Animal Physiology: Osmoregulation and Excretory Systems

Introduction to Osmoregulation and Excretory Systems

• These notes cover the excretory system and osmoregulation, focusing on aquatic environments.
• The main learning objectives include understanding the evolutionary story of the excretory system and its influence on osmoregulation in animals.
• Emphasis will be placed on properties, principles, and evolutionary adaptations related to osmoregulation in various animals.

Aquatic Environments and Osmoregulation

• The excretory system is closely linked to osmoregulation, which is the regulation of body fluids, water content, and electrolyte balance.
• Aquatic environments include:
  • External Environment: Water and dissolved salts (e.g., lakes, shallow marine areas, large rivers).
  • Water contains dissolved and non-dissolved particulates and solutes, primarily sodium and chloride ions.
  • Other electrolytes like magnesium and calcium are also present but less emphasized.
  • Tissue Fluid: Internal fluid within organisms that bathes cells and maintains homeostasis of electrolytes and fluids.

River Confluence Example: Amazon and Rio Negro

• The confluence of the Amazon River and the Rio Negro in South America illustrates differences in water composition.
• The Rio Negro is a black water river due to decaying plant material leaching dark staining tannins into the water.
• The Amazon appears cloudy due to sediments picked up from the Andes Mountains through erosion.
• These rivers remain separated for a distance due to differences in composition, dissolved particulates, and densities.

Basic Principles: Diffusion and Osmosis

• Diffusion: Movement of solutes from an area of high concentration to an area of low concentration.
• Osmosis: Movement of water from an area of low solute concentration to an area of high solute concentration.
• Rule: Water moves towards areas with more solutes.
• Example: Semipermeable membrane separating water with different solute concentrations.
  • Water moves through the membrane towards the higher solute concentration side, increasing volume on that side.
  • The generated pressure is called osmotic pressure.

Osmotic Terms

• Isoosmotic:
  • The osmolarity of the organism's internal fluid matches the external environment.
  • There is no major difference in concentration gradients.
• Hyperosmotic:
  • The organism has a higher solute concentration internally than its external environment.
• Hypoosmotic:
  • The organism has a lower solute concentration than its external environment.

Osmoregulation in Different Environments

• Different organisms in various environments face unique challenges in maintaining fluid and electrolyte balance.
• Organisms must actively regulate water and solute movement to prevent dehydration or excessive solute uptake.
• The plan is to discuss osmoregulation in freshwater, saltwater, and terrestrial organisms, focusing on:
  • Differences between body fluid and environment.
  • Regulation mechanisms.
  • Ionic composition (e.g., Na^+, K^+, Cl^-, Ca^{2+}).
  • Volume of urine produced (dilute vs. concentrated).

Freshwater Animals

• Freshwater fishes (e.g., goldfish) are used as examples.
• Internal fluid composition is hyperosmotic relative to the surrounding freshwater.
• Hyperosmotic regulators maintain a relatively constant internal condition despite external differences.
• Challenges:
  • Loss of solutes to the environment.
  • Constant influx of water into the bloodstream.
• Solutions:
  • Produce extremely dilute urine to remove excess water.
  • Active transport cells in gills actively bring solutes back into the bloodstream (requires energy).
• Even when a fish is not moving, these internal mechanisms are actively working to maintain balance.

Ocean Invertebrates

• Ocean invertebrates are thought to reflect ancestral conditions.
• Most ocean invertebrates are isosmotic with respect to marine water.
• This means their osmolarity is similar to the ocean, requiring little energy for regulation.
• There is relatively little water or solute movement in or out of the organism.
• Osmolarity is measured in osmoles (OSM), with seawater being about 1 osmol.
• The isoosmotic condition in marine invertebrates is considered an ancestral trait.
• Examples include echinoderms (sea stars), corals, and nudibranchs.

Ocean Bony Fishes

• Ocean bony fishes are hypo osmotic to seawater; their solute concentration is lower than the surrounding seawater.
• Seawater is about 1 OSM, while bony fish body fluids range from 0.3 to 0.5 OSM.
• Challenges:
  • Rapid gain of ions due to concentration gradients.
  • Tendency to lose water to the environment.
• Solutions:
  • Produce very concentrated urine to conserve water.
  • Actively drink seawater to compensate for water loss.
  • Specialized pumps in the digestive tract remove solutes from the ingested seawater.
• These mechanisms require energy; pumping out ions continuously requires 8-17% of daily metabolic energy.
• Chloride cells and Mitochondria-rich cells are specialized gill membrane cells that excrete ions.

Evolutionary Hypothesis

• Ocean invertebrates are isosmotic with marine conditions, while freshwater and ocean bony fishes are not.
• This is explained through an evolutionary hypothesis:
  • Invertebrates have body fluid around 1 osmol, similar to the marine environment, requiring little energy.
  • Ancestral condition: Ocean invertebrates colonized freshwater.
  • Freshwater osmolarity is close to zero. Colonizing freshwater created a significant osmolarity disparity.
  • Adjustments to freshwater resulted in lower body fluid osmolarity through natural selection, reducing energy expenditure.
  • Fishes like the arapaima and darters exemplify adaptations in freshwater environments.
• Land vertebrates have osmolarity levels similar to freshwater organisms.
• Human blood osmolarity is around 0.3 osmoles.
• Modern ocean bony fishes originated from freshwater fishes that recolonized the marine environment.
  • These fishes retained the 0.3-0.5 osmolarity, explaining why ocean bony fishes are hypo osmotic.
• Tiktaalik is a key evolutionary intermediate between sarcopterygian fishes and tetrapods.

Terrestrial Animals and Marine Vertebrates

• Terrestrial animals' environment is similar to freshwater, so the same osmoregulation properties apply.
• Marine environment exceptions:
  • Air-breathing vertebrates that live in a marine environment (marine iguanas, sea turtles, seagulls).
  • These animals consume marine organisms high in solutes (algae, cnidarians, echinoderms).
  • Excess salt is removed through salt glands.
• Salt glands are ATP-dependent and located around the head region.
  • Seabirds have glands above their eyes that empty salt outwards towards the nostrils.
  • Ocean lizards (marine iguanas) expel salt through their nostrils.
  • Sea turtles secrete salt in the form of tears.

Animals Moving Between Environments

• Animals that move between saltwater and freshwater behave as freshwater regulators in freshwater and saltwater regulators in saltwater.
• These animals can shift physiological mechanisms based on the environment, including ion pump direction.
• Regulation is largely controlled by hormones, coinciding with reproductive cycles.
• Examples include American eels and sturgeons. Brackish water areas are interfaces between marine and freshwater environments with varying salinity.

Brackish Water and Osmoregulation

• Brackish water environments, like river mouths, have varying salinity due to the mixing of fresh and marine water.
• Salinity fluctuations depend on seasonality and climate.
  • Rainy seasons result in lower osmolarity due to increased freshwater input.
  • Dry seasons increase salinity as marine water creeps upstream.

Osmoregulation Strategies in Ocean Invertebrates

• Ocean invertebrates employ different strategies:
  • Osmoconformers: Internal osmolarity changes with the environment; tissues may face stress due to changing conditions.
    • Example: Marine mussel, where internal salinity mirrors environmental salinity.
  • Osmoregulators: Maintain a stable internal osmolarity over a range of environmental conditions.
    • Example: Blue crab, which maintains a relatively flat internal osmolarity until extreme conditions are reached.
• Many invertebrates are isotonic with their environment.

Mammalian Excretory System

• The mammalian excretory system is more complex.
• Kidneys are the primary organs for filtering blood.
• Blood is sent into the kidneys, filtered, and modified to retain necessary components and excrete waste through urine.
• Nephrons are the smallest functional units of the kidney that filter blood and modify filtrate.
• The kidney filters blood and modifies the filtrate, taking back what the body needs and depositing extra waste the body does not need.
• Key structures within the nephron include:
  • Glomerulus: Network of blood capillaries where blood filtration occurs under high pressure.
  • Bowman's Capsule: Collects filtrate pushed out of the capillaries.
  • Proximal Convoluted Tubule: The first region of the tubule where the filtrate is collected.