Water Regulation (=Osmoregulation)

  1. Introduction

    1. Life processes are carried out in an aqueous medium

      1. blood serum
      2. cytoplasm of cells
      3. digestive reactions and other enzymatic processes
      4. interstitial water

    1. Water balance (water taken in : water lost) must be regulated fairly precisely

      Water gain
      1. Drinking
      2. Eating
      3. Metabolic water (a product of oxidation of organic molecules)
        C6H12O6 + 6O2--> 6CO2 + 6H2O
      Water loss
      1. Respiration
      2. Urination
      3. Defecation
      4. Sweating
      5. Lactation

  1. Dealing with, dry hot environments

    1. Desert conditions are stressful to the mammalian body

      Water loss (sweating/respiration) =1-3 % of body weight/hr
      Death occurs when water loss=15-20% of body weight

    1. Dealing with very dry environments

      1. Take in more water

        1. Drink more
        2. Eat moist food

          Moisture content of food depends upon

          • type of food (eg, animal, plant, seeds)
          • humidity
          • time of day (hygroscopic plants)

      1. Reduce evaporative water loss

        1. Heat loading
        2. Lower metabolic rate when hot
        3. Cooler nasal mucosa
        4. Be nocturnal
        5. Be fossorial
        6. Summer estivation

      1. Reduce water excreted in urine

        1. Urine

          • Carries nitrogenous wastes from the body (urea, uric acid)
          • Carries other wastes (excess ions, vitamins, etc.)
          • Vertebrate urine is normally hypotonic to blood, but mammals (and birds) have the capability to make hypertonic urine

        1. Basic anatomy and physiology of the kidney

          • Filtration of blood plasma through glomeruli
          • Active secretion of waste molecules
          • Reabsorption of solutes (glucose, amino acids) and water
          • Final urine concentration depends on the lengths of the loops of Henle
          • Facultative adjustment of the urine concentration

        Mammalian blood plasma 300 mosmols
        Human urine 1200 mosmols
        Banner-tailed K rat urine 2700 mosmols
        Oryx urine 2900 mosmols
        Fennec fox urine 4000 mosmols
        Vampire bat urine (digesting) 6000 mosmols

      1. Reduce water in feces
      1. Reingestion of infant wastes by lactating mothers (at about 2:00)

    1. Water budget for a desert animal

      Kangaroo Rat
      25 degrees C, 20% humidity

    Water gains Water losses
    Oxidation water 54.0 ml Urine 13.5 ml
    Absorbed water 6.0 ml Feces 2.6 ml
    Evaporation 43.9 ml
    Total water gain 60.0 ml Total water loss 60.0 ml

    Comparison to humans

  1. Osmoregulation in bats

    1. The problem: Flying increases evaporative water loss
    2. The solution:

      1. Urine concentration increases during increased evaporative cooling
      2. Roosting in groups results in less evaporative water loss during the day (higher humidity in roost)

    1. Vampire bats: Blood meals are high in water and protein

      1. Produce dilute urine during feeding
      2. Produce small quantities of highly concentrated urine in the roost

  1. Marine Mammals: The physiological desert

    1. The problem: Both drinking and eating result in an intake of salts that must be flushed out with water

      1. Sea water is hypertonic
      2. Most foods are hypertonic (except teleost fish)

    1. Solutions

      1. Eat teleost fish

        Seals/sea lions mostly teleost fish, some eat inverts
        Sirenia aquatic plants
        Cetaceans teleost fish or marine inverts and plankton

      1. Kidneys produce urine hypertonic to sea water

        Sea water consumed Urine produced Water balance
        Volume Cl- conc. Volume Cl- conc Change
        (ml) (mmol/l) (ml) (mmol/l) (ml)
        Human 1000 535 1350 400 -350
        Whale 1000 535 650 820 +350

      1. Produce very concentrated milk (eg. seals)

        Animal H2O (g/100g milk) Fat (g/100g milk)
        Human 87.6 3.8
        Cow 87.3 3.7
        Harp seal 45.3 42.7
        Weddell seal 43.6 42.2