Final Exam - Part 1 of Ion and Water Balance

• Control of ion and water balance

  • Animals use different combinations of tissues to control ion and water balance in the various internal and external environments
  • These tissues regulate three homeostatic processes: osmotic regulation, ionic regulation, and nitrogen excretion
• Osmotic regulation


  Animals cannot actively pump water, control of osmotic pressure requires movement of solutes
• Ionic regulation


  Control of ionic composition: blood - interstitial - cellular
• Nitrogen excretion


  Protein catabolism creates ammonia, which is excreted as ammonia, urea, or uric acid
• Types of fluid


  • Interstitial fluid
  • Blood
  • Lymph
  • Hemolymph
• Ionoconformer


  • Exert little control over solute concentration within the extracellular space, exclusively marine animals
• Ionoregulator


  • Control the ion profile of the extracellular space
• Osmoconformer

  • Internal and external osmolarity are similar, marine invertebrates
• Osmoregulator


  • Osmolarity is constant regardless of the external environment
• Stenohaline


  Can tolerate a narrow range of salinity
• Euryhaline


  Can tolerate a wide range of salinity
• The crab eating frog is the exception among amphibians that can live in saline environments and maintain hypoosmotic internal fluids
• Sources of water


  • Dietary water
  • Metabolic water
  • Drinking (freshwater vs seawater)
  • Dietary solutes
• Classification of solutes


  • Perturbing (disrupt function, e.g. Na+, K+, Cl-, SO4+, charged amino acids)
  • Compatible (little effect, e.g. polyols, uncharged amino acids)
  • Counteracting (disruptive on their own but offset each other in combination)
• Animals use multiple tissues to control water and ion balance
• Fish gills


  • Contain mucus secreting cells, chloride cells (mitochondrial rich), and pavement cells (mitochondrial rich and poor), direction of ion transport depends on water salinity
• Fish gills in freshwater

  • Take up ions against gradient, mitochondrial rich pavement cells have apical H+ ATPase, Na+ channel, Na+/H+ exchanger, and basolateral Na+/K+ ATPase, chloride cells have apical Cl-/HCO3- exchange and basolateral Cl- channel
• Diadromous fish migration


  • Catadromous (live in freshwater, migrate to saltwater to spawn), anadromous (live in saltwater, migrate to freshwater to spawn), ion transport functions of epithelia must change during migration, controlled by hormones
• Smoltification


  The process of changing from saltwater to freshwater or vice versa, involves physiological, morphological, biochemical, and behavioural changes in preparation for migration and life in the new environment
• Salt glands


  • Reptiles and birds possess salt glands near the eye that excrete hyperosmotic solutions of Na+ and Cl-, use ion pumps and countercurrent system
• Salt gland - rectal gland

  • Accessory excretory organ in elasmobranchs that actively transports Na+ and Cl- from the blood, uses basolateral Na/K/2Cl, basolateral Na/K ATPase, basolateral K channel, and apical Cl channel for salt excretion, similar process to marine teleost gill, salt gland of birds/reptiles