Temperature, Osmotic Regulation and the Urinary System

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Temperature, Osmotic Regulation and the Urinary
System

• Chapter 50

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Classification of Organisms

• For many years, animals were classified according
  to whether they maintained a constant body
  temperature
• -Homeotherms Regulate their body temperature
  about a set point
• -Also called warm-blooded
• -Poikilotherms Allow their body temperature to
  conform to the environment
• -Also called cold-blooded

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Classification of Organisms

• Limitations to this dichotomy led to another view
  based on how body heat is generated
• -Endotherms Use metabolism to generate body
  heat and maintain temperature above ambient
  temperature
• -Ectotherms Do not use metabolism to produce
  heat and have body temperature that conforms to
  ambient temperature

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Ectotherms

• Ectotherms regulate temperature using behavior

-Insects, such as moths, use a shivering reflex
to warm thoracic muscles for flight
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Ectotherms

• Many marine animals, such as killer whales, limit
  heat loss in cold water using countercurrent heat
  exchange
• -Warm blood pumped from within the body in
  arteries warms the cooler blood returning from
  the skin within veins

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Ectotherms
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Ectotherms

• Reptiles place themselves in varying locations of
  sunlight and shade
• -Some can maximize the effect of behavioral
  regulation by also controlling blood flow
• In general, ectotherms have low metabolic rates,
  which have the advantage of low energy intake
• -However, they are not capable of sustained
  high-energy activity

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Endotherms

• A high metabolic rate can be used to warm the
  endotherm if it is cold
• The simplest way to regulate body temperature is
  by the control of blood flow to the surface of
  the animal
• -Vasodilation increases blood flow, thereby
  increasing heat dissipation
• -Vasoconstriction decreases blood flow, thus
  limiting heat loss

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Endotherms

• When ambient temperatures rise, many endotherms
  take advantage of evaporative cooling in the form
  of sweating or panting
• The advantage of endothermy is that it allows
  sustained high-energy activity
• -The tradeoff is that the high metabolic rate
  requires constant and high energy intake (food)

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Endotherms

• In animal physiology, size does matter!
• -Smaller animals have much higher metabolic
  rates per unit body mass relative to larger
  animals
• -Small endotherms in cold environments require
  significant insulation to maintain their body
  temperature
• -Large endotherms in hot environments usually
  have little insulation

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Endotherms

• When temperatures fall below a threshold, animals
  resort to thermogenesis, or use of normal energy
  metabolism to produce heat
• -Shivering thermogenesis uses muscles to
  generate heat, without producing useful work
• -Nonshivering thermogenesis alters fat
  metabolism to produce heat instead of ATP
• -Brown fat is utilized

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Control of Body Temperature

• Mammalian thermoregulation is controlled by the
  hypothalamus
• -A rise in body temperature is detected by
  neurons, which stimulate the heat-losing center
  in the hypothalamus
• -Sympathetic nerves cause dilation of
  peripheral blood vessels, and production of
  sweat from sweat glands

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Control of Body Temperature

• -A drop in body temperature is detected by
  neurons, which stimulate the heat-promoting
  center in the hypothalamus
• -Sympathetic nerves cause constriction of
  peripheral blood vessels, and inhibit sweating
  to prevent evaporative cooling
• -Hypothalamus releases hormones that stimulate
  the thyroid to produce thyroxin, which
  stimulates metabolism

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Control of Body Temperature
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Control of Body Temperature

• Pyrogens are substances that cause a rise in
  temperature
• -Act on the hypothalamus to increase the normal
  set point to a higher temperature
• -Produce the state we call fever
• -A normal response to infection

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Control of Body Temperature

• Torpor is a state of dormancy produced by a
  reduction in both metabolic rate and body
  temperature
• -Allows an animal to reduce the need for food
  intake
• Hibernation is an extreme state in which torpor
  lasts for weeks or months
• -Practiced usually by mid-sized animals

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Osmolarity and Osmotic Balance

• To maintain osmotic balance, the extracellular
  compartment of an animals body must be able to
  take water from and excrete excess water into the
  environment
• -Inorganic ions must also be exchanged to
  maintain homeostasis
• -These exchanges occur across specialized
  epithelial cells, and, in most vertebrates,
  through the kidney

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Osmolarity and Osmotic Balance

• Osmotic pressure is the measure of a solutions
  tendency to take in water by osmosis
• Osmolarity is the number of osmotically active
  moles of solute per liter of solution
• Tonicity is the measure of a solutions ability
  to change the volume of a cell by osmosis
• -Solutions may be hypertonic, hypotonic, or
  isotonic

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Osmolarity and Osmotic Balance

• Osmoconformers are organisms that are in osmotic
  equilibrium with their environment
• -Include most marine invertebrates, and
  cartilaginous fish (sharks and relatives)
• All other vertebrates are osmoregulators
• -Maintain a relatively constant blood osmolarity
  despite different concentrations in their
  environment

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Osmolarity and Osmotic Balance

• Freshwater vertebrates are hypertonic to their
  environment
• -Have adapted to prevent water from entering
  their bodies, and to actively transport ions back
  into their bodies
• Marine vertebrates are hypotonic to their
  environment
• -Have adapted to retain water by drinking
  seawater and eliminating the excess ions through
  kidneys and gills

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Osmoregulatory Organs

• In many animals, removal of water or salts is
  coupled with removal of metabolic wastes through
  the excretory system
• A variety of mechanisms have evolved to
  accomplish this
• -Single-celled protists use contractile vacuoles

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Osmoregulatory Organs

• Invertebrates use specialized cells tubules
• -Flatworms use protonephridia which branch into
  bulblike flame cells
• -Open to the outside of the body, but not to
  the inside
• -Earthworms use nephridia
• -Open both to the inside and outside of the
  body

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Osmoregulatory Organs

• Insects use Malpighian tubules, which are
  extensions of the digestive tract
• -Waste molecules and K are secreted into
  tubules by active transport
• -Create an osmotic gradient that draws water
  into the tubules by osmosis
• -Most of the water and K is then
  reabsorbed into the open circulatory
  system through hindgut epithelium

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Osmoregulatory Organs

• The kidneys of vertebrates consist of thousands
  of repeating units, nephrons
• -Create a tubular fluid by filtering the blood
  under pressure through the glomerulus
• -Filtrate contains many small molecules, in
  addition to water and waste products
• -Most of these molecules and water are
  reabsorbed into the blood
• -Waste products are eliminated from
  the body in the form of urine

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Evolution of the Vertebrate Kidney

• Kidneys are thought to have evolved among the
  freshwater teleosts, or bony fishes
• -Body fluids are hypertonic with respect to
  surrounding water, causing two problems
• 1. Water enters body from environment
• -Fishes do not drink water and
  excrete large amounts of dilute urine
• 2. Solutes tend to leave the body
• -Reabsorb ions across nephrons

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Evolution of the Vertebrate Kidney

• In contrast, marine bony fishes have body fluids
  that are hypotonic to seawater
• -Water tends to leave their bodies by osmosis
  across their gills
• -Drink large amounts of seawater
• -Actively transport monovalent ions out of the
  blood across the gill surfaces
• -Excrete urine isotonic to body fluids
• -Contains divalent cations

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Evolution of the Vertebrate Kidney
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Evolution of the Vertebrate Kidney

• Cartilaginous fish, including sharks and rays,
  reabsorb urea from the nephron tubules
• -Maintain a blood urea concentration that is 100
  times higher than that of mammals
• -Blood is isotonic to surrounding sea
• -These fishes do not need to drink seawater or
  remove large amounts of ions from their bodies

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Evolution of the Vertebrate Kidney

• The amphibian kidney is identical to that of
  freshwater fish
• The kidneys of reptiles are very diverse
• -Marine reptiles drink seawater and excrete an
  isotonic urine
• -Eliminate excess salt via salt glands
• -Terrestrial reptiles reabsorb much of the salt
  and water in their nephron tubules
• -Dont excrete urine, but empty it into cloaca

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Evolution of the Vertebrate Kidney

• Mammals and birds are the only vertebrates that
  can produce urine that is hypertonic to body
  fluids
• -Accomplished by the loop of Henle
• Birds have relatively few or no nephrons with
  long loops, and so cannot produce urine as
  concentrated as that of mammals
• -Marine birds excrete excess salt from salt
  glands near the eyes

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Evolution of the Vertebrate Kidney
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Nitrogenous Wastes

• When amino acids and nucleic acids are
  catabolized, they produce nitrogenous wastes that
  must be eliminated from the body
• -First step is the removal of the amino
  (-NH2) group, and its combination with H to form
  ammonia (NH3) in the liver
• -Toxic to cells, and thus it is only safe in
  dilute concentrations

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Nitrogenous Wastes

• Bony fishes and amphibian tadpoles eliminate most
  of the ammonia by diffusion via gills
• Elasmobranchs, adult amphibians, and mammals
  convert ammonia into urea, which is soluble in
  water
• Birds, terrestrial reptiles, and insects convert
  ammonia into the water-insoluble uric acid
• -Costs most energy, but saves most water
• Mammals also produce uric acid, but from
  degradation of purines, not amino acids
• -Most have an enzyme called uricase, which
  convert uric acid into a more soluble derivative
  called allantoin

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The Mammalian Kidney

• Each kidney receives blood from a renal artery,
  and produces urine
• -Urine drains from each kidney through a ureter
  into a urinary bladder
• Within the kidney, the mouth of the ureter flares
  open to form the renal pelvis
• -Receives urine from the renal tissue
• -Divided into an outer renal cortex and inner
  renal medulla

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The Mammalian Kidney
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The Mammalian Kidney

• The kidney has three basic functions
• -Filtration Fluid in the blood is filtered out
  of the glomerulus into the tubule system
• -Reabsorption Selective movement of solutes
  out of the filtrate back into the blood via
  peritubular capillaries
• -Secretion Movement of substances from the
  blood into the extracellular fluid, then into the
  filtrate in the tubular system

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The Mammalian Kidney

• Each kidney is made up of about 1 million
  functioning nephrons
• Blood is carried by an afferent arteriole to a
  tuft of capillaries in cortex, the glomerulus
• -Blood is filtered as it is forced through
  porous capillary walls

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The Mammalian Kidney

• Blood components that are not filtered drain into
  an efferent arteriole, which empties into
  peritubular capillaries
• Glomerular filtrate enters the first region of
  the nephron tubules, Bowmans capsule
• -Goes into the proximal convoluted tubule
• -Then moves down the medulla and back up into
  cortex in the loop of Henle

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The Mammalian Kidney

• After leaving the loop, the fluid is delivered to
  a distal convoluted tubule in the cortex
• -Drains into a collecting duct
• -Merges with other collecting ducts to empty
  its contents, now called urine, into the renal
  pelvis

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Reabsorption and Secretion

• Most of the water and dissolved solutes that
  enter the glomerular filtrate must be returned to
  the blood by reabsorption
• -Water is reabsorbed by the proximal convoluted
  tubule
• -Reabsorption of glucose and amino acids is
  driven by active transport carriers
• Secretion of waste products involves transport
  across capillary membranes and kidney tubules
  into the filtrate

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Excretion

• A major function of the kidney is elimination of
  a variety of potentially harmful substances that
  animals eat and drink
• -In addition, urine contains nitrogenous wastes,
  and may contain excess K, H and other ions that
  are removed from blood
• Kidneys are critically involved in maintaining
  homeostasis

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Transport in the Nephron

• A mechanism is needed to create an osmotic
  gradient between the glomerular filtrate and the
  blood, to allow reabsorption of water
• -Virtually all nutrient molecules in the
  filtrate, and two-thirds of the NaCl and water,
  are reabsorbed by proximal convoluted tubule
• -Active transport of Na out of proximal
  tubule is followed by passive movement of K
  and water

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Transport in the Nephron

• The function of the loop of Henle is to create a
  gradient of increasing osmolarity from the cortex
  to the medulla
• -Active extrusion of NaCl from the ascending
  loop creates an osmotic gradient
• -Allows reabsorption of water from descending
  loop and collecting duct

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Transport in the Nephron

• Filtrate that reaches distal convoluted tubule
  and enters the collecting duct is hypotonic
• -The hypertonic interstitial fluid of the renal
  medulla pulls water out of the collecting duct
  and into the surrounding blood vessels
• Kidneys also regulate electrolyte balance in the
  blood by reabsorption and secretion
• -K, H, and HCO3

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Hormones Control Osmoregulation

• Kidneys maintain relatively constant levels of
  blood volume, pressure, and osmolarity
• -Also regulate the plasma K and Na
  concentrations and blood pH within narrow limits
• -These homeostatic functions of kidneys are
  coordinated primarily by hormones

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Hormones Control Osmoregulation

• Antidiuretic hormone (ADH) is produced by the
  hypothalamus and secreted by the posterior
  pituitary gland
• -Stimulated by an increase in the osmolarity of
  blood
• -Causes walls of distal tubule and collecting
  ducts to become more permeable to water
• -Increases reabsorption of water

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Hormones Control Osmoregulation

• Aldosterone is secreted by the adrenal cortex
• -Stimulated by low levels of Na the blood
• -Causes distal tubule and collecting ducts to
  reabsorb Na
• -Reabsorption of Cl and water follows
• Low levels of Na the blood are accompanied by a
  decrease in blood volume
• -Renin-angiotensin-aldosterone system is
  activated

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