Final Exam - Part 2 of Ion and Water Balance

• Phototaxtic Responses

  Visual pigment changes from porphyrospin to mostly rhodopsin
• Parr-smolt Transformation

  • Photoperiod is the primary environment cue
  • Complex endocrine control mediated by many hormones
• All tetrapods depend on keratinized stratum corneum to prevent water loss
• Nitrogen Excretion

  • Ammonia
  • Urea
  • Uric acid
• Ammonia Production

  • Advantage: requires little energy to produce
  • Disadvantage: highly toxic and requires large volumes of water to store and excrete
• Nitrogen Waste

  • Ammonia
  • Urea
  • Uric Acid
• Ammonia

  • Produced from breakdown of proteins
  • Low concentrations are toxic
  • Exists as either NH3 or NH4+ (depending on pH)
  • Excretion of ammonia requires water – only fish (but not elasmobranchs)
• Urea

  • 2 nitrogen groups
  • Soluble and much less toxic than NH3
  • Made via the ornithine urea cycle
  • Requires significant energy input to make urea
  • Mammals excrete urea, elasmobranch retain it
• Uric Acid

  • 4 nitrogen groups, also non-toxic
  • Insoluble, excreted as a solid
  • Energetically expensive to make
  • Birds, reptiles and some insects
• Urea Production

  Made in the liver via ornithine urea cycle
• Energetic costs of N excretion

  • Ammonia (fish) - no cost, no conversion, requires lots of water
  • Uric acid (birds and reptiles) - energetically expensive, less expensive then urea
  • Urea (mammals) - most expensive
• Ammonia Excretion in Fish

  • Mostly at the gill
  • As NH3 via diffusion down concentration gradient
  • Also as NH4+ exchange with Na+ (apical Na uptake)
• Urea Excretion in Non-Elasmobranch Fish

  • Most fish are ammoniotelic
  • Non-elasmobranch exceptions: some early development stages of teleost fish, lungfish when out of water, fish living at high pH
• Dormancy

  • Hypometabolism = reduction in metabolic rate
  • Allows the animal to survive adverse environmental conditions
  • Types: torpor, hibernation, estivation
  • Most dormant mammals accumulate urea and urine because of protein breakdown
• Micturition


  Voiding the urine from the bladder, under conscious control
• Urine Production Involves

  • Filtration
  • Reabsorption
  • Secretion
  • Excretion
• Transport in Proximal Tubule

  • Glucose reabsorption
  • Na and Cl reabsorption
  • Reabsorbed molecules are taken up by the blood
• Transport in Loop of Henle

  • Descending limb - water resorption, primary urine becomes more concentrated
  • Ascending limb - recovery of ions, primary urine becomes less concentrated
  • Vasa recta - maintains the gradient in the medulla, counter current to tubular flow
• Ion and Water Transport in the Loop of Henle

  • Descending limb is permeable to water, water is reabsorbed, volume of primary urine decreases, primary urine becomes more concentrated
  • Ascending limb is impermeable to water, ions are reabsorbed, primary urine becomes dilute
  • Reabsorption ions accumulate in interstitial fluid, creating an osmotic gradient in the medulla