Chapter 44: Osmoregulation and Excretion

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 44: Osmoregulation and Excretion


• A balancing act

• The physiological systems of animals

– Operate in a fluid environment

• The relative concentrations of water and solutes in this environment

– Must be maintained within narrow limits

Homeostasis


• Freshwater animalsAdaptations for reducing water uptake and conserve solutes

• Desert & marine animals desiccating environment, conserve water

Figure 44.1


Osmoregulation

• Balances uptake and loss of water and solutes

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


Osmosis

• Osmotic gain and loss of water at the level of the cell

• Balanced by osmoregulation


Osmotic Challenges

• Osmoconformers

– Isoosmotic with their surroundings and do not regulate their osmolarity, e.g. marine inverts.

• Osmoregulators expend energy to control water uptake and loss, e.g. marine verts.


• Marine bony fishes are hypoosmotic to sea water

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

– Sm. Volume, highly conc. urine

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


Freshwater Animals

• Take in water from their hypoosmotic environment

• Lose salts by diffusion


• Freshwater animals excrete 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


Land Animals

• Manage their water budget by drinking and by using metabolic water

Figure 44.5

Waterbalance in

a human(2,500 mL/day

= 100%)

Waterbalance in a

kangaroo 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


Transport Epithelia

• Specialized cells that regulate solute movement


• e.g. salt glands of marine birds

– remove excess NaCl 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 the

nostrils, 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.


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

Nitrogenous Waste

• Nitrogenous breakdown products of proteins and nucleic acids

Figure 44.8


Ammonia

• Animals that excrete ammonia need access to lots of water


Urea

• Liver of mammals convert ammonia to less toxic urea

kidneys excreted (w/ minimal loss of water)


Uric Acid

• Insects, land snails, reptiles, birds

• Little water loss


Excretory Processes

• Urine produced 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


– Filtration, pressure-filtering of body fluids

– Reabsorption, reclaiming valuable solutes

– Secretion, addition of toxins and other solutes to the filtrate

– Excretion, the filtrate leaves the system


Nucleusof cap cell

Cilia

Interstitial fluidfilters throughmembrane wherecap cell and tubulecell interdigitate(interlock)

Tubule cell

Flamebulb

Nephridioporein body wall

Tubule

Protonephridia(tubules)

Protonephridia: Flame-Bulb Systems, flatworm

Figure 44.10


Metanephridia, earthworm

Figure 44.11 Nephrostome Metanephridia

Nephridio-pore

Collectingtubule

Bladder

Capillarynetwork

Coelom


Digestive tract

Midgut(stomach)

Malpighiantubules

Rectum

IntestineHindgut

Salt, water, and nitrogenous

wastes

Feces and urineAnus

Malpighiantubule

Rectum

Reabsorption of H2O,ions, and valuableorganic molecules

HEMOLYMPH

Malpighian Tubules, insects

Figure 44.12


Vertebrate Kidneys

• Function in both excretion and osmoregulation


• Nephrons functional unit


Figure 44.13a

Posterior vena cava

Renal artery and vein

Aorta

Ureter

Urinary bladder

Urethra

(a) Excretory organs and major associated blood vessels

Kidney


(b) Kidney structure

UreterSection of kidney from a rat

Renalmedulla

Renalcortex

Renalpelvis

Figure 44.13b

• 2 distinct regions

– An outer renal cortex and an inner renal medulla


Nephron

Figure 44.13c, d

Juxta-medullarynephron

Corticalnephron

Collectingduct

To renalpelvis

Renalcortex

Renalmedulla

20 µm

Afferentarteriolefrom renalartery Glomerulus

Bowman’s capsule

Proximal tubule

Peritubularcapillaries

SEM

Efferentarteriole fromglomerulus

Branch ofrenal vein

Descendinglimb

Ascendinglimb

Loopof

Henle

Distal tubule

Collectingduct

(c) Nephron

Vasarecta

(d) Filtrate and blood flow


• Filtration occurs in the glomerulus


• Glomerulus of Bowman’s capsule proximal tubule the loop of Henle distal tubule collecting duct


Proximal tubule

Filtrate

H2OSalts (NaCl and others)HCO3

–

H+

UreaGlucose; amino acidsSome drugs

Key

Active transport

Passive transport

CORTEX

OUTERMEDULLA

INNERMEDULLA

Descending limbof loop ofHenle

Thick segmentof ascendinglimb

Thin segmentof ascendinglimb

Collectingduct

NaCl

NaCl

NaCl

Distal tubule

NaCl Nutrients

Urea

H2O

NaCl

H2O

H2OHCO3 K+

H+ NH3

HCO3

K+ H+

H2O

1 4

32

3 5

Filtrate becomes urine

Figure 44.14


• The mammalian kidney’s ability to conserve water is a key terrestrial adaptation


• NaCl and urea, contribute to the osmolarity of the interstitial fluid

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

H2O

Urea

H2O

Urea

H2O

H2O

H2O

H2O

1200

1200

900

600

400

300

600

400

300

Osmolarity of interstitial

fluid(mosm/L)

300


Regulation of Kidney Function

• Osmolarity of the urine regulated by nervous and hormonal control


• Antidiuretic hormone (ADH)

– Increases water reabsorption in the distal tubules and collecting ducts

Figure 44.16a

Osmoreceptorsin hypothalamus

Drinking reducesblood osmolarity

to set point

H2O reab-sorption helps

prevent furtherosmolarity 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 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


• 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)

Chapter 44: Osmoregulation and Excretion

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