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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)