Homeostasis
Chapter 30
Homeostasis
 Homeostasis refers to maintaining internal stability
within an organism and returning to a particular stable
state after a fluctuation.
Homeostasis
 Changes to the internal environment come from:
 Metabolic activities require a supply of materials (oxygen,
nutrients, salts, etc) that must be replenished.
 Waste products are produced that must be expelled.
Homeostasis
 Systems within an organism function in an integrated
way to maintain a constant internal environment around
a setpoint.
 Small deviations in pH, temperature, osmotic pressure,
glucose levels, & oxygen levels activate physiological
mechanisms to return that variable to its setpoint.
 Negative feedback
Osmoregulation & Excretion
 Osmoregulation regulates solute concentrations and
balances the gain and loss of water.
 Excretion gets rid of metabolic wastes.
Osmosis
 Cells require a balance between osmotic gain and loss
of water.
 Water uptake and loss are balanced by various
mechanisms of osmoregulation in different
environments.
Osmosis
 Osmosis is the movement of water across a selectively
permeable membrane.
 If two solutions that are separated by a membrane differ
in their osmolarity, water will cross the membrane to bring
the osmolarity into balance (equal solute concentrations
on both sides).
Osmotic Challenges
 Osmoconformers, which are only marine animals, are
isoosmotic with their surroundings and do not regulate
their osmolarity.
 Osmoregulators expend energy to control water
uptake and loss in a hyperosmotic or hypoosmotic
environment.
Osmotic Regulation
 Most marine invertebrates are osmotic conformers –
their bodies have the same salt concentration as the
seawater.
 The sea is highly stable, so most marine invertebrates are
not exposed to osmotic fluctuations.
 These organisms are restricted to a narrow range of
salinity – stenohaline.
 Marine spider crab
Osmotic Regulation
 Conditions along the
coasts and in estuaries
are often more variable
than the open ocean.
 Animals must be able to
handle large, often abrupt
changes in salinity.
 Euryhaline animals can
survive a wide range of
salinity changes by using
osmotic regulation.
 Hyperosmotic regulator
(body fluids saltier than
water)
 Shore crab.
Osmotic Regulation
 The problem of dilution is solved by pumping out the
excess water as dilute urine.
 The problem of salt loss is compensated for by salt
secreting cells in the gills the actively remove ions from
the water and move them into the blood.
 Requires energy.
Osmotic Regulation Freshwater
 Freshwater animals face an even more extreme
osmotic difference than those that inhabit estuaries.
Osmotic Regulation Freshwater
 Freshwater fishes have skin covered with scales and
mucous to keep excess water out.
 Water that enters the body is pumped out by the kidney
as very dilute urine.
 Salt absorbing cells in the gills transport salt ions into
the blood.
Osmotic Regulation Freshwater
 Invertebrates and
amphibians also
solve these problems
in a similar way.
 Amphibians actively
absorb salt from the
water through their
skin.
Osmotic Regulation – Marine
 Marine bony fishes are
hypoosmotic
regulators.
 Maintain salt
concentration at 1/3
that of seawater.
 Marine fishes drink
seawater to replace
water lost by diffusion.
 Excess salt is carried to the gills where salt-secreting
cells transport it out to the sea.
 More ions voided in feces or urine.
Osmotic Regulation – Marine
 Sharks and rays retain urea (a metabolic waste usually
excreted in the urine) in their tissues and blood.
 This makes osmolarity of the shark’s blood equal to
that of seawater, so water balance is not a problem.
Osmotic Regulation –
Terrestrial
 Terrestrial animals
lose water by
evaporation from
respiratory and body
surfaces, excretion
(urine), and
elimination (feces).
 Water is replaced by
drinking water, water
in food, and retaining
metabolic water.
Osmotic Regulation –
Terrestrial
 The end-product of protein metabolism is ammonia,
which is highly toxic.
 Fishes can excrete ammonia directly because there
is plenty of water to wash it away.
Osmotic Regulation –
Terrestrial
 Terrestrial animals must convert ammonia to uric acid.
 Semi-solid urine – little water loss.
 In birds & reptiles, the wastes of developing embryos are
stored as harmless solid crystals.
Osmotic Regulation –
Terrestrial
 Marine birds and
turtles have a salt
gland capable of
excreting highly
concentrated salt
solution.
Excretory Processes
 Most excretory
systems produce
urine by refining a
filtrate derived from
body fluids (blood,
hemolymph, or
coelomic fluid).
Excretory Processes
 Key functions of most excretory systems are:
 Filtration, pressure-filtering of body fluids producing
a filtrate.
 Reabsorption, reclaiming valuable solutes from the
filtrate.
 Secretion, addition of toxins and other solutes from
the body fluids to the filtrate.
 Excretion, the filtrate leaves the system.
Invertebrate Excretory
Structures
 Contractile vacuoles are found in protozoans and
freshwater sponges.
 An organ of water balance – expels excess water gained
by osmosis.
Invertebrate Excretory
Structures
 The most common type of
invertebrate excretory
organ is the nephridium.
 The simplest arrangement
is the protonephridium of
acoelomates and some
pseudocoelomates.
 Fluid enters through flame
cells, moves through the
tubules, water and
metabolites are recovered
and wastes are excreted
through pores that open
along the body surface.
 Highly branched due to lack
of circulatory system.
Invertebrate Excretory
Structures
 The metanephridium is
an open system found in
annelids, molluscs, and
some smaller phyla.
 Tubules are open at
both ends.
 Water enters through
the ciliated, funnel
shaped nephrostome.
 The metanephridium is
surrounded by blood
vessels that assist in
reclaiming water and
valuable solutes.
Invertebrate Excretory
Structures
 In arthropods,
antennal glands are
an advanced form of
the nephridial organ.
 No open
nephrostomes,
hydrostatic pressure
of the blood forms an
ultrafiltrate in the end
sac.
 In the tubule,
selective resorption
of some salts and
active secretion of
others occurs.
Invertebrate Excretory
Structures
 Insects and spiders have
Malpighian tubules that are
closed and lack an arterial
supply.
 Salts (especially potassium) are
secreted into the tubules from
the hemolymph (blood).
 Water & other solutes (including
uric acid) follow.
 Water & potassium are
reabsorbed.
 Uric acid is expelled in feces.
Vertebrate Kidneys
 Kidneys, the excretory organs of vertebrates,
function in both excretion and osmoregulation.
Vertebrate Kidneys
 Nephrons and associated blood vessels are
the functional unit of the mammalian kidney.
 The mammalian excretory system centers on
paired kidneys which are also the principal site
of water balance and salt regulation.
Vertebrate Kidneys
 Each kidney is
supplied with
blood by a renal
artery and
drained by a
renal vein.
Vertebrate Kidneys
 Urine exits each kidney through a duct called
the ureter.
 Both ureters drain into a common urinary
bladder.
Structure and Function of the Nephron
and Associated Structures
 The mammalian kidney has two distinct
regions:
 An outer renal cortex
 An inner renal medulla
Renal
medulla
Renal
cortex
Renal
pelvis
Ureter
Section of kidney from a rat
(b) Kidney structure
Structure and Function of the Nephron
and Associated Structures
 The nephron, the
functional unit of
the vertebrate
kidney consists of a
single long tubule
and a ball of
capillaries called
the glomerulus.
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:
 Proximal tubule
 Loop of Henle
 Distal tubule
 Fluid from several
nephrons flows into a
collecting duct.
From Blood Filtrate to Urine: A
Closer Look
 Filtrate becomes urine as it flows through the
mammalian nephron and collecting duct.
 The composition of the filtrate is modified through
tubular reabsorption and secretion.
 Changes in the total osmotic concentration of urine
through regulation of water excretion.
From Blood Filtrate to Urine: A
Closer Look
 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.
From Blood Filtrate to Urine: A
Closer Look
 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.
Conserving Water
 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.
Solute Gradients and Water
Conservation
 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.
Solute Gradients and Water
Conservation
 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.
Regulation of Kidney Function
 The osmolarity of the urine is regulated by
nervous and hormonal control of water and salt
reabsorption in the kidneys.
Regulation of Kidney Function
 Antidiuretic hormone
(ADH) increases water
reabsorption in the distal
tubules and collecting
ducts of the kidney.
Temperature Regulation
 Animals must keep their bodies within a range of
temperatures that allows for normal cell function.
 Each enzyme has an optimum temperature.
 Too low and metabolism slows.
 Too high and metabolic reactions become unbalanced.
Enzymes may be destroyed.
Temperature Regulation
 Poikilothermic animals’ body temperatures
fluctuate with environmental temperatures.
 Homeothermic animals’ body temperatures
are constant.
Temperature Regulation
 All animals produce heat from cellular metabolism, but
in most this heat is lost quickly.
 Ectotherms – lose metabolic heat quickly, so body
temperature is determined by the environment.
 Body temp may be regulated environmentally.
 Endotherms – retain metabolic heat and can
maintain a constant internal body temperature.
Ectothermic Temperature Regulation
 Many ectotherms regulate body temperature
behaviorally.
 Basking to increase temperature.
 Shelter in shade or coolness of a burrow to
decrease temperature.
Ectothermic Temperature Regulation
 Most ectotherms can also adjust their metabolic rates
to the environmental temperature.
 Activity levels can remain unchanged over a wider
range of temperatures.
Endothermic Temperature
Regulation
 Constant temperature in endotherms is maintained by
a delicate balance between heat production and heat
loss.
 Heat is produced by the animal’s metabolism.
 Producing heat requires energy – supplied by food.
 Endotherms must eat more in cold weather.
Endothermic Temperature
Regulation
 If an animal is too
cool, it can generate
heat by increasing
muscular activity
(exercise or
shivering). Heat is
retained through
insulation.
 If an animal is too
warm it decreases
heat production and
increases heat loss.
Adaptations for Hot Environments
 Small desert mammals are mostly fossorial (living
underground) or nocturnal.
 Burrows are cool and moist.
 Adaptations to derive water from metabolism and
produce concentrated urine & dry feces.
Adaptations for Hot Environments
 Larger desert mammals
(camels, desert
antelopes) have
different adaptations.
 Glossy, pallid color
reflects sunlight.
 Fat tissue is
concentrated in a
hump, rather than being
evenly distributed in an
insulating layer.
 Sweating and panting
are ways of dumping
heat.
Adaptations for Cold Environments
 In cold environments,
mammals reduce heat
loss by having a thick
insulating layer of fat,
fur, or both.
 Heat production is
increased.
 Extremities are allowed
to cool.
 Heat loss is
prevented through
countercurrent
heat exchange.
Adaptations for Cold Environments
 Small mammals are not as well insulated.
 Many avoid direct exposure to the cold by living in
tunnels under the snow.
 Subnivean environment.
 This is where food is located.
Adaptive Hypothermia
 Endothermy is energetically expensive.
 Ectotherms can survive weeks without eating.
 Endotherms must always have energy supplies.
Adaptive Hypothermia
 Some very small
mammals & birds (bats
or hummingbirds)
maintain high body
temperatures when
active, but allow
temperatures to drop
when sleeping.
 Daily torpor
Adaptive Hypothermia
 Hibernation is a way to solve
the problem of low temperatures
and the scarcity of food.
 True hibernators store fat, then
enter hibernation gradually.
 Metabolism & body slows to a
fraction of normal.
 Body temperature decreases.
 Shivering helps increase
temperatures when they are
waking up.
Adaptive Hypothermia
 Other mammals, such as bears, badgers, raccoons and
opossums enter a state of prolonged sleep, but body
temperature does not decrease.
Adaptive Hypothermia
 Adverse conditions can also occur during the summer.
 Drought, high temperatures.
 Some animals enter a state of dormancy called
estivation.
 Breathing rates and metabolism decrease.
 African lungfish, desert tortoise, pigmy mouse, ground
squirrels.