Osmoregulation and excretion

 

  1. Osmoregulation
    1. Basic problem
      1. typically, fish have different ionic concentrations than their environment
      2. they also have highly permeable membranes
      3. osmotic movement of ions and water will occur
      4. physiological processes impaired when internal ionic concentrations change far from norm
    2. Osmoconformers vs. osmoregulators
      1. most fish are osmoregulators
        1. regulate internal ionic concentrations to fairly constant level
        2. example: California killifish, Fundulus californianus, can ismoregulate up to twice salinity of seawater, and can survive up to almost 4X seawater salinity (handout fig 6.2)
      2. Hagfish is an osmoconformer, internal ion concentration mimics the environment
        1. not surprisingly, this fish is restricted to marine habitats
  2. Organs involved
    1. The kidney
      1. Basic scheme shared by hagfish, lamprey and higher verts (drawing and handout fig. 6.10)
        1. Glomerulus
          1. Filtration of blood
        2. tubules
          1. Resorption of ions from filtrate
          2. often resorption of water from filtrate
          3. secretion of larger waste products into filtrate from blood
          4. transport both passive (downgradient) and active (against gradient, requiring energy)
      2. Kidney functions both in excretion and ion exchange
      3. Quite a bit of diversity in structure: glomerulus vs. none, different elaboration of tubules (handout fig. 6.12)
    2. The gills
      1. Large surface area
      2. Specialized cells: chloride cells (fig. 7.4, Helfman et al.)
        1. tubules maximize contact of cytoplasm with extracellular fluid
        2. apical membrane in contact with water, also has numerous pits
        3. cytoplasm has many mitochondria, so high metabolic rate required
        4. Ion pumps: ions are transported across membranes against a concentration gradient, expending energy
  3. Marine bony fish osmoregulation
    1. Hypoosmotic
      1. Total ion concentration <1/2 that of water (Table 7.1, Helfman et al.)
    2. Lose water to saltier medium at gills (fig. 7.3)
    3. Drink water (10% of body wt per day; 7 quarts of water a day if human size) to balance water losses
    4. But salts absorbed in esophagus and gut
    5. system couples water uptake to salt uptake
    6. At gills
      1. active secretion of monovalent ions (sodium, chloride)
    7. At kidneys
      1. secretion of bigger ions (divalents: calcium, magnesium)
      2. some marine fish have aglomerular kidney to prevent water loss.
      3. Urine: as little as 3 ml/kg/day. 150 ml of urine, or less than a cup, scaled to human.
  4. Freshwater teleosts
    1. Hyperosmotic;
      1. total ion concentration >100X that of ambient (Table 7.1)
    2. Water gain and ion loss at all permeable membranes (fig. 7.3)
    3. At kidneys
      1. Excrete dilute urine (ca. 5-12% of body weight)
      2. kidney has more glomeruli, and bigger organ than in sw fishes
      3. some ion uptake in tubules
    4. At gills
      1. Chloride cells function in uptake of sodium and chloride
  5. Marine cartilaginous fishes
    1. Isosmotic or a bit hyperosmotic
      1. Inorganic salt concentration lower than seawater, though higher than marine bony fish (Table 7.1)
      2. but high levels of organic salts: urea and TMAO (fishy smell)
    2. At gills:
      1. Inorganic salts come into body, downgradient
    3. At kidney
      1. urea filtered at glomeruli
      2. recovered from tubules, which are complex
      3. lots of filtration and urine production, because they are taking up water
    4. Renal gland
      1. an organ with choride cells, for excreting monovalent ions
    5. Much less costly system than marine teleost approach of salt excretion.
  6. Excretion
    1. Lipid and carbohydrates
      1. Turn into water and carbon dioxide
    2. Protein
      1. In addition to water and carbon dioxide, nitrogen as ammonia
      2. Although toxic, ammonia usually not a serious problem for fish
        1. excreted at gills
      3. Sharks and coelacanths make urea
  7. pH regulation
    1. What controls blood pH?
      1. In metabolizing tissues, CO2 produced
      2. enzymatic hydration
        1. conversion to carbonic acid
        2. dissociation to proton and bicarbonate ion
      3. this, and production of lactic acid, lowers pH
      4. one control of pH hydration of CO2, or dehydration of carbonic acid
      5. but environments vary in pH. What other controls?
    2. pH control and ion exchange are linked at gills (freshwater fish)
      1. protons (going out) exchanged for sodium (coming in)
      2. ammonium (also going out) is also exchanged for sodium
      3. bicarb (going out) exchanged for chloride (coming in)
    3. If pH of water increases
      1. harder to dump protons
      2. less ability to take up sodium
      3. lowered blood sodium chloride