsible for the osmotic gradient that concentrates the urine.

Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid.- Causes the reabsorption of water in the kidney and

concentrates the urine.

Figure 44.15

H2O

H2O

H2O

H2O

H2O

H2O

H2O

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

300

300 100

400

600

900

1200

700

400

200

100

Activetransport

Passivetransport

OUTERMEDULLA

INNERMEDULLA

CORTEX

H2O

Urea

H2OUrea

H2O

Urea

H2O

H2O

H2O

H2O

1200

1200

900

600

400

300

600

400

300

Osmolarity of interstitial

fluid(mosm/L)

300

• The countercurrent multiplier system involving the loop of Henle– Maintains a high salt concentration in

the interior of the kidney, which enables the kidney to form concentrated urine.

• The collecting duct, permeable to water but not salt:

–Conducts the filtrate through the kidney’s osmolarity gradient, and more water exits the filtrate by osmosis.

• Urea diffuses out of the collecting duct:

–As it traverses the inner medulla

• Urea and NaCl:

–Form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood.

• Antidiuretic Hormone (ADH)– Increases water reabsorption in the distal tubules

and collecting ducts of the kidney

Figure 44.16a

Osmoreceptorsin hypothalamus

Drinking reducesblood osmolarity

to set point

H2O reab-sorption helpsprevent further

osmolarity increase

STIMULUS:The release of ADH istriggered when osmo-receptor cells in the

hypothalamus detect anincrease in the osmolarity

of the blood

Homeostasis:Blood osmolarity

Hypothalamus

ADH

Pituitarygland

Increasedpermeability

Thirst

Collecting duct

Distaltubule

(a) Antidiuretic hormone (ADH) enhances fluid retention by makingthe kidneys reclaim more water.

• The Renin-Angiotensin-Aldosterone System (RAAS)

– Is part of a complex feedback circuit that functions in homeostasis

Figure 44.16b

Increased Na+

and H2O reab-sorption in

distal tubules

Homeostasis:Blood pressure,

volume

STIMULUS:The juxtaglomerular

apparatus (JGA) respondsto low blood volume or

blood pressure (such as dueto dehydration or loss of

blood)

Aldosterone

Adrenal gland

Angiotensin II

Angiotensinogen

Reninproduction

Renin

Arterioleconstriction

Distal tubule

JGA

(b) The renin-angiotensin-aldosterone system (RAAS) leads to an increasein blood volume and pressure.

• The South American vampire bat, which feeds on blood:– Has a unique excretory system in which its

kidneys offload much of the water absorbed from a meal by excreting large amounts of dilute urine.

Figure 44.17

• Concept 44.6: Diverse adaptations of the vertebrate kidney have evolved in different environments.

• The form and function of nephrons in various vertebrate classes:

– Are related primarily to the requirements for osmoregulation in the animal’s habitat.

Terrestrial Animals• Land animals manage their water budgets

– By drinking and eating moist foods and by using metabolic water.

Figure 44.5

Waterbalance in a human

(2,500 mL/day= 100%)

Waterbalance in akangaroo rat

(2 mL/day= 100%)

Ingested in food (0.2)

Ingested in food (750)

Ingested in liquid(1,500)

Derived from metabolism (250)

Derived from metabolism (1.8)

Water gain

Feces (0.9)

Urine(0.45)

Evaporation (1.46)

Feces (100)

Urine(1,500)

Evaporation (900)

Water loss

• Desert animals:– Get major water savings from simple anatomical

features

Figure 44.6

Control group(Unclipped fur)

Experimental group(Clipped fur)

4

3

2

1

0

Wat

er lo

st p

er d

ay(L

/100

kg

body

mas

s)

Knut and Bodil Schmidt-Nielsen and their colleagues from Duke University observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals’ skin remained more than 30°C cooler. The Schmidt-Nielsens reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they compared the water loss rates of unclipped and clipped camels.

EXPERIMENT

RESULTSRemoving the fur of a camel increased the rateof water loss through sweating by up to 50%.

The fur of camels plays a critical role intheir conserving water in the hot desertenvironments where they live.

CONCLUSION

• Exploring environmental adaptations of the vertebrate kidney

Figure 44.18

MAMMALS

Bannertail Kangaroo rat(Dipodomys spectabilis)

Beaver (Castor canadensis)

FRESHWATER FISHES AND AMPHIBIANS

Rainbow trout(Oncorrhynchus mykiss)

Frog (Rana temporaria)

BIRDS AND OTHER REPTILES

Roadrunner(Geococcyx californianus)

Desert iguana(Dipsosaurus dorsalis)

MARINE BONY FISHES

Northern bluefin tuna (Thunnus thynnus)