15 - Osmoregulation & Excretion

PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
!Indispensable for all biochemical and physiological
processes. The physicochemical nature of Earth life is
largely a reflection of the physical properties of water!
!Elimination of metabolic waste products
accompanying digestion and respiration and/
or maintenance of water and electrolytes !
Excretory organs:!
  Maintain proper balance of water and salts
(osmoregulation) by eliminating excess
quantities of either water or electrolytes!
  Maintain proper internal pH by removing
excess acids and bases!
  Eliminate byproducts of nitrogen metabolism!
Polar solvent well-suited to dissolving biomolecules and electrolytes!
Due to hydrogen bond formation, very high melting/boiling point
for a molecule of its size - attraction between the two hydrogen
atoms of one water molecule to the oxygen atoms of two adjacent
water molecules (energy store of 20 kJ mol-1)!
When water is frozen, almost all the water molecules have formed
these hydrogen bonds (to break them requires heat of fusion of ice: 6
kJ mol-1)!
The remaining hydrogen bonds must be broken before evaporation
of water into water vapor occurs (much greater energy required: 44
kJ mol-1 @ 25 °C, 40 kJ mol-1 @ 100 °C)!
Bounces readily off of surfaces, but is difficult to compress (allows
use in hydrostatic skeletons). Also allows animals to live at greatest
ocean depths (no solidification)!
Figure 27.19 Terrestrial vertebrates living freely in their natural habitats: Their total daily rates of
water turnover in relation to body size
Physics of Water Movement!
!Spring 2010!
Rate of diffusion described by Fick s diffusion equation:!
ds/dt = –DA(dc/dx),! where:!
ds/dt is the instantaneous rate of movement of a substance!
  D is the diffusion coefficient (a measure of the ease of
diffusion, e.g., membrane permeability)!
  A is the diffusion area, and!
  dc/dx is the concentration gradient of the substance moving
(moles/unit distance)!
Similar to heat exchange:!
Rate of diffusion is largely determined by the surface area
  However, instead of thermal gradients, there are
concentration gradients!
Figure 27.15 The temperature of air exhaled from the nostrils as a function of ambient air
temperature in mammals and birds
Figure 27.17 The total rate of evaporative water loss varies greatly among different types of
vertebrates and arthropods
Topic 15 - 1!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
!Spring 2010!
Figure 26.8 The fundamental principles of cell volume regulation
!Movement of a solvent (i.e., water) between solutions:"
Two solutions separated by a thin and semipermeable membrane:
solvent can pass through but solutes cannot (or not as readily)!
  Because of high water concentration on the left-hand side, the
number of random collisions of the water molecules with the
membrane per unit area and per unit time will be greater on the left
side than on the right!
  Net flow of water will be from low to high solute concentration!
  Osmotic flow often exceeds diffusion due to facilitated diffusion
mechanisms and active transport (e.g., exocytosis)!
Human Blood/Tissue Composition!
Ionic and Osmotic Balance!
Life requires that appropriate quantities of both water and
solutes be maintained in both the extracellular and
intracellular body compartments!
Life evolved several billion years ago in shallow, salty seas.
Concentrations of extracellular fluids likely reflect the osmotic
conditions of that environment!
The ability to regulate internal osmotic environment that is
possessed by more advanced forms of life is an important
evolutionary trait!
Buffers the tissues from abrupt changes in environmental
conditions and increasing the efficiency of metabolism by
maintaining homeostasis!
  Animals are restricted in their geographic distribution by their
osmotic requirements!
  Maintenance of internal osmotic environment allowed colonization
of freshwater and terrestrial habitats!
Problems of Osmoregulation!
Figure 26.2 The three major types of body fluids are closely juxtaposed
Because of oxygen and waste elimination needs, animals
must be able to exchange materials with the environment!
Exchanges of water and ions accompany these!
Some tissues require an extracellular ionic environment
that is an approximation of seawater!
High Na+ and Cl–!
Low in other major ions, e.g., K+ and divalent cations!
  For many marine animals, seawater (1000 mosmol kg-1) can act
as the extracellular medium!
However, vertebrates (except for hagfishes) have an ionic
concentration about 1/3 (300 mosmol kg-1) that of seawater!
Also, much of the MgSO4 is removed, and some of the Cl– is
replaced by HCO3– !
  Reflects their freshwater origin (including marine teleosts)!
Topic 15 - 2!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
!Spring 2010!
Osmotic Strength of Aquatic Environments!
Concentration of solutions almost always expressed in
molarity (moles of solute/liter of solvent)!
  Occasionally expressed in molality (1 molal = 1 mole of
solute dissolved in 1 kg of solvent)!
Little difference in dilute aqueous solutions, but significant
difference in concentrated solutions!
Osmotic concentrations in tissue fluids of animals are often
expressed in osmoles (1 molal = 1 osmol)!
Because osmotic activity depends on the number of particles in
solution, osmolality depends on the molality of the solution as
well as to the extent to which the solute dissociates!
  For NaCl, for example, the osmolality is 2× the molality, although
in practice the dissociation is never complete!
  Generally measured indirectly by measuring another colligative
property (e.g., depression of vapor pressure or freezing point)!
! !(mmoles/kg)"
! Cl
!Mg !sulfate !bicarbonate!
soft lake water !0.17 !0.03 !0.22
river water
!0.39 !0.23 !0.52 !0.04 !0.15
hard river water !6.13 !13.44 !5.01 !0.11 !0.15
!470.2 !548.3 !10.23 !9.96 !53.57 !28.25
Bad Water
(Death Valley)!
Dead Sea
!840 !6662 !583 !152 !0.15
Major Inorganic Ions of Tissues!
HPO42–; HCO3–
!Main extracellular cation
!Main cytosolic cation
!Low concentration in cells
!Intra- and extracellular
!Intra- and extracellular
!Main extracellular anion
!of tissues!
Osmotic Strength in Animals!
! Major Functions!
!Is major source of osmotic pressure."
!Provides potential energy for"
!transport of substances across cell"
!membranes. Provides inward current"
!for membrane excitation!
!Is source of cytosolic osmotic"
!pressure. Establishes resting"
!potential. Provides outward current"
!for membrane repolarization!
!Regulates exocytosis and muscle"
!contraction. Is involved in"
! cementing cells together. Regulates"
!many enzymes and other cell"
!proteins, acts as second messenger!
!Acts as cofactor for many enzymes"
!(e.g., ATPases)!
!Buffers H+ concentration!
!Is counter-anion for inorganic cations
in Aquatic Animals!
Terrestrial animals adjust internal ionic environment in
response to desiccation, but not in response to osmotic
inflow of water!
  Aquatic (freshwater and marine) animals must deal with
ionic diffusion, as well as diffusion of water, between the
animal and the fluid medium!
  Osmotic strength of animal body fluids:!
Isosmotic animals ( ionic conformists ) - regulate ionic ratios
  Hyperosmotic animals - maintain a high body solute
concentration compared with concentrations in the
  Hyposmotic animals - maintain a low body solute
concentration compared with concentrations in the
Figure 27.10 Types of osmotic regulation
Strict osmoconformers - always have an internal osmolarity"
that is the same as the medium in which they are immersed!
  Limited osmoregulators -"
maintain an internal"
osmolarity different"
from the medium in"
SW eurywhich they are"
FW euryhaline fish!
immersed, but can"
haline fish!
do so only over a"
limited range!
  Strict osmoregulators -"
maintain an internal"
(stenohaline fish)!
osmolarity different from"
Osmolarity of the Environment!
the medium in which they"
are immersed!
Topic 15 - 3!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Intra/Extracellular Osmoregulation!
Intracellular environment of most animals is low in
Na+, but high in K+, HPO42–, and proteins!
  There are only minor and transient osmotic
differences between the intracellular and extracellular
fluids of animals!
  Thus, the cell membrane maintains ionic, but not
osmotic differences between the intracellular and
extracellular fluids!
  In contrast, the epithelium surrounding the body
often maintains both ionic and osmotic differences
between the animal and its environment!
In most animals, entire body surface does not participate
in ionic and osmotic regulation!
  Instead, specialized body surfaces (e.g., gills) or internal
organs (e.g., kidneys, salt glands) regulate salt and water!
Osmoregulatory Mechanisms!
Hober (1902): handle osmotic problems and
regulate the differences between:!
Water enters with food and drink (terrestrial and
marine animals) or through respiratory epithelium or
integument (freshwater animals)!
  Water leaves in urine, feces, and by evaporation from
lungs and integument!
Osmotic Exchanges!
Examples: trans-epithelial diffusion, ingestion, egestion,
metabolic water production!
Regulated osmotic exchanges - physiologically controlled
and serve to maintain homeostasis!
Two classes of osmotic exchanges between an
animal and its environment:!
Obligatory Exchanges of Ions and Water!
Obligatory osmotic exchanges - occur mainly in response
to physical factors over which the animal has little or no
physiological control!
Example: active epithelial transport!
Compensate for obligatory exchanges!
Influenced by same factors:!
Fluxes across membranes determined by concentration
gradients, surface area of membrane, thickness of membrane
(diffusion distance)!
  Small animals will dehydrate or hydrate more rapidly (and must
expend more energy on active regulatory processes)!
!Permeability of the integument - barrier between the
extracellular compartment and the environment!
  Movement of water occurs between cells (paracellular) and
through cells (transcellular)!
  Pure phospholipid bilayers not very water-permeable:
transcellular movement depends on presence of protein water
channels (e.g., aquaporin, a tetramer which forms a channel
through the plasma membranes of many animal cells)!
  Water exchange rates vary greatly among animal taxa: low in
reptiles, birds, and mammals, high in fishes and amphibians
(also have active transport of salts through skin and gills).
Amphibians have other water-retaining adaptations: !
Water is an end product of cellular metabolism of food. Produced in
small enough quantities that elimination is no problem (actually the
major source of water for desert animals):"
!g metabolic
water/g food!
!kJ expended/
g food!
!g metabolic
water/kJ expended!
In terrestrial animals, regulation of plasma ion concentrations
and elimination of metabolic wastes is accompanied by
unavoidable water losses:!
Insects and arachnids are particularly efficient at conserving water
while eliminating nitrogenous and inorganic wastes!
  Can absorb ions via the rectum or eliminate them in feces!
In vertebrates (especially mammals, which have no other
organs for this purpose), the kidney is the main
osmoregulatory (and nitrogen excretion) organ!
Diet may contain excess water or salts: need to eliminate excess salts
and nitrogenous wastes!
Example: seals become fat eating fish but thin eating invertebrates!
Can reabsorb water, Na+, and Cl– from urinary bladder!
Pelvic patches (highly absorbent skin on underside of pelvis)!
Hormonal (arginine vasotocin) control over skin permeability!
Toads have microchannels through skin to soak up water!
Excretory Water Losses!
Feeding and Metabolic Water Production!
Intracellular and extracellular compartments!
Extracellular compartment and the environment!
May be hourly or daily variations, but generally
animal is at long-term steady-state (inputs and
outputs of water and salts are equal)!
!Spring 2010!
Invertebrates have similar salt content to seawater, fish are less salty!
Seal burns fat to produce both energy and water (to excrete excess salt)
when feeding in invertebrates, but stores fat when eating fish !
Topic 15 - 4!
Kidneys of birds and mammals utilize countercurrent
multiplication to produce a hyperosmotic urine that is more
concentrated than plasma!
  Specialization (loop of Henle) of individual nephrons which
allows terrestrial existence. Reaches its highest level in desert
animals (e.g., kangaroo rats). Less efficient in birds, absent in other
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Figure 27.23 An index of water balance: Metabolic water production (MWP) as a ratio of total
evaporative water loss (EWL)
Figure 27.22 A kangaroo rat water budget
Summary of Desert Adaptations in the"
Kangaroo Rat (Dipodemys merriami)!
Water Balance in the kangaroo rat!
Don t sweat!
!Water Losses!
Lose less water in feces than other rodents (3× less than rats)!
Urinates in burrow to increase relative humidity!
Lose less water in respiration!
Complicated nasal passages increase turbulence, slows down
expired air and allows it to cool. Moisture condenses on surfaces of
nasal passages, drips down throat, and is swallowed.!
Figure 26.3 Osmotic regulation and conformity
Osmoregulators maintain strict extracellular homeostasis in
the face of large environmental variation in electrolyte
Internal tissues are generally unable to cope with more than minor
changes in extracellular osmolarity!
  Cells must depend entirely on osmotic regulation of extracellular
fluid to maintain cell volume!
Water obtained from metabolizing food!
Water collected on surface of food (at high humidity, up to 6× the
amount of water available in the food)!
Increased water retention!
Evaporative cooling from lungs? Not at reasonable temperatures!
Don t seem to lose water under heat stress!
Don t drink!
Water Gains
Metabolic water
Free water in dry food
!Spring 2010!
Osmoconformers demonstrate a high degrees of intracellular
osmotic tolerance!
Cells can cope with high plasma osmolarities by increasing their
intracellular osmolarities using organic molecules, thus increasing
total osmolarity and maintaining cell volume!
  This reduces the need to maintain osmotic pressure with other
substances (e.g., ions) that would produce other physiological
problems (e.g., changes in protein conformation)!
Topic 15 - 5!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Figure 26.9 Many animal cells achieve cell volume regulation principally by altering their content of
organic solute molecules
!Spring 2010!
Organic Osmolytes!
In some marine invertebrates and vertebrates, organic
osmolytes also are present in the blood and interstitial
fluids, as well as inside cells!
  Thus, both intracellular and extracellular osmolarity are
close to seawater!
  Best-known examples are urea and trimethylamine oxide
Urea tends to cause dissociation of multi-subunit proteins!
TMAO reduces this dissociation!
Both used by marine elasmobranchs, chimeras
(Holocephali), the coelacanth (Latimeria chalumnae), and
the brackish-water crab-eating frog Rana cancrivora!
General Patterns of Water and Salt"
Regulation in Marine Animals!
Figure 26.10 Counteracting solutes in a stingray
!Liability to
! Liability to
!Blood conc
!Urine conc"
! evaporative
!relative to
!relative to"
!water loss
! water loss
!medium conc
!medium conc!
!– (+++ if
!intertidal) "
No osmoregulation!
Do not drink; excrete hypertonic NaCl from rectal gland!
SW teleosts
Drink copiously; excrete NaCl from gills!
FW teleosts
Do not drink; produce large amount of very dilute urine!
Sea turtles
! isotonic "
Drink seawater; produce hypertonic tears !
! hypertonic"
Drink seawater; produce hypertonic nasal secretion; weakly hypertonic urine!
! hypertonic"
mammals "
Do not drink; produce strongly hypertonic urine!
Figure 28.5 Major solute and water fluxes in the late distal convoluted tubule during diuresis and
Figure 28.6 The human kidney, emphasizing the nephrons and their blood supply (Part 1)
Topic 15 - 6!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
!Spring 2010!
Figure 28.6 The human kidney, emphasizing the nephrons and their blood supply (Part 2)
Figure 28.1 The structural and functional basis for formation of primary urine by ultrafiltration in the
vertebrate kidney
Figure 28.2 Formation of primary urine by active solute secretion
Figure 28.16 Major molecular mechanisms of NaCl reabsorption and associated processes in three
parts of the mammalian kidney tubule
Nephron Function!
Figure 28.6 The human kidney, emphasizing the nephrons and their blood supply (Part 2)
Topic 15 - 7!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
!Spring 2010!
Figure 28.12 Countercurrent multiplication in the loop of Henle
Figure 28.6 The human kidney, emphasizing the nephrons and their blood supply (Part 2)
Figure 27.18 Urine concentrating ability in mammals: The maximum concentration is in part a
function of body size
Figure 28.7 Evolutionary development of the renal papilla in mammals native to different habitats
Figure 28.8 Maximum urine concentration correlates with the relative thickness of the medulla
Figure 28.9 The relation between relative medullary thickness and body size depends on whether
mammals are native to arid, mesic, or aquatic habitats
Topic 15 - 8!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Non-mammalian Vertebrate Kidneys!
Non-mammalian Vertebrate Kidneys!
Nephrons of hagfishes possess glomeruli, but no tubules:
Bowman s capsules empty directly into collecting ducts!
Freshwater teleosts have larger and more abundant
glomeruli than marine teleosts!
  Some marine teleosts are aglomerular (lack both glomeruli
and Bowman s capsules)!
Osmoconformers: Extracellular fluids almost identical to seawater!
Kidneys are used largely to excrete divalent cations (Ca2+, Mg2+,
SO42–). Little or no osmoregulation!
The elasmobranchs have a complex tubule organization that
should allow countercurrent multiplication!
Unlike mammalian kidney (which excretes urea and retains water
to produce a hypertonic urine), this structure retains urea and does
not produce a concentrated urine!
  Appears to be an adaptation to conserve urea for use as an
osmolyte. The freshwater skate Raja erinacea does not reabsorb
filtered urea, and its kidneys lack these tubular bundles, indicating
that these structures are the site of urea absorption!
Urine formed entirely by secretion, because no specialized
mechanism to produce a filtrate!
Amphibians and reptiles lack countercurrent loop of Henle!
Figure 28.3 Amphibian nephrons and their connections to collecting ducts
!Spring 2010!
Incapable of producing hyperosmotic urine relative to plasma!
Birds have a mixture of reptilian-type and mammaliantype nephrons!
Some nephrons lack a loop of Henle!
In some birds, the loop is located perpendicular to the collecting
duct: less efficient concentrating mechanism!
Figure 28.4 Urine formation in amphibians during diuresis
Extrarenal Osmoregulatory Organs!
Figure 28.19 A lobe of a bird s kidney in cross section
Found in a variety of animals:!
Rectal glands (elasmobranchs)!
Nasal salt glands (birds and reptiles) , also tongue of crocodilians!
  Gills of teleosts!
Common feature is the presence of epithelial chloride cells!
These cells have extensive, folded basolateral (serosal) membrane,
with much larger surface area ( brush border ) than that of the
apical (mucosal) membrane!
  Many mitochondria - lots of active transport!
  Cells excrete salt in a manner similar to Na-absorbing cells in
kidney, but!
  Blood is not filtered - salt is removed from ECF!
Often these cells form a large number of blind-ending
tubules that drain into a duct, which opens into a lumen that
is exposed to the environment!
Topic 15 - 9!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Teleostean Gill Structure!
The branchial apparatus of teleosteans consists of four pairs
of gill arches, each with two rows of projecting filaments!
The lamellae extend as flattened leaflets from each side of the
Also shown: afferent (a.v.)"
and efferent (e.v.)"
vessels with"
(r.c.); cartilaginous"
spine supporting"
the filament (c.);"
central vascular"
compartment (c.c.);"
mucous cell (m.c.);"
chloride cell (ch.);"
pavement or pillar cells (p.c.)!
!Spring 2010!
Teleostean Gill Cells!
Osmotic liability: large surface area in contact with
water. Epithelium separating blood from water
consists of several cell types:!
Pavement (or pillar) cells!
Chloride cells!
Flat (3-5 mm thick)!
Some mitochondria!
Minimal barrier to diffusion: used for gas exchange!
Thicker and more columnar in shape!
Deeply infolded!
Many mitochondria and active salt transport enzymes!
Tight junctions joining these to chloride cells!
Mucous cells!
Secrete protective film of mucus!
Figure 27.2 Ion exchanges mediated by active Na+ and Cl– transport in the gill epithelium of
freshwater teleost fish
Salt Cell Function!
(or gland"
Na+/K+ ATPase!
Figure 27.6 A section of a chloride cell of a marine teleost fish
The apical pit (or apical crypt) characterizes these cells in seawater-adapted fishes!
Na+! Na+/2 Cl–/K+
2 Cl–!
Salt Secretion by SW Teleosts!
Chloride cells similar to those described earlier:!
  High levels of Na+/K+ ATPase associated with Na/2 Cl/K
cotransporters in the basolateral membrane and Cl– channels
in the apical membrane!
  Each is associated with an accessory cell (distinct from a
pavement cell): Na+ diffuses from blood to seawater through
the less tight paracellular channel between the chloride and
accessory cell!
  Salt secretion occurs against an osmotic gradient, and there is
no movement of water following the movement of salt!
  Also mediate the exchange of other ions, including Ca2+ (Ca2+
in water is taken up via Ca2+ channels in the apical
membrane, and is then actively transported into the cells via
large numbers of Ca2+ ATPase molecules in the basolateral
Topic 15 - 10!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Summary of Osmoregulation in"
SW and FW Teleosts!
Salt Uptake by FW Teleosts!
Pavement cells have a proton (H+) ATPase and Na+ channels
in the apical membrane!
A Na+/K+ ATPase in the basolateral membrane pumps Na+
out of the cell into the blood, and K+ cycles through channels
in the membrane!
  Gill epithelium also contains chloride cells, which also
mediate the uptake of Ca2+ from the water. These are
different from the chloride cells of marine teleosts:!
Saltwater teleosts:!
Drink seawater copiously!
Pump salt into body - water follows!
  Pump out salt via gills!
  If you remove too much salt, water follows, so kidney produces
small volume of isosmotic urine!
Proton ATPase is presumably electrogenic: pumping protons out
of the gills and generating an electrical potential that draws Na+
into the cell (similar to frog skin and mammalian kidney)!
  However, not as yet a demonstration of a relationship between the
activity of this enzyme and apical membrane potential!
!Spring 2010!
Freshwater teleosts:!
Don t drink (except incidentally to feeding)!
Take up some salt from blood (not nearly as much as SW teleosts)!
  Don t take up water (gut is nearly impermeable to H2O)!
  Take up ions at the gills!
  Kidney produces large volume of dilute urine!
  Take up salts from bladder by altering permeability of epithelium!
Have an anion transporter on the apical membrane!
High levels of proton ATPase within the cell!
  May take up Cl–!
Overview of Osmoregulation in SW
and FW Teleosts!
G = glomerulus"
PS = proximal segment"
DS = distal segment!
UB: urinary bladder!
GB: gall bladder!
F = cortisol"
PL = prolactin segment"
NH = neurohypophyseal"
UH = urophyseal factor (urotensin)!
⇒ = water movement"
→ = sodium movement!
Black circles = inhibitory responses!
Adaptive Osmoregulation in SW and FW Teleosts!
Many fishes migrate daily, or at different life stages,
between FW and SW!
  Similar adaptations to those described above for FW and
SW Fishes!
Active uptake of NaCl in FW, excretion in FW!
Physiological adaptation of gills involves:!
Synthesis and/or destruction of molecular components of
epithelial transport systems!
  Changes in morphology and number of chloride cells!
  Thus, may take several days to regain homeostasis!
Adaptation mediated by endocrine hormones that
influence epithelial cell differentiation and metabolism!
Figure 27.11 Molecular phenotypic plasticity in gills of trout transferred between freshwater and
Adrenal corticosteroid hormone (cortisol) and pituitary peptide
(GH) responsible for transition from FW to SW!
Posterior pituitary peptide (prolactin) responsible for
transition from SW to FW!
Movement from FW → SW!
1. !Proton ATPase that powers active NaCl uptake
is down-regulated!
2. !↑Na+ influx into body → ↑plasma [Na+] →
↑plasma cortisol and GH levels!
3. !↑cortisol and GH → ↑number of chloride cells,
as well as the degree of elaboration (↑
infolding) of their basolateral membranes!
4. !As a result, ↑Na+/K+ ATPase activity and NaCl
5. !Plasma Na+ levels return to normal!
Topic 15 - 11!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Movement from SW → FW!
!Spring 2010!
Figure 27.9 Water–salt relations in a marine shark
1. !Low external Na+ closes paracellular gaps
between chloride and accessory cells → rapid
↓NaCl efflux!
2. !↑plasma prolactin levels!
3. !↑prolactin → ↓number of chloride cells, apical
pits disappear!
4. !As a result, ↓Na+/K+ ATPase activity!
5. !Proton ATPase that powers active NaCl uptake
is up-regulated Fish returns to freshwater
Rectal Gland Cell Function!
Rectal Glands!
Basolateral membrane contains high amounts of Na+/K+ ATPase:
pumps Na+ out and K+ into cell (K+ leaks back out through the
many channels present in the membrane)!
  Generates large Na gradient, which drive NaCl uptake via a"
Na/2 Cl/K cotransport system also present in the membrane!
  As Na+ and K+ are exchanged, intracellular [Cl–] rises above that
of the tubule lumen; eventually exiting via chloride channels in
the apical (mucosal) membrane!
  Overall effect is to move Cl from serosal (blood) side into the
lumen. This creates an electrical potential with the serosal side
positive and the lumen negative!
  The resulting electrochemical gradient for Na permits diffusion
of Na+ from the serosal side through paracellular pathways into
the lumen!
  Water follows NaCl secreted into lumen of the tubule, but tubule
wall is impermeable to urea and TMAO. Resulting secretion is
isosmotic with blood, but has much higher NaCl concentration!
Found in elasmobranchs (sharks and rays)!
Principal (perhaps only)"
extrarenal salt removal site"
in these animals!
  Large number of blind-"
ending tubules that drain"
into a duct, which opens"
into the intestine near the"
  Walls of tubule consist of"
single type of salt cell !
Salt Glands!
Figure 27.8 Avian salt glands
Found in nearly all marine birds,"
ostriches, crocodilians, desert"
and marine reptiles (sea turtles,"
marine iguanas, and sea snakes), !
  Work in a similar fashion to"
rectal glands, but unlike rectal"
glands, secretions are 2-3× the osmolarity of plasma!
  Active secretion takes place across the epithelium of
the secretory tubules, which have characteristic salt
  Basolateral membrane heavily folded to increase
surface area relative to apical membrane, many
mitochondria, tight junctions between cells to
prevent leakage!
Topic 15 - 12!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Salt Gland Structure!
!Spring 2010!
Animals body pH is slightly more alkaline than neutrality
(i.e., there are few hydrogen (H+) than hydroxyl (OH–) ions in
the body!
  Concentrations of both H+ and OH– ions are low in aqueous
solutions because water is only weakly dissociated
(functioning as a weak acid)!
  Human blood plasma @ 37 °C has a pH of 7.4 (H+ activity of
40 nM l-1). Normal function in mammals at 37 °C can be
maintained at a range of pH 7.0-7.8 (i.e., between 100 and 16
nM H+ l-1)!
  This is actually a large percentage variation of tolerance
compared with Na+ or K+ levels!
  However, important to remember that the absolute changes
in concentration are small, as are total body H+
Marine birds maintain osmotic balance by excretion of a concentrated salt solution from glands located
above the orbit. (A) These glands consist of a longitudinal arrangement of many lobes, each about 1 mm
in diameter. Secretions from these lobes drain via a central canal into the nasal passages of the beak,
where they exit from the body via the nostrils. (B) Each lobe consists of branching tubules, which are
surrounded by capillaries in which blood flows countercurrent to the secretory fluid in the tubule. This
facilitates salt transfer from blood to tubule, because the uphill gradient of salt concentration is
minimized at each point along the length of the tubule. The tubules and capillaries are arranged radially
around the central canals. (C) Typical secretory cells of the salt gland tubules.!
Body pH!
Blood pH in vertebrates is midway between the pK of
the carbon dioxide/bicarbonate and ammonia/
ammonium reactions:!
! !NH3 + H+
! NH4+
! !CO2 + OH–
Effects of Changes of Body pH!
!pK = 9.58 (15 °C)!
!pKa =6.08 (15 °C)!
Most cell membranes are not very permeable to HCO3
and NH4+, but are very permeable to CO2 and NH3 !
A body pH between these two pKs thus ensures adequate
rates of excretion of these two major end products of
metabolism !
  Also, because these pKs vary with temperature, so does pH
of blood, ensuring adequate rates of excretion at different
H+ Production and Excretion!
Also external effects on pH (e.g., hypoxia, in which animals become
acidotic because of imbalances in H+ production by hydrolysis of ATP
into ADP vs. proton consumption by NAD during anaerobic
Intracellular pH will be stable if the rate of acid loading from
metabolism or influx to the cell is equal to the rate of acid
  Difference in permeabilities of molecules used in regulation!
Metabolism (especially metabolic production of CO2):!
Net balance of CO2 production and excretion!
Excess CO2 increases acidity by carbonic acid formation
(deficit CO2 can produce respiratory alkalosis)!
  Can be regulated by terrestrial vertebrates!
Ingestion in food:!
Citric and other plant acids (ingesting plants generally
increases intake of bases)!
  Meats contain amino acids!
Excretion of H+!
Stabilize cell volume!
Regulate enzyme activity!
Activate other cellular processes (e.g., sperm activation in sea urchins,
stimulation of glycolysis in muscles by insulin)!
Factors Influencing Intracellular pH!
increases due to!
Enzymatic activity!
Subunit aggregation!
Membrane characteristics!
Contributes to osmotic pressure of body compartments (because charge on
proteins is a major contributor to the total fixed charge within cells)!
Facing continuous metabolic release of H+, animals therefore regulate
their internal pH to:!
Alters dissociation of weak acids, and thus the ionization of proteins
(especially α-amydasal side group of histidine). Net charge on
proteins determines:!
Via the kidney in (most terrestrial vertebrates), and/or
across body surfaces (e.g., gills of fishes, skins of
H+ permeability generally greater than that of K+,!
Band II cells in RBCs and collecting ducts of nephrons exchange
for HCO3– for Cl– at high rates!
Several mechanisms for counteracting cell acidity:!
Buffering by physical buffers (e.g., proteins and phosphates)
located within the cell!
  Reactions to HCO3– with H+ ions, forming CO2, which then
diffuses out of the cell!
  Passive diffusion or active transport of H+ ions from the cell!
  Cation-exchange (Na+/H+) or anion-exchange (HCO3–/ Cl–)
mechanisms, or both, in the cell membrane!
Topic 15 - 13!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Factors Influencing Body pH!
Acid Balance in Fishes!
In mammals, body pH is maintained by adjusting the
excretion of CO2 via the lungs and excretion of H+ or
HCO3– by the kidneys:!
Collecting duct has A-type (acid excreting) and B-type (base
excreting) cells, the activity of which can be altered to
determine net acid-base excretion!
  Skins of frogs and gills of fishes have a proton ATPase to
excrete protons on the apical surface of the epithelium!
Gill surfaces too small!
Solubility of CO2 in water too great!
  Too much water flow over gills is necessary!
Requires several hours to adapt to ΔpH!
↓ pH (e.g., from acid deposition) → ↓ [HCO3–] (↑ CO2) →
↓ PO (Bohr effect - shift in O2 dissociation curve to right ↑
O2 unloading) → ↑ tissue anoxia (↑ anaerobic metabolism)
→ ↑ lactic acid → ↓ pH (positive feedback loop)!
  Also lose capacity to control Cl– excretion (counterexchange with HCO3–). Lose ability to control salt levels!
Increasing temperature also decreases pH!
Increases dissociation of water and the pH of neutrality!
Affects pK of plasma proteins and the CO2/ HCO3– system!
Must alter pH by changing HCO3– concentrations!
Fish gills also have a HCO3–/Cl– exchange mechanism!
Fish can t alter PCO2:!
!Spring 2010!
Nitrogen Excretion!
Figure 28.24 Some nitrogenous compounds excreted by animals
Protein catabolism:"
! !O"
!deamination ! !+ "
! !NH3!
After deamination of the protein, the α-ketoacid formed can
be fully oxidized into CO2 and H2O and excreted via
respiratory and integumentary systems!
  The NH3 radical, however, is very toxic!
In aquatic (ammonotelic) animals, the highly water-soluble radical
is easily excreted into surrounding water, but!
  Not easily handled in terrestrial animals!
In many (ureotelic) animals, the NH3 radical must be converted
into urea before it can be excreted!
Nitrogen Excretion!
Nitrogen Excretion!
Urea synthesis:"
!2 NH3 + CO2
! !NH2 "
! !/! !"
C=O + H2O "
! !\ "
! !NH2!
Small and reasonably soluble (can act as a means of maintaining
osmotic pressure, e.g., elasmobranchs)!
  Some animals (e.g., frogs), switch from ammonotelic to ureotelic
modes during their life cycle!
  However, lower solubility relative to ammonia means that
elimination requires loss of lots of water (disadvantage in dry
environments). Also cannot be used within the amniote cleidoic
egg environment!
  In such cases, animals use uric acid (uricotelism) as metabolic end
Much less toxic than ammonia: many animals (e.g.,
elasmobranchs) tolerate high levels in tissues!
Uric acid structure:"
  Very insoluble in water: can be excreted as a
solid paste or even a dry pellet!
  Requires much more energy than urea for
  Not excreted in acid form, but as a salt (urate)!
Topic 15 - 14!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
Box 27.3 Synthesis of urea from protein nitrogen by the ornithine–urea cycle in adult vertebrates: A
phylogenetic perspective
!Spring 2010!
General Trends in Nitrogen Excretion!
!Major end product
! of protein metabolism
Aquatic invertebrates
! Aquatic!
! Ureotelic
! Aquatic!
! Ureotelic
! Aquatic!
!some ureotelic
! Aquatic!
Larval amphibians
! Ammonotelic
! Aquatic!
Adult amphibians
! Aquatic!
! Cleidoic egg!
Land snails
!Uric acid
! Cleidoic egg!
! Ammonotelic, some uric acid !Semi-aquatic ! Cleidoic egg !
!Urea and uric acid
! Cleidoic egg !
Lizards and snakes
! Uricotelic
! Cleidoic egg!
! Uricotelic
! Terrestrial
! Cleidoic egg!
! Ureotelic
! Aquatic!
Differences in Nitrogen Excretion in Chelonians!
! NH3
Figure 28.25 Bullfrogs shift from ammonotelism to ureotelism as they undergo metamorphosis
! % of urinary nitrogen"
!Urate !Amino !Unaccounted"
Trachemys scripta
Pelusios derbianus
!Semi-aquatic, feeds"
!on land in marshes !14.4
!Damp terrestrial
!frequently enters"
!Drier than K. erosa
!Damp, swampy
!Almost desert
!Almost desert
! 56.1
Kinixys youngii
T. graeca
T. elegans
Differences in Ammonia Excretion at
Different Life Stages in Anurans!
Figure 28.20 The antennal gland and urine formation in a freshwater crayfish
!% Total Ammonia!
!Bufo bufo ! !Xenopus laevis!
Life Stage
!(Terrestrial) ! !(Fully aquatic)!
No hindlimbs
!85 !
Hindlimbs 3/4 developed
Hindlimbs functional
Forelimbs free, tail remaining !50
Tail atrophying
Topic 15 - 15!
PSLY 4210/5210
!Comparative Animal Physiology
!Osmoregulation and Excretion!
!Spring 2010!
Excretion and Water Balance in Insects!
Figure 28.22 The posterior gut and Malpighian tubules of an insect
Both insects and spiders utilize Malpighian tubules in conjunction with
rectal glands!
• Malpighian tubules vary in number but join between the midgut and hindgut!
• The blind ends of the tubules float freely in the hemocoel bathed in hemolymph!
Potassium is actively secreted into the tubules; other solutes follow the
  The main waste product is uric acid; it flows across at the upper end that is
mildly alkaline!
  In the lower end of"
the tubule, K+"
combines with CO2"
and is reabsorbed!
  Rectal glands then"
reabsorb Cl–, Na+,"
and H2O; the wastes"
pass on out of the body!
Topic 15 - 16!