Biology II
Most animals can
survive environmental fluctuations more extreme than any of their individual
cells could tolerate. This is possible
because mechanisms of homeostasis
maintain internal environments within ranges tolerable to body cells.
*Are
long-term adaptations that evolved in populations facing environmental
problems.
*Include
cellular mechanisms and short-term physiological adjustments.
I.
Homeostasis
mechanisms protect an animal’s internal environment from harmful fluctuations.
The
majority of cells in most animals (all but sponges and cnidarians) are not to
be exposed to the external environment, but are bathed by an extracellular
fluid.
*Animals
with open circulatory systems have an extracellular compartment containing
hemolymph, which bathes the cells.
*Animals
with closed circulatory systems have two extracellulare compartments – interstitial fluid and blood plasma.
*By moderating changes in the extracellular fluid, an animal’s homeostatic mechanisms prevent environmental fluctuations from having a harmful impact.
I.
Cells require a
balance between water uptake and loss.
Walls
may enter the body of a terrestrial animal through food, drinking, and
oxidative metabolism; water exits the body via evaporation and excretion. Aquatic animals are not affected by
evaporation, but face the problem of osmosis where water may enter (freshwater)
or leave (marine) the body.
*Even
animals with specialized body coverings that retard water gain or loss have
some unprotected structures exposed to the environment for gas exchange (lungs,
gills).
Animals’
cells cannot survive a net gain (swell and burst) or loss (shrivel and die) of
water.
*Solutes
in the blood help ensure the proper water balance in the cell.
Osmosis = Diffusion of water across a
selectively permeable membrane.
*Occurs when two solutions separated by a membrane differ in osmolarity (total solute concentration).
*If
a selectively permeable membrane separates two solutions of differing
osmolarities, water flows from the hypotonic solution to the hypertonic
solution.
Hypertonic
Solution = When comparing
two solutions, the solution with a greater solute concentration; net water
movement occurs into the solution.
Hypotonic
Solution = When comparing
two solution, the solution with lower solute concentration; net water movement
occurs out of the solution.
Isotonic
Solution = When comparing
two solution, a solution with a solution with a solute concentration equal to
that of the other solution; no net water movement occurs between the solutions.
An
animal may be an osoconformer or osmoregulator depending on how they
balance water loss with water gain.
Osmoconformers = Anmals that do not actively adjust
their internal osmolarity.
*Many salt water animals.
*Body fluids are isotonic with surroundings.
Osmoregulators = Animals that regulate internal
osmolarity by discharging excess water or taking in additional water.
*Many
saltwater animals, all freshwater animals, and terrestrial animals.
*Net movement of water in or out, requires a concentration gradient- the maintenance of which requires energy.
*Osmoregulation
permits animals to live in a variety of habitats, but the tradeoff is that it
requires an energy expenditure by the animal.
A large change in
external osmolarity is fatal to most animals, although some can survive radical
fluctuations.
Stenohaline animal = Animal that cannot survive a wide
fluctuation in external osmolarity.
Euryhaline animal = Animal that can survive wide fluctuations
in external osmolarity; may be:
*Osmoconformers
*Osmoregulators
which minimize the osmotic shock by a variety of mechanisms.
A. Maintaining Water Balance in Different
Environments
Marine animals lice in a saline environment
consisting of about 96.5% water and 3.5% dissolved substances.
*The
dissolved substances are collectively referred to as salts and the total amount
of these dissolved substances in the water is salinity.
Most marine
invertebrates are osmoconformers.
*Body fluids are isotonic to the
environment.
*Body
fluid composition usually differs from the external medium due to internal
regulation of specific ions.
Some vertebrates of
the Class Agnatha (hagfishes) are also osmoconformers. Most cartilaginous fishes, including sharks,
maintain internal salt concentrations lower than seawater by pumping salt out
through rectal glands and through the kidneys, yet their osmolarity is slightly
hypertonic to seawater.
*Sharks retain urea as a dissolved
solute in the body fluids.
*Sharks
also produce and retain trimethylamine oxide (TMAO), which protects their
proteins from the denaturation by urea.
*Retention
of these organic solutes (urea, TMAO) in the body fluids actually makes the
slightly hypertonic to seawater.
*Do
not drink water, but balance osmotic uptake of water by excreting urine.
Marine bony fishes
are hypotonic to seawater.
*Compensate
for osmotic water loss by drinking large amounts
of seawater and pumping excess salt out with
their gill
epithelium.
*Excrete
only a small amount of urine.
Freshwater animals are hypertonic to their environment
and constantly take in water by osmosis.
*Freshwater
protozoa compensate with contractile
vacuoles
that pump out excess
water.
*Many
freshwater animals, including fish, compensate by
excreting large amounts of very dilute
urine.
*Since
salts are lost in this process, salt is replenished
either by eating substances with a higher
salt content
or, in the case of some fish, by active
uptake of sodium
and chloride ions from the surrounding water
by gill
epithelium.
*Anadromous
fishes such as salmon are euryhaline
and
migrate between seawater and fresh
water.
*While in the ocean, they osmoregulate like other
marine fishes.
*When
in fresh water, they alter their
osmoregulation to that of freshwater fishes.
Temporary waters present a special problem to animals
that live in such environments.
*Anhydrobiosis is an adaptation found in
a small number of
aquatic invertebrates which permits them to
survive when
their habitat dries up.
*Best exemplified by the tardigrades.
*Hydrated
animals are about 1 mm long and are about
85% water.
*As
the water around the animal disappears , water is
lost from the tissues.
*Tardigrades
can survive many years in this state and will
rehydrate and become active when water
returns.
*Dehydrated
and frozen animals face the problem of keeping
their cell membranes intact.
*Researchers
have found that dehydrated anhydrobiotic
animals contain large amounts of the
disaccharide
trehalose along with other sugars.
*Trehalose
appears to replace the water associated with
membranes and proteins.
*Trehalose
is also found in insects that survive freezing.
Terrestrial animals live in a dehydrating environment and
cannot survive desiccation.
*Humans die if 12% of their body water
is lost.
Osmoregulatory
mechanisms in terrestrial animals include protective outer layers, drinking and
eating moist foods, behavioral adaptations and excretory organ adaptations that
conserve water.
*Anthropods
ave waxy cuticles, land snails possess shells, and
vertebrates are covered by a multi-layer
skin comprised of
dead, keratinized cells.
*Drinking
and eating moist foods replaces much of the water
lost during gas exchange.
*Some
desert animals are nocturnal; being active only at night
reduces dehydration and some like the
kangaroo rat produce
large amounts of metabolic water.
*The
excretory organs of terrestrial animals are adapted to
conserve water while eliminating wastes.
I.
Osmoregulation depends
on transport epithlia
Osmoregulators
utilize transport epithelia to regulate the movement of solutes between their
internal fluids and the external environment.
*Usually
a single sheet of cell, joined by impermeable
tight junctions, facing the external
environment.
*May
be a channel that leads to the exterior through an
opening on the body surface.
*The
molecular composition of the epithelium’s plasma
membrane determines the specific
osmoregulatory
functions.
(Remember that gill epithium pumps salt out
of marine fishes and pumps salts into
freshwater
fishes.)
*The
transport epithelium in the nasal glands of marine
birds are very efficient at eliminating the
excess slats
obtained from drinking seawater.
*May
also function in excretion of nitrogenous wastes in
some animals.
I.
Tubular systems
function in osmoregulation and excretion in many invertebrates
A. Protonephridia: The Flame-Bulb System
of Flatworms
Flatworms,
which have nether circulatory systems nor coeloms, have a simple tubular
excretory system called a protonephridium.
Protonephridium = A network of closed tubules lacking
internal openings that branch throughout the body; the smallest branches are
capped by a cellular flame bulb.
*Interstitial
fluid passes through a flame bulb and is
propelled by a tuft of cilia (in the flame
bulb) along the
branched system of tubules.
*In
planaria, this fluid drains into excretory ducts that
empty out of the body through numerous
nephridiopores.
*Transport
epithelium lining the tubules function in
osmoregulation by absorbing salts before the
fluid exits
the body.
*Some
parasitic flatworms are isotonic to their hosts and
this closed system is used mainly to excrete
nitrogenous
wastes.
Protonephridia
are also found in other invertebrates and lancets.
A. Metanephridia of Earthworms
Each
segment of most annelids, including earthworms, contains a pair of metanephridia, excretory tubules that
have internal openings to collect body fluids.
*Coelomic
fluid enters the funnel-shaped nephrostome
which is surrounded with cilia.
*The
fluid passes through the metanephridium and
empties into a storage bladder that empties
outside the
body through the nephridiopore.
*The
nephrostome collects coelomic fluid from the body
segment just anterior.
*A
network of capillaries envelops each metanephridium.
*These
capillaries reabsorb essential salts pumped
out of the collecting tubules by transport
epithelium bordering the lumen.
*Excretion
of hypotonic, dilute urine offsets the continual
osmosis of water from damp soil across the
skin.
A. Malpighian Tubules of Insects
Malpighian
tubules = Excretory organs of insects and other terrestrial arthropods that
remove nitrogenous wastes from the hemolymph and function in osmoregulation.
*Are
outpocketings of the gut that open into the digestive
tract at the midgut-hindgut juncture.
*The
tubules dead-end at the tips away from the
gut and are bathed in the hemolymph.
*Transport
epithelium lining each tubule moves solutes
(salts and nitrogenous wastes) from the
hemolymph into
the tubule’s lumen.
*Accumulates
nitrogenous wastes from the hemolymph
and water follows by osmosis.
*The
fluid in the tubule then passes through the hindgut
to the rectum.
*Salts
and water are reabsorbed across the epithelium of
the rectum and dry nitrogenous wastes are
excreted
with feces.
I.
The kidneys of most
vertebrates are compact organs with many excretory tubules.
The
invertebrate chordate ancestors of vertebrates probably possessed segmentally
arranged excretory structures arranged throughout the body.
*Extant
hagfishes have segmentally arranged
excretory tubules associated with their
kidneys.
The
kidneys of vertebrates (other than hagfishes) are compact organs and contain
large numbers of non-segmentally arranged tubules.
*Kidney
structure also includes a dense capillary network
intimately associated with the tubules.
*The
tubules function in both excretion and
osmoregulation in vertebrates that
osmoregulate.
The
vertebrate excretory system is comprised of the kidneys, blood vessels serving
in kidneys, and the structures that carry urine from the kidneys out of the
body.
*Variations
of the basic system are found among the
vertebrate classes.
A. The Mammalian Excretory System
The
human kidneys are a pair of bean-shaped organs about 10 cm long.
*Blood
enters each kidney via the renal artery
and exits
via the renal
vein.
*About 20% of the blood pumped by each heartbeat
passes through the kidneys.
Urine exits each kidney through a ureter and both drain into a common urinary bladder.
Urine leaves the body from the urinary bladder through the urethra.
*Sphincter muscles near the junction of the urethra and
bladder control urination.
A. The Nephron and Associated Structures
The two distinct regions of the kidney are the outer renal cortex and inner renal medulla.
*Each region contains many microscopic nephrons and
collecting ducts.
*Associated with each excretory tubule is a network of
capillaries.
Nephron = Functional unit of the kidney, consisting of a single long tubule and its associated capillaries.
*The blind end of the renal tubule that receives filtrate
from the blood forms a cup-shaped Bowman’s capsule
which embraces a ball of capillaries, the glomerulus.
*Water, salts, urea and other small molecules are
separated from the blood passing through the
glomerulus by blood pressure.
*This
filtrate enters the lumen of the
Bowman’s
capsule.
*Filtrate
then passes through the proximal tubule,
the
loop
of Henle (a long hairpin turn with descending limb
and an ascending limb) and the distal tubule, which
empties into a collecting duct.
*The
collecting duct receives filtrate from many
nephrons.
*Filtrate,
now called urine, flows from the
collecting ducts
into the renal
pelvis. Urine then drains from the
chamberlike pelvis into the ureter.
Two
tlypes of nephrons are found in mammals and birds: cortical nephrons and juxtamedullary
nephrons.
Cortical
nephron = Nephrons that
have reduced loops of Henle and are confined to the renal cortex. 80% of the nephrons in humans are cortical
nephrons.
Juxtamedullary
nephrons = Nephrons that
have long loops that extend into the renal medulla. 20% of the nephrons are juxtamedullary nephrons.
*The nephrons in other vertebrates lack loops of
Henle.
The
nephron and collecting duct are lined by transport epithemlium that processes
the filtrate into urine.
*About
1100-1200 L of blood flow through the
human kidneys each day.
*The
nephrons process 180 L of filtrate per day,
and the transport epithlium processes this
filtrate
to form the approximately 1.5 L urine
excreted
daily.
*The
rest of the filtrate is reabsorbed into the
blood.
The
Bowman’s capsules and the proximal and distal convoluted tubules are located in
the cortex.
*The
loops of Henle and collecting tubules extend
into the medulla.
Each nephron is closely
associated with blood vessels:
*Afferent arteriole is a branch of the renal artery that
divides to
form the capillaries
of the glomerulus.
*Efferent arteriole forms from the converging capillaries
as they
leave the glomerulus. This subdivides to form the peritubular
capillaries which intermingle with the
proximal and distal
tubules.
*Vasa recta is the capillary system branching
downward from
the peritubular capillaries that serves the
loop of Henle.
Materials are exchanged beween capillaries and nephrons through interstitial fluids.
I.
The kidney’s
transport epithelia regulate the composition of blood
The
composition of blood is regulated by transport epithelia of the nephrons and
collecting ducts through three processes: Filtration,
secretion, and reabsorption.
A.
Production of Urine
form a Blood Filtrate
1. Filtration of the Blood
Blood
pressure forces fluid from the glomerulus across the Bowman’s capsule
epithelium into the lumen of the nephron tubule.
*Porous
capillaries and podocytes
(specialized cells
of the capsule) nonselectively filter out
blood cells
and large molecules; any molecule small enough
to
be forced through the capillary wall enters
the
nephron tubule.
*Filtrate at this point contains a
mixture of glucose,
salts, vitamins, nitrogenous wastes, and
small
molecules in concentrations similar to that
in blood
plasma.
2. Secretion
Filtrate
is joined by substances transported across the tubule epithlium from the
surrounding interstitial fluid as it moves through the nephron tubule.
*Adds
plasma solutes to the filtrate
*The
proximal and distal tubules are the most
common sites of secretion.
*A
very selective process involving both
passive and active transport.
*For
example, controlled secretion of H+
ions helps maintain constant body fluid
pH.
3.
Reabsorbtion
Reabsorbtion
is the selective transport of filtrate
substances across the excretory tubule epithelium from the filtrate back
to the interstitial fluid.
*Reclaims small molecules essential to
the body.
*Occurs in the
proximal tubule, distal
tubule, loop of Henle, and collecting
duct.
*Nearly all sugar,
vitamins, organic
nutrients and, in mammals and birds,
water are reabsorbed.
The composition of the filtrate is
modified by
selective secretion and reabsorption.
*The concentration
of beneficial
substances in the filtrate is reduced as
they are returned to the body.
*The concentration
of wastes and
nonuseful substances is increased and
excreted from the body.
The kidneys are central to the
process of
homeostasis as they clear metabolic
wastes from
the blood and respond to body fluid
imbalances by
selectively secreting ions.
A. Transport Properties of the Nephron and
Collecting Duct
Reclamation
of small molecules and water from the filtrate as it flows through the nephron
and collecting duct converts the filtrate into urine.
1. The proximal
tubule alters the volume and composition of filtrate by reabsorption and
secretion.
*In
this area ammonia, drugs and poisons
processed in the liver are secreted to join
the filtrate.
*Helps
maintain a constant body fluid pH by
controlled secretion of H+ and reabsorption
of bicarbonate.
*Nutrients
such as glucose and amino
acids are reabsorbed (by active transport)
from the filtrate and returned to the
interstitial fluid from which they enter the
blood.
The
reabsorption of NaCl and water is also an important function of the proximal
tubule.
*Salt
diffuses into the transport epithelium
cells; the membranes facing the interstitial
fluid then actively pump Na+ out of the
cells, which is balanced by passive
transport
of Cl-.
Water follows passively by osmosis.
*Cells
facing the interstitial fluid (outside the
tubule) have a small surface area to
minimize leakage of salt and water back into
the filtrate.
1. In the descending limb of the loop of Henle, the
transport epithlium is freely permeable to
water but
not to salt and other small solutes.
*Filtrate
moving down the tubule from the cortex to
the medulla continues to lose water by
osmosis
since the interstitial fluid in this region
increases in
osmolarity.
*The
NaCl concentration of the filtrate increases
due to the water loss.
1. In the ascending limb of the loop of Henle, the
transport epithelium is very permeable to
salt, but not
to water.
*As the filtrate ascends through the thing segment
near the loop tip. NaCl diffuses out and
contributes to the high osmolarity of interstitial
fluids of the medulla.
*In
the think segment leading to the distal
convoluted tubule, NaCl is actively
transported out
into the interstitial fluid.
*The
filtrate becomes more dilute due to the
removal of salts without loss of water.
1. The distal
tubule is an important site of selective
secretion and absorption.
*It
regulates K+ and NaCl concentrations of body
fluids by regulating K+ secretion into the
filtrate
and NaCl reabsoption from the filtrate.
*This
region also contributes to pH regulation by
controlled secretion of H+ and reabsorption
of
bicarbonate.
5.The
collecting duct carries filtrate back
through the
medulla into the renal pelvis.
*The transport epithelium here is permeable to
water by not to salt.
*The
filtrate loses water by osmosis to the
hypertonic fluid outside the duct which
results in a concentration of urea in the
filtrate.
*The
lower portion of the duct is permeable to
urea, some of which diffuses out.
*This
contributes to the high osmolarity of the
interstitial fluid of the renal medulla,
which
enables the kidney to conserve water by
excreting a hypertonic urine.
I.
The water-conserving
ability of the mammalian kidney is a key
terrestrial adaptation.
Cooperative
action between the loop of Henle and the collecting duct maintain the
osmolarity gradient in the tissues of the kidney.
*The
two solutes responsible for the gradient are NaCl
(deposited by the loop of Henle) and urea
which leaks
across the epithelium of the collecting
ducts.
*The
urine formed is up to four times as concentrated as
the blood (about 1200 mosm/L).
A. Conservation of Water by Two Solute
Gradients
The
jextamedullary nephron, with its urine-concentrating features, is a key
adaptation to terrestrial life that enables mammals to excrete nitrogenous
waste without squandering water.
Filtrate passing from the Bowman’s Capsule to the proximal tubule has
about the same osmolarity as blood (300 mosm/L).
*A
large amount of water and salt is reabsorbed as
filtrate passes through the proximal tubule
which is
located in the renal cortex.
*The
osmolarity remains about the same during the
decrease in volume.
*Water
moves out of the filtrate by osmosis as it flows
from the cortex into the medulla through the
descending
loop of Henle.
*The
osmolarity steadily increases due to loss of
water until it peaks at the apex.
*As
filtrate moves up the ascending lop of Henle back to
the cortex, salt leaves the filtrate first
by passive then
by active transport.
*Osmolarity
decreases to about 100 mosm/L at this
point.
*Osmolarity
changes very little as filtrate flows through
the distal tubule as it is hypotonic to the
interstitial fluids
of the cortex.
*After
entering the collecting duct, the filtrate passes
back through the medulla.
*The filtrate loses water which
increases osmolarity.
*Some
urea also leaks out of the lower portion of
the collecting duct.
*The
passage of the filtrate back through the hypertonic
medulla causes a gradual increase in
osmolarity to
about 1200 mosm/L; the remaining molecules
are
excreted in a minimal amount of water as
urine which
passes to the renal pelvis to the ureter.
NOTE:
The loss of salt form the filtrate
passing through the ascending limb of the loop of Henle to the interstitial fluid of the medulla contributes to the high
osmolarity of the medulla. This, in
turn, helps conserve water.
The
vasa recta (capillary network of the
renal medulla) does not dissipate the crucial osmolarity gradient in the
kidney.
*As the descending vessel conveys blood toward
the inner medulla, water is lost from the blood and
NaCl diffuses into the blood.
*These
fluxes are reversed as blood flows back
toward the cortex in the ascending vessel.
*This
countercurrent system allows the vasa recta
to supply the tissues with necessary
substances
without interfering with the osmolarity
gradient.
Urine,
as its most concentrated, is isotonic to the interstitial fluid of the inner
medulla, but is hypertonic to body fluids elsewhere.
A. Regulation of Kidney Function of
Feedback Circuits
The
kidney is a versatile osmoregulatory organ subject to a combination of nervous
and hormonal controls.
*It excretes hypertonic or hypotonic urine
as necessary.
Three mechanisms regulate the kidney’s
ability to change
the osmolarity, salt concentration,
volume, and blood
pressure: 1) antidiuretic
hormone; 2) juxtaglomerular
apparatus; and 3) atrial natriuretic factor.
Antidiuretic hormone (ADH) enhances fluid retention by
increasing the water permeability of epithelium of the distal tubules and the
collecting duct.
*ADH is produced in the hypothalamus and stored and
released from the pituitary gland.
*ADH release is triggered when osmoreceptor cells in the
hypothalamus detect increased blood osmolarity due to
an excessive loss of water from the body.
*Increased
water reabsorption reduces blood osmolarity
and reduces stimulation of the osmoreceptor
cells.
*Results in less ADH being secreted.
*Ingestion
of water returns blood osmolarity to
normal.
*When
a large volume of water has been ingested, little
ADH is released and the kidneys produce a
dilute urine
since little water is absorbed.
*Alcohol
can inhibit ADH release, causing dehydration.
The
juxtaglomerular apparatus (JGA) is a
specialized tissue near the afferent arterioles which crry blood to the
glomeruli. It responds to a decrease in
blood pressure or blood volume by releasing the enzyme renin into the blood.
*Renin
converts inactive angiotensin to
active angiotensin
II
which funcitons as a hormone.
*Angiotensin
II directly increases blood pressure by:
*Causing arteriole constriction.
*Stimulating
the proximal tubules to absorb more
NaCl and water. Reduction in the amounts of salt
and water excreted raises blood volume and
pressure.
*Angiostensin II acts indirectly
by:
*Signaling
adrenal glands to release aldosterone,
which stimulates Na+ reabsorption by the
distal
tubules (water follows by osmosis).
*Increases
blood pressure and blood volume.
*The
increased blood pressure and blood volume
suppresses further release of renin.
*The
renin-aniotensin-aldosterone system
(RAAS) is part
of a complex feedback circuit that functions
in
homeostasis.
ADH
and RAAS cooperate in homeostasis.
*ADH
is released in response to increased blood
osmolarity, but this does not compensate for
excessive loss of salts and body fluids if
blood
osmolarity does not change.
*RAAS
responds to a decrease in blood volume
caused by fluid loss.
*ADH
alone would lower blood Na+ concentration
by increasing water reabsorption but the
RAAS
maintains balance by stimulating Na+
reabsorption.
The hormone atrial natriuretic factor (ANF) opposes the
RAAS.
*Released by the heart’s atrial walls in
response to
increased blood volume and pressure.
*Inhibvits release of renin from the
JGA, inhibits NaCl
absorption by the collecting ducts,
and reduces
aldosterone release from the adrenal glands. This
decreases volume and lowers blood pressure.
I.
Diverse adaptations
of the vertebrate kidney have evolved in
different habitats
Nephrons vary in structure and
physiology and help different
vertebrates
to osmoregulate in their various habitats.
*Desert
mammals have very long loops of Henle to maintain
steep osmotic gradients that conserve water
by allowing urine
to
become very concentrated.
*Mammals
living in aquatic environments (e.g. beavers)
nephons with very short loops of Henle
resulting in production
of dilute urine.
*Birds
have shorter loops of Henle and produce a more dilute
urine than mammals.
*Reptiles
have only cortical nephrons and produce isotonic
urine, but the epithelium of their cloaca
conserves fluid by
reabsorbing water from urine and feces.
*Also most excrete nitrogenous wastes as
uric acid which
conserves water.
*Freshwater
fish (hypertonic to surroundings) nephrons use
cilia to sweep the large volume of very
dilute urine from the
body.
*Salts are conserved by efficient ion
reabsorption from filtrate.
*Amphibians
excrete dilute urine and accumulate certain salts
from the water by active transport across
the skin. On land,
body fluid is conserved by water
reabsorption across urinary
bladder epithelium.
*Bony
marine fishes (hypotonic to surroundings) excrete very
little concentrated urine (many lack
glomeruli and capsules).
*The kidneys function mainly to rid the body
of divalent ions
such as Ca2+, Mg2+ and SO42- taken in by
drinking
seawater.
*Monovalent ions like Na+ and Cl-, and the
majority of
nitrogenous waste (in the form of
ammonium) is excreted
mainly by the gills.
I.
An animal’s
nitrogenous wastes are correlated with its phylogeny and habitat
The
metabolism of proteins and nucleic acids produces ammonia, a small and very
toxic waste product. Some animals
excrete the ammonia directly, while other first convert it to urea or uric acid
which are less toxic.
A. Ammonia
Ammonia is excreted directly by most aquatic
animals.
*Easily
permeates membranes since molecules are very
water soluble.
*In
soft-bodied invertebrates, ammonia just diffuses out
across the body surface.
*In
fishes, ammonium ions are excreted as ammonium ions
across gill epithelium.
*The gill epithelium of freshwater fish
exchange Na+ from
the water for NH4+, thus maintaining a
higher Na+
concentration in the blood.
A. Urea
Ammonia
excretion is unsuitable for animals in a terrestrial habitat.
*It
requires large amounts of water and is so toxic it must
be eliminated quickly.
Urea
is the nitrogenous waste excreted by mammals and most adult amphibians.
*Can
be more concentrated in the body since it is much less
toxic than ammonia; reduces water loss for
terrestrial
animals.
*Produced
in liver by a metabolic cycle combining ammonia
with CO2.
It is transported to kidneys via the circulatory
system.
*Some
urea is retained in the kidney where it contributes to
osmoregulation by maintaining the osmolarity
gradient in
the medulla.
*Sharks
also produce and retain urea in the blood as an
osmoregulatory agent.
*Amphibians
that undergo metamorphosis and move to land
as adults switch from excreting ammonia to
excreting urea.
A. Uric Acid
Uric acid is the primary form of nitrogenous
waste excreted by land snails, insects, birds and many reptiles.
*Much less soluble in water than ammonia or
urea and can
be excreted as a precipitate after
reabsorption of nearly all
the water from the urine.
*Eliminated
in a pastelike form through the cloaca (mixed
with feces) in birds
and reptiles.
The
mode of reproduction is an important factor in determining whether uric acid or
urea excretion evolved in a particular group.
*If
an embryo released ammonia or urea within a shelled
egg, the soluble waste would accumulate to
toxic
concentrations: uric acid precipitates out of solution to be
stored as a solid.
The
animal’s habitat, along with the phylogenetic position, influences the type of
nitrogenous waste produced.
*Terrestrial
reptiles excrete mostly uric acid; crocodiles
excrete ammonia and uric acid; aquatic
turtles excrete urea
and ammonia.
Some
animals can modify their nitrogenous wastes when the temperature or water
availability changes.
I.
Thermoregulation
maintains body temperature within a range
conducive to metabolism
Metabolism and membrane properties are
very sensitive to
changes in an animal’s internal
temperature.
*Each animal lives in, and is adapted
to, an optimal
temperature range in which it can
maintain a constant internal
temperature when external
temperatures fluctuate.
*Maintaining the body temperature within
a range that permits
cells to function efficiently is
known as thermoregulation.
A. Heat Transfer Between Organisms and
Their Surroundings
An
organism exchanges heat with its environment by four physical processes:
1. Conduction is the direct transfer of thermal
motion (heat) between molecules of the environment and a body surface.
*Heat
is always conducted from a body of higher
temperature to one of lower temperature.
*Water
is 50-100 times more effective than air in
conducting heat.
*For
example, on a hot day, an animal in cold water cools
more rapidly than one on land.
1. Convection is the transfer of heat by the
movement of air or liquid past a body surface.
*For
example, breezes contribute to heat loss from an
animal with dry skin.
1. Radiation is the emission of
electromagnetic waves produced by all objects warmer than absolute zero.
*Can
transfer heat between objects not in direct contact.
*For
example, an animal can be warmed by the heat
radiating from the sun.
1. Evaporation is the loss of heat from a liquid’s
surface that is losing some molecules as gas.
*Cooling
increases greatly by production of sweat.
*Can
only occur if surrounding air is not saturated with
water molecules.
*Along
with convection, is most variable cause of heat
loss.
I.
Ectotherms derive
body heat mainly from their surroundings and endotherms derive it mainly from
metabolism
Animals
may be classified as either ectotherms
or endotherms depending on their
major source of body heat.
Ectotherm=An animal that wams its body mainly by
absorbing heat from surroundings.
*Most invertebrates, fishes, reptiles and amphibians.
*May
derive a small amount of body heat from metabolism.
Endotherm=An animal that derives
most or all of its body heat
from
its own metabolism.
*Mammals,
birds, some fishes, and numerous insects.
*Many
maintain a consistent internal temperature even as the
environmental temperature fluctuates.
The
main source of body heat distinguishes endotherms from
ectotherms, not the body temperature.
*Distinction
is not absolute.
*Most
endothermic insects and some fishes are partial
endotherms.
*These organisms retain metabolic heat to
warm only certain
body parts (i.e. locomotor muscles).
*Some
birds and mammals acquire additional body heat by
basking in the sun.
A
terrestrial life style presents certain problems which have been solved by the
evolution of endothermy.
*Environmental
temperatures fluctuate more in terrestrial
habitats than in aquatic habitats.
*Endothermic vertebrates are usually warmer
than their
surroundings, but also have mechanisms to
cool their bodies.
*Maintaining
a warm body temperature requires an active
metabolism which contributes to a high level
of cellular
respiration
*This permits endotherms to be more
physically active for a
longer period of time in comparison to
most ectotherms.
Endothermy
requires more energy than ectothermy.
*At
20°C, a human has a resting metabolic rate of 1300 to
1800 kcal/day while an alligator (ectotherm)
of similar weight
has a resting metabolic rate of about 60
kcal/day.
*Endotherms
usually consume more food than ectotherms of
similar size to offset the energy
requirements.
The
bioenergetic connections between body temperature, an active metabolism, and mobility
were important to the evolution of endothermy.
*The
evolution of endothermy and a high metabolic rate were
accompanied by an increase in efficiency in
circulatory and
respiratory systems as seen in the birds and
mammals.
*Terrestrial
ectotherms exhibit their own adaptations for
adjusting to temperature changes in the
terrestrial
environment.
I.
Thermoregulation
involves physiological and behavioral
Adjustments
Endotherms
and ectotherms use a combination of
strategies to thermoregulate. For
example:
1. Adjusting the
rate of heat exchange between the animal and its surroundings.
Heat
loss is reduced by the presence of hair, feathers, and fat just below the skin.
Adaptations
found in the circulatory system also help regulate heat exchange.
*The
amount of blood flowing to the skin can be changed
by many endotherms and some ectotherms.
*Vasodilation
increases the blood flow to the skin due to an
increase in the diameter of blood vessels
near the body
surface.
*Nerve signals cause muscles in the walls of
the blood
vessels to relax which permits more blood
to flow
through the vessel.
*As blood flow increases, more heat is
transferred to the
environment by conduction, convection, and
radiation.
*Vasoconstriction reduces blood flow to
the skin due to a
decrease in the diameter of blood vessels
near the body
surface.
*Reduced blood flow decreases the amount of
heat
transferred to the environment.
Heat
exchange with the environment is also altered by a countercurrent heat
exchanger. (See Cambell, Figure 40.16)
*This
is a special arrangement of arteries and veins found in
the extremities of many endothermic animals.
*Arteries
carrying warm blood from the body to the legs of
a bird or flipper of a dolphin are in close
contact with veins
carrying blood from the appendage into the
body.
*This
vessel arrangement enhances heat transfer from
arteries to veins along the entire length of
the vessel.
*This is possible since venous blood
returning from the tip
of the appendage is always cooler than the
arterial blood.
*The
venous blood entering the body has been warmed to
almost core temperature by the exchange.
*In
some species, an alternative set of vessels which by-
pass the exchanger is present.
*The rate of heat loss is controlled by the
relative amount
of blood that enters the appendage by the
two paths.
1. Cooling by
evaporative heat loss.
Water
is lost by terrestrial endotherms and ectotherms through breathing and across
the skin.
*In
low humidity, water evaporates and heat is lost by
evaporative cooling.
*Painting
increases evaporation from the respiratory system.
*Sweating
in mammals increases evaporative cooling
across the skin.
3. Behavioral
responses
Relocating
allows animals to increase of decrease heat loss from the body.
*In
winter, many animals bask in the sun or on warm rocks.
*In
summer, many animals burrow or move to damp areas.
*Some
animals migrate to more suitable climates.
4.Changing the rate of metabolic heat
production.
This is found only in birds and mammals.
*Increased
skeletal muscle activity and non-shivering
thermogenesis can greatly increase the
amount of
metabolic heat produced.
I.
Comparative
physiology reveals diverse mechanisms of thermoregulation
A. Invertebrates
Most
invertebrates have little control over body temperature, but some do adjust
temperature by behavioral or physiological mechanisms.
*The
desert locust orients the body to maximize heat
absorption from the sun.
*Some
large flying insects (e.g. bees) can generate
internal heat by contracting all flight
muscles in
synchrony (functionally analogous to
shivering).
*Little
wing movement occurs by large amounts of
heat are produced.
*Allows activity even on cold days and
at night.
*Endothermic
insects such as honeybees, bumblebees,
and noctuid moths have a countercurrent heat
exchanger that maintains a high temperature
in the
thorax where flight muscles are
located.
Honeybees
also use social organization to regulate temperature.
*Increase
movements and huddle to retain heat in
cold weather.
*Maintain
constant temperature by changing
the density of huddling; heat is distributed
by the movement of individuals from the
core to the margins of the huddle.
*In
warm weather, they cool hives by transporting
water to it and fanning with their wings to
promote evaporation and convection.
A. Amphibians and Reptiles
Amphibians
produce little heat and most lose heat rapidly by evaporative cooling from
their body surface, making thermoregulation difficult.
*The
optimal temperature range varies greatly depending
on the species.
*Seek
cooler or warmer microenvironments as necessary
(behavioral adaptation).
*Some
can vary the amount of mucus they secrete to
regulate evaporative cooling.
Reptiles
are generally ectotherms that warm themselves mainly by behavioral adaptations.
*Orient
the body toward the heat sources to increase
uptake and maximize the body surface exposed
to, the
heat source.
*Along
with orientations and body surface exposure, they
regulate body temperature by seeking warm
places or
other favorable microclimates in the
environment.
Some
reptiles have physiological adaptations that help regulate heat loss.
*When
swimming in cold water, the Galapagos iguana
utilizes superficial vasoconstriction to
reduce heat loss.
*Female
pythons that are incubating eggs increase their
metabolic rate by shivering.
*Maintains
their body temperature 5° - 7°C above
ambient temperature.
C. Fishes
Fish
are generally ectotherms although some are endothermic.
*The
body temperature of most is within 1° - 2°C of the
water temperature.
*Metabolic
heat produced by the swimming muscles is
lost to the water as blood passes through
the gills.
*The
dorsal aorta carries blood directly from the
gills to the tissues, cooling the body core.
Endothermic fishes have and adaptation
to reduce heat loss.
*Includes
large active species such as the bluefin tuna,
swordfish, and great with shark.
*Large
arteries carry most of the blood from the
gills to tissues just under the skin.
*Branches
carry blood to the deep muscles where
smaller vessels are arranged into a
countercurrent
heat exchanger.
*The
swimming muscles produce enough metabolic heat
to raise temperatures at the body core.
*Adaptations
in the circulatory system help retain the
heat.
*Swimming
muscles remain several degrees warmer than
tissues closer to the animal’s surface.
*Endothermy thus enhances the activity
level of
these fishes.
A. Mammals and Birds
These
organisms maintain high body temperatures within a narrow range.
*36° - 38°C for most mammals.
*40° - 42°C for most birds.
Birds
and mammals regulate the rate of metabolic heat production and balance it with
the rate of heat loss or gain from their surroundings.
The
rate of heat production may be increased by:
*The increased contraction of muscles.
*Moving or shivering produce
metabolic heat.
*The action of hormones that increase the
metabolic rate
and the production of heat instead of
ATP.
*Nonshivering
thermogenesis is the hormonal
triggering of heat production.
*Found in numerous mammals and a few
birds.
*Occurs throughout the body and some
mammals
have brown fat between the
shoulders and in the
neck that is specialized for rapid heat production.
Birds
and mammals also use other mechanisms to regulate environmental heat exchange.
*Vasodilation and vasoconstriction permit
the
maintenance of a proper core temperature
even when
the extremities are cooler.
*Fur and feathers trap a layer of insulating
air next to the
body.
*Most land mammals and birds raise
their hair or
feathers in response to cold.
*This traps a thicker layer of air
next to the body,
thus providing more insulation.
*A layer of fat just below the skin provides
insulation.
The
insulating power of hair is greatly reduced in water, and marine mammals show
various adaptations to reduce heat loss.
For example, whales and seals live in water colder than their body
temperture.
*They are insulated by a very thick layer of
fat called
blubber
under the skin.
*Helps them maintain a body temperature
of 36° -
38°C even in Arctic and Antarctic waters.
*Countercurrent heat exchangers reduce heat
loss in the
extremities where no blubber is found.
Thermoregulation
by endotherms also involves cooling ghe body.
Many birds and mammals have behavioral and physiological adaptations to
cool the body.
*When whales and other marine mammals move
into
warm waters, excess heat is eliminated by vasodilation.
*A large number of blood vessels in
the outer skin
dilate which permits increased blood flow (and heat
loss by conduction) to occur.
*Terrestrial birds and mammals
depend on evaporative
cooling.
*Panting increases heat
loss.
*The fluttering of
vascularized pouches in the floor
of the mouth in some birds increases evaporative
cooling.
*Many terrestrial mammals have
sweat glands.
*Some mammlas spread saliva
(some kangaroos and
rodents) or a combination of
saliva and urine (some
bats) over the body to
increase evaporative heat loss.
A. Thermoregulation in Humans
Thermoregulation
in humans involves a complex homeostatic system and feedback mechanisms. (See
Campbell, Figure 40.21)
*The hypothalamus
contains nerve cells which control
thermoregulation and many other aspects
of
homeostasis.
*The hypothalamus responds to changes in
body
temperature which are above or below
the normal
range. (36.1° - 37.8°C).
The
body’s temperature is monitored by nerve cells in the skin, hypothalamus, and
other parts of the body.
*Some function as warm receptors that signal
the
hypothalamus when the skin or blood
temperature
increases.
*Others function as cold receptors that
signal the
hypothalamus when there is a decrease in
temperature
of the skin or blood.
When
the body’s temperature increases above the normal range, cooling mechanisms are activated.
*Vasodilation of skin vessels occurs and
capillaries fill with
blood; the heat radiates from the skin’s
surface.
*Sweat glands are activated which increases
evaporative
cooling.
*These changes result in a decrease in body
temperature
to within the normal range.
*The cooling mechanisms are “turned off” by
the
hypothalamus when normal body
temperature is
achieved.
When
the body’s temperature decreases below the normal range due to a cold environment,
warming mechanisms are activated.
*Vasoconstriction of skin vessels occurs.
*A smaller volume of blood passes to
the skin, thus
reducing heat loss from the body
surface.
*The warm blood is diverted from the
skin to
deeper tissues.
*Skeletal muscles are activated and
shivering occurs
which generates metabolic heat.
*Nonshivering thermogenesis increases heat
production.
*Changes resulting in an increase in body
temperature to
within the normal range.
*The warming mechanisms are “turned off” by
the
hypothalamus when normal body
temperature is
achieved.
A. Torpor: Conserving Energy During Environmental Extremes
When
food supply is low and/or environmental temperatures are extreme, many
endotherms will enter a state of torpor.
Torpor=Alternative physiological state in
which metabolism decreases and the heart and respiratory systems slow down.
*This is a mechanism to conserve energy.
Torpor
may be a long-term state (hibernation, aestivation) or short-term as in the
daily period of torpor seen in many small mammals and birds.
Hibernation
and aestivation are often triggered by seasonal changes in daylength.
*In hibernation,
the body temperature is lowered; this
allows an animal to survive long periods
of cold and
diminished food supplies.
*Some animals will begin to eat huge
quantities of
food as the amount of daylight
decreases.
*Aestivation
is characterized by slow metabolism and
inactivity.
*Allows the animal to survive long
periods of high
temperature and diminished water
supply.
*Also known as summer torpor.
Daily
periods of torpor appear to be adapted to feeding patterns.
*They allow many relatively small endotherms
to survive
on stored energyduring hours when they
cannot feed.
*These animals have a very high
metabolic rate and
rate of energy consumption when
active.
*Most bats and shrews feed at night and
enter torpor
during daylight hours.
*Chickadees and hummingbirds feed during the
day and
undergo torpor on cold nights.
*The body temperature of chickadees
in the cold,
northern forests may drop 10°C
during winter
nights.
An
animal’s biological clock appears to control its daily cycle and torpor.
*For example, shrews continue to undergo a
daily torpor
even when food is continually available.
*Human sleep periods and the corresponding
drop in
body temperature may be a remnant of a
daily torpor in
ancestral mammals.
F. Temperature Range Adjustments
A
physiological response called acclimatization allows many animals to adjust to
a new range of environmental temperature.
*The adjustment takes place over a period of
days or
weeks.
*The process is important to animals that
must adjust to
seasonal changes in temperature.
Acclimatization
of an animal to a new external temperature range may involve several
physiological changes.
*May involve changes in the cooling and
warming
mechanisms which maintain the internal
temperature.
*May involve adjustments at the cellular
level.
*Cells may increase production of
certain enzymes
to compensate for the lowered
activity of each
molecule at lower temperatures.
*Cells may produce enzyme variant
having the
same function but different
temperature optima.
*Membranes may remain fluid by changing the
proportions of saturated and unsaturated
lipid in their
composition.
Cells
can also make rapid adjustments to temperature changes.
*Severe stress due to a large temperature
increase (or
other stress) stimulate the accumulation
of stress-
induced proteins.
*Found in animal cells, yeast, and
bacteria.
*Help prevent denaturation of other
proteins by
high temperatures.
*Help prevent cell death and may
help maintain
homeostasis while the organism
is adjusting to the
external environment.
XIV. Regulatory
systemsinteract in the maintenance of homeostasis
Maintaining homeostasis in the internal
environment involves a
number of regulatory systems within an
animal’s body. For
example:
*Regulation of body temperature
involves mechanisms
that control osmolarity,
metabolic rate, blood pressure,
tissue oxygenation, and body
weight.
While the regulatory systems normally
work in concert to
maintain homeostasis, under extreme
physiological stress,
demands of one regulatory system may
conflict with those of
other systems.
*Water conservation takes
precedence over evaporative
heat loss in very warm, dry climates.
*Many desert animals must
tolerate occasional
hyperthermia (abnormally
high body temperature) in
order to maintain body
water.
A. Roles of the Liver in Homeostasis
The
functions of the vertebrate liver are critical to maintaining homeostasis and
involve interactions with most of the body’s organ systems. (See Campbell,
Figure 40.1).
*Excretion by the kidneys is supported by
the liver which
synthesizes ammonia, urea, or uric acid
from the
nitrogen of amino acids.
*The liver also detoxifies many
chemical poisons.
*The bioenergetics of the body are influenced
by many of
the liver’s activities.
*Liver cells take up glucose from
the blood and
store excess amounts as
glycogen.
*When glucose is needed by the body,
the liver
converts glycogen back to
glucose and releases it
into the blood.
*Blood glucose levels are closely
controlled by
feedback circuits which regulate
the homeostatic
mechanisms.
*This, while the liver performs the
essential functions,
feedback circuits are in turn controlled
by the nervous
and endocrine systems.