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

      Water gain Drinking Eating Metabolic water (a product of oxidation of organic molecules) C6H12O6 + 6O2--> 6CO2 + 6H2O

      Water loss Respiration Urination Defecation Sweating Lactation

2. 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

   2. 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)
      2. 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
      3. 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
         2. 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

      4. Reduce water in feces
      5. Reingestion of infant wastes by lactating mothers (at about 2:00)
   3. 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

3. 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)
   3. 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

4. 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)
   2. 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

      2. 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

      3. 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