Homeostasis

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Homeostasis

• Chapter 30

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Homeostasis

• Homeostasis refers to maintaining internal
  stability within an organism and returning to a
  particular stable state after a fluctuation.

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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.

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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

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Osmoregulation Excretion

• Osmoregulation regulates solute concentrations
  and balances the gain and loss of water.
• Excretion gets rid of metabolic wastes.

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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.

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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).

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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.

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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

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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.

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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.

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Osmotic Regulation - Freshwater

• Freshwater animals face an even more extreme
  osmotic difference than those that inhabit
  estuaries.

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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.

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Osmotic Regulation - Freshwater

• Invertebrates and amphibians also solve these
  problems in a similar way.
• Amphibians actively absorb salt from the water
  through their skin.

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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.

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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 sharks blood equal
  to that of seawater, so water balance is not a
  problem.

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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.

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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.

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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.

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Osmotic Regulation Terrestrial

• Marine birds and turtles have a salt gland
  capable of excreting highly concentrated salt
  solution.

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Excretory Processes

• Most excretory systems produce urine by refining
  a filtrate derived from body fluids (blood,
  hemolymph, or coelomic fluid).

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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.

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Invertebrate Excretory Structures

• Contractile vacuoles are found in protozoans and
  freshwater sponges.
• An organ of water balance expels excess water
  gained by osmosis.

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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.

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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.

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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.

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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.

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Vertebrate Kidneys

• Kidneys, the excretory organs of vertebrates,
  function in both excretion and osmoregulation.

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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.

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Vertebrate Kidneys

• Each kidney is supplied with blood by a renal
  artery and drained by a renal vein.

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Vertebrate Kidneys

• Urine exits each kidney through a duct called the
  ureter.
• Both ureters drain into a common urinary bladder.

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Structure and Function of the Nephron and
Associated Structures

• The mammalian kidney has two distinct regions
• An outer renal cortex
• An inner renal medulla

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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.

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Filtration of the Blood

• Filtration occurs as blood pressure forces fluid
  from the blood in the glomerulus into the lumen
  of Bowmans capsule.

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Pathway of the Filtrate

• From Bowmans 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.

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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.

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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.

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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.

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Conserving Water

• The mammalian kidneys ability to conserve water
  is a key terrestrial adaptation.
• The mammalian kidney can produce urine much more
  concentrated than body fluids, thus conserving
  water.

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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.

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Solute Gradients and Water Conservation

• The collecting duct, permeable to water but not
  salt conducts the filtrate through the kidneys
  osmolarity gradient, and more water exits the
  filtrate by osmosis.

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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.

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Regulation of Kidney Function

• The osmolarity of the urine is regulated by
  nervous and hormonal control of water and salt
  reabsorption in the kidneys.

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Regulation of Kidney Function

• Antidiuretic hormone (ADH) increases water
  reabsorption in the distal tubules and collecting
  ducts of the kidney.

46
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.

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Temperature Regulation

• Poikilothermic animals body temperatures
  fluctuate with environmental temperatures.
• Homeothermic animals body temperatures are
  constant.

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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.

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Ectothermic Temperature Regulation

• Many ectotherms regulate body temperature
  behaviorally.
• Basking to increase temperature.
• Shelter in shade or coolness of a burrow to
  decrease temperature.

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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.

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Endothermic Temperature Regulation

• Constant temperature in endotherms is maintained
  by a delicate balance between heat production and
  heat loss.
• Heat is produced by the animals metabolism.
• Producing heat requires energy supplied by
  food.
• Endotherms must eat more in cold weather.

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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.

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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.

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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.

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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.

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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.

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Adaptive Hypothermia

• Endothermy is energetically expensive.
• Ectotherms can survive weeks without eating.
• Endotherms must always have energy supplies.

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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

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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.

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Adaptive Hypothermia

• Other mammals, such as bears, badgers, raccoons
  and opossums enter a state of prolonged sleep,
  but body temperature does not decrease.

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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.