Osmoregulation & Excretion

Osmoregulation

  • Definition: Process of controlling solute concentrations and water balance in organisms.

  • Types of Osmoregulators:

    • Osmotic Regulators: Actively manage internal osmolarity.

    • Osmotic Conformers: Match their internal osmolarity to that of their environment; most marine animals fall into this category.

  • Energy Considerations: For marine regulators, osmoregulation is energetically favorable when water moves out of the organism.

Marine and Freshwater Osmoregulation

  • Marine Fish:

    • Drink large amounts of water.

    • Eliminate excess salt through gills and kidneys.

  • Freshwater Regulators:

    • Do not drink water; excrete dilute urine.

    • Replenish lost salts through gills.

  • Salmon Adaptation: Can switch between freshwater and seawater using specialized epithelial cells that actively transport salt out against the concentration gradient.

Transport Epithelia and Waste Disposal

  • Transport Epithelia: Specialized cell layers for moving solutes in specific directions; typically organized in networks of tubes with high surface area.

  • Salt Glands: Marine birds concentrate salt in super-saturated secretions.

  • Consequences of Drinking Saltwater: Leads to fatal dehydration as excess salt necessitates consuming more freshwater.

Costs of Osmoregulation

  • Energy Investment: Costs of osmoregulation increase with larger differences between external environment and internal osmolarity.

  • Conformers: Spend less energy; for example, brine shrimp use ~30% of BMR for osmoregulation in highly saline environments.

  • Terrestrial Animals:

    • Adaptations to prevent water loss include reducing urine output and minimizing evaporation through skin and gas exchange organs.

    • Desiccation can be fatal; losing just 12% of body water can lead to death.

Questions on Osmoregulation & Excretion

  • Differences between malnutrition and undernutrition.

  • Mechanisms for maintaining osmolarity in freshwater versus marine animals.

  • Classes of nitrogenous waste products and their differences.

  • Structures of the kidney and their functions in urine creation.

  • Differences between selective secretion and selective reabsorption.

Excretion and Homeostasis

  • Excretion: Removal of metabolic wastes and is central to homeostasis, aiming to maintain fluid and solute balance.

  • Nitrogenous Waste: Breakdown of proteins and nucleic acids results in ammonia, which is highly toxic and requires significant dilution for excretion.

Types of Nitrogenous Waste

  • Ammonia:

    • Requires large amounts of water; tolerable only in low concentrations.

    • Common in aquatic animals.

  • Urea:

    • Formed by combining ammonia and carbon dioxide in the liver.

    • Low toxicity but energy-intensive to produce.

  • Uric Acid:

    • Non-toxic and produced in a semi-solid paste form, requiring less water for excretion.

    • Can lead to gout due to inflammation from uric acid crystals in joints.

Urine Formation Process

  • Urine is typically a dilute solution of urea and other wastes.

  • Filtration Process: Body fluid, such as blood, comes into contact with transport epithelium to filter out waste products.

  • Nephrons: Functional units in the kidneys that consist of tubules responsible for reabsorption and waste transport.

  • Hormonal Regulation: Hormones regulate water reabsorption and control active transport of salt based on hydration levels.

Kidney Structure

  • Nephron Structure:

    • End of nephron allows passage of water and small solutes but blocks larger molecules like proteins and blood cells.

    • Proximal tubules reabsorb essential salts and molecules; can utilize both active and passive transport to return substances to the blood.

  • pH Regulation: Bicarbonate is reabsorbed to maintain filtrate pH; ammonia can be secreted to combine with H+ ions in filtrate.

  • Loop of Henle:

    • Descending Loop: More water reabsorption facilitated by aquaporins.

    • Ascending Loop: More salt reabsorption occurs without aquaporins, diluting the filtrate; an example ratio is 1600 L of blood producing 1.5 L of urine with 99% water retention.