Aquatic Physiology and Osmoregulation

  • Course Expectations

  • Emphasis on understanding over memorization.

  • Knowledge application and ability to convey concepts is crucial.

  • Freshwater vs. Saltwater Animals

  • Discussion on animals in different aquatic environments and their cellular adaptations.

  • First life believed to originate from deep sea vents with sulfur-rich hydrogen sulfide gas.

  • Most life forms on Earth rely on oxygen for metabolism, unlike those in extreme environments that utilize sulfur.

  • Osmolarity and Adaptations

  • Saltwater osmolarity: approximately 1,000 milliosmol

  • Animals in saltwater are often osmoconformers, matching their body fluid osmolarity to their environment.

  • Transitioning to freshwater lowers osmolarity, presenting osmotic challenges for the organisms.

  • Additionally, challenges with internal salt levels arise for animals adapting from saltwater to freshwater.

  • Osmotic Considerations

  • Movement of water molecules from low to high osmolarity can cause cells to swell and burst.

  • Evolution: Organisms that can adapt to osmotic changes pass on advantageous genetic traits to offspring.

  • Example: Marine vertebrates evolved from osmoconforming ancestors to osmotic regulators as they adapted to varying osmolarity environments (including moving back into saltwater).

  • Key Osmoregulation Terms

  • Hyperosmotic regulator: Body fluid osmolarity higher than environment (e.g., freshwater fish).

  • Hypoosmotic regulator: Body fluid osmolarity lower than environment (e.g., saltwater fish).

  • Osmotic Challenges

  • Freshwater vertebrates face an ionic challenge: Losing salts to the environment.

  • They become hyperosmotic, constantly gaining water and losing salts.

  • Solutions include:

    • Peeing large amounts of dilute urine (low osmolarity, <300).
    • Active transport mechanisms using ATP to reclaim lost ions.

  • Carbon Dioxide and pH Regulation

  • CO2 produced from aerobic metabolism must be meticulously managed to avoid acidosis.

  • CO2, when dissolved in water, forms carbonic acid and can lower pH through proton release.

  • Need for carbonic anhydrase in cells to facilitate the conversion of CO2 to bicarbonate and protons for effective waste removal.

  • Transport Mechanisms in Gills

  • Salt reclamation through countertransport and active transport in gills allows fish to regain lost Na+ and Cl- ions.

  • Primary transporters: Sodium-potassium ATPase pump, proton pump, and distinct channels for Na+ and Cl-.

  • Active transport is essential for maintaining osmotic balance despite the ionic challenges faced in varying water chemical environments.

  • Energy and Metabolism

  • The significant energy usage for osmoregulation in fish (7% of total metabolism).

  • Freshwater fishes' gill epithelial cells, rich in mitochondria, reflect the need for high energy output to manage osmoregulatory functions.

  • Comparison of Urination Rates

  • Example: Goldfish generates substantial urine volume (approximately 33 mL per 100 grams of body weight per day).

  • The concept of osmoregulatory UP ratios: Osmolarity of urine to plasma indicating how dilute urine is.

  • Marine Vertebrates

  • Opposite osmotic challenges compared to freshwater fish (face a risk of dehydration due to higher salt intake).

  • Basic mechanisms of osmoregulation understand that their blood plasma is hypoosmotic relative to the seawater, creating ongoing challenges to maintain fluid balance.

  • Consume saltwater to retain water, necessitating special adaptations in gill cells to excrete excess salt efficiently.