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Water and Electrolyte Balance in Animals

  • Life-sustaining chemical reactions occur in aqueous solutions.
  • Disruption in water and solute balance can halt these reactions.
  • Water Balance: Achieved when water intake equals water loss, a key aspect of homeostasis.
  • Electrolytes: Dissociate into ions in water and are crucial for maintaining electrical and osmotic balance.
    • Examples: Na⁺, Cl⁻, K⁺, Ca²⁺.
    • Imbalances can cause muscle spasms, fatigue, irregular heart rhythms, or death.

Water Loss and Replacement in Terrestrial Animals

  • Terrestrial animals lose water via breathing, defecation, urination, and sometimes sweating.
  • Methods to replace water:
    • Drinking.
    • Absorbing water from food.
    • Producing metabolic water during cellular respiration.

Osmoregulation Across Habitats

  1. Marine Animals:
    • Marine bony fishes live in a hyperosmotic environment (seawater has higher solute concentration).
    • Challenges: Water loss via osmosis and salt gain via diffusion.
    • Adaptations:
      • Drink seawater.
      • Excrete excess salts via gills and small quantities of concentrated urine.
  2. Freshwater Animals:
    • Freshwater is hyposmotic to their tissues (water moves in, salts move out).
    • Challenges: Water gain and electrolyte loss.
    • Adaptations:
      • Excrete large amounts of dilute urine.
      • Actively absorb salts through gills or from food.
  3. Terrestrial Animals:
    • Constant water loss through evaporation, urine, and feces.
    • Adaptations:
      • Minimize water loss using specialized structures like kidneys.
      • Replace lost water through drinking, food, and metabolic processes.

Key Mechanisms for Osmoregulation

  • Diffusion: Passive movement of solutes along a gradient.
  • Osmosis: Water moves across semipermeable membranes to balance solute concentrations.
  • Active Transport:
    • Primary (e.g., Na⁺/K⁺ pump).
    • Secondary (uses established gradients, e.g., symporters and antiporters).
  • Water movement: No active transport; follows osmotic gradients via aquaporins.

Osmoconformers vs. Osmoregulators

  • Osmoconformers: Marine invertebrates (e.g., jellyfish) match their internal osmolarity to the environment.