Water Balance & Osmoregulation

Life & Water

Water Balance & Osmoregulation

Useful Vocabulary to Remember

• Osmosis:

• Diffusion from an area of high water concentration to low.

• Movement of water from low osmotic pressure to high.

• Osmotic Pressure:

• Hydrostatic pressure that counterbalances the osmotic movement of water.

• Hyperosmotic:

• Low solute concentration.

• Hyposmotic:

• High solute concentration.

Importance of Water Balance

• Understanding water balance is crucial because:

• Concentration of free water outside cells can affect cell structure.

• Cells may swell if excess water flows in.

• Total Body Mass Distribution:

• Female: 45% solids, 55% fluids (2/3 intracellular fluid, 1/3 extracellular fluid).

• Male: 40% solids, 60% fluids.

• 20% of ECF is plasma.

Tolerance to Varying Habitats

• Euryhaline Animals:

• Tolerate a wide range of salt concentrations (e.g., migratory fish).

• Stenohaline Animals:

• Tolerate a narrow range of salt concentrations (e.g., amphibian larvae).

Osmoregulation Strategies

• Osmoregulators:

• Maintain internal fluid composition regardless of surroundings.

• Osmoconformers:

• Conform to osmotic pressure of surroundings.

• Ionoconformers:

• Concentration of all ions same inside and out.

• Ionoregulators:

• Concentration of some ions differs inside and out (common in vertebrate fish).

Osmoregulatory Organs

• Kidney:

• Primary organ for osmoregulation in vertebrates.

• Functions to remove excess water, salts, and solutes.

• Nephron is the functional unit for urine production.

Analyzing Urine: Osmotic U/P Ratios

• U/P Ratio: osmotic pressure of urine/osmotic pressure of plasma.

• Implications for Excretion:

  • U/P=1 (isosmotic urine):

  • Water & solutes are excreted in same relation as blood plasma.

  • U/P<1 (hyposmotic urine):

  • Water is preferentially excreted.

  • U/P>1 (hyperosmotic urine):

  • Water is held back; solutes are preferentially excreted.

Freshwater Osmoregulators

• Freshwater animals are typically hyperosmotic regulators:

• Problems:

  • Gain excess water from high internal solute concentration.

  • Lose ions/salts to lower concentration in environment.

• Solutions:

  • Produce dilute urine.

  • Ion uptake in gills (requires ATP).

Marine Osmoregulators

• Marine Teleost Fish:

• Hyposmotic regulators.

• Problems:

  • Water loss due to high solute concentration externally.

  • Gain salt due to sea water.

• Solutions:

  • Drink sea water.

  • Active transport via gills to excrete salt.

  • Produce concentrated salty urine.

• Euryhaline Fish:

• Can tolerate wide solute concentration ranges (e.g., migratory spawning).

Marine Reptiles & Birds

• Produce isosmotic or slightly hyperosmotic urine.

• Use salt glands for excess salt excretion (active transport of NaCl).

Marine Mammals

• Produce hyperosmotic urine.

• Obtain water primarily from food sources without salt glands.

Osmoconformers: Elasmobranchs (Sharks & Rays)

• Problem: Urea can denature proteins.

• Solution: Urea balances water influx, TMAO counters negative effects.

• Excess salts removed via rectal salt glands.

Water Balance on Land

• Behavior: First line of defense.

• Humidic Animals: Restricted to humid environments.

• Xeric Animals: Capable of living in dry areas.

Water Loss on Land

• Caused by: Evaporation, urine, feces, secretions.

Water Gain on Land

• Caused by: Drinking, consuming food, specialized surfaces, metabolic processes.

Evaporative Water Loss

• Cutaneous Water Loss: Through skin; permeability differs among species.

• Respiratory Water Loss: Minimized by internal structures (e.g., elaborate nasal passages).

Excretory Water Loss

• Problem: Water is essential for excreting many ions.

• Solutions:

1. Concentrate urine (most efficient in mammals).

2. Reduce nitrogenous waste in urine:

   • Ammonia (fish) - highly toxic.

   • Urea (amphibians & mammals) - medium toxicity.

   • Uric Acid (reptiles & birds) - low toxicity, insoluble.

Physiological Regulation for Amphibians

• Tied to water throughout their life cycle:

• Hyperosmotic when in water, combat water gain by diluting urine and active salt transport.

• Away from water, use a bladder for water retention.

Adaptations for Hot/Dry Habitats

• Behavioral adaptations (e.g., exploiting rain storms).

• Rapid development in temporary pools, waxy skin reduces water loss.

Water Management in Reptiles

• Manage intake through food, drinking, and metabolic water production.

• Avoid desiccation via behavior (e.g., uric acid production).

Water Management in Birds

• Increased mobility solves many water management issues.

• Produce uric acid waste, higher tolerance to water loss.

Water Management in Mammals

• Utilize various strategies:

• Concentrated urine production.

• Oily skin, high-water food consumption.

• Specialized respiratory structures, fur, and metabolic water production.