Freshwater and Terrestrial Vertebrates: Osmoregulation and Excretion

Freshwater Vertebrates

• Freshwater vertebrates are hyperosmotic, meaning they have higher solute concentrations in their blood compared to their environment.
• Consequently, they constantly gain water and lose solutes.
• To counteract this, they have evolved several osmoregulatory solutions:
  • Produce dilute urine to excrete excess water.
  • Kidneys actively reabsorb ions to retain essential solutes.
  • Active transport mechanisms to uptake ions from the environment.

Freshwater Teleosts (Bony Fish)

• Teleosts in freshwater environments maintain higher solute levels in their blood, making them hyperosmotic.
• They constantly face water gain and solute loss, specifically sodium ($Na^+$) and chloride ($Cl^-$) ions.
• Unlike marine teleosts, freshwater teleosts do NOT drink water.
• Their primary osmoregulatory organs include:
  • Gills: Actively uptake sodium ($Na^+$) and chloride ($Cl^-$) ions from the surrounding water. This uptake is linked to carbon dioxide ($CO_2$) elimination.
  • Kidneys: Produce dilute urine to excrete excess water.

Osmoregulation in Freshwater Teleosts: A Detailed Look

• Water Intake: No drinking occurs; water enters osmotically through the gills.
• Salt Loss: Diffusional loss of salts occurs across the gills.
• Salt Uptake:
  • Gills: Active uptake of NaCl from the environment compensates for diffusional losses.
  • Gut: Isosmotic salt influx through the gut.
• Urine Production: Kidneys produce copious amounts of hyposmotic (dilute) urine.
• Feces: Salts and water are lost in feces.
• Osmolarity: The fish's internal osmolarity (250-400 mOsm) is hyperosmotic relative to the surrounding freshwater (<10 mOsm).

Freshwater Amphibians

• Similar to freshwater teleosts, amphibians in freshwater are hyperosmotic.
• They also constantly gain water and lose solutes (sodium ($Na^+$) and chloride ($Cl^-$) ions).
• They do NOT drink water.
• Key osmoregulatory organs:
  • Integument (skin): Actively uptakes sodium ($Na^+$) and chloride ($Cl^-$) ions from the environment, linked to $CO_2$ elimination.
  • Kidneys: Produce dilute urine to excrete excess water.

Terrestrial Vertebrates

• The main challenge for terrestrial vertebrates is water loss/conservation.

Challenges Leading to Water Loss:

• Thermoregulation: Evaporative coolingmechanisms (sweating, panting) lead to water loss.
• Eliminating nitrogenous waste: Production and excretion of urea requires water.
• Ventilation: Exhalation during breathing results in water loss.

Terrestrial Amphibians

• To conserve water, terrestrial amphibians employ several strategies:
  • Behavioral adaptations: Reducing evaporative water loss by seeking humid environments or being nocturnal.
  • Excreting nitrogenous waste as uric acid: Uric acid is less toxic than ammonia and requires less water for excretion.
  • Relatively Impermeable integument: Reducing water loss through the skin.
  • Reducing metabolic rate: Lower metabolic rate reduces water loss.
  • Water store: Some species, like Ranoidea sp., can store water.
  • Examples: The Sonoran desert toad (Incilius alvarius) is adapted to arid environments.

Terrestrial Reptiles

• Reptiles are well-adapted to terrestrial life due to their conservation of water through:
  • Dry, scaly skin: Reduces evaporative water loss.
  • Excreting nitrogenous waste as uric acid: Conserves water compared to urea or ammonia excretion.

Terrestrial Mammals

• Mammals employ diverse mechanisms to minimize water loss:
  • Lack of sweating or panting (in some species): Reduces evaporative cooling.
  • Production of very dry feces: Reabsorbs water in the large intestine.
  • Production of very concentrated urine: Kidneys efficiently reabsorb water.
  • Nocturnal behavior and burrowing: Avoids daytime heat and reduces water loss.
  • Nasal countercurrent exchange: Cools exhaled air, condensing water vapor for reabsorption.
  • Examples: The kangaroo rat (Dipodomys merriami) is highly adapted to dry environments.
• Water is obtained through metabolism.

Vertebrate Kidney

• The kidney is the main osmoregulatory organ of terrestrial mammals.
• The ability of the kidney to concentrate urine is linked to the animal's environment.
• The concentrating ability of the kidney is related to the number and type of nephrons, specifically:
  • Juxtamedullary nephrons: Long-looped nephrons that extend deep into the medulla, associated with desert rodents and efficient water reabsorption.
  • Cortical nephrons: Short-looped nephrons located primarily in the cortex, found in animals like beavers and pigs that have less need for water conservation because rodents,beavers and pigs live in opposite environment where rodents lives in desert while the others live in watery areas.
• Vasopressin (ADH):
  • Released by the posterior pituitary gland.
  • Controls the permeability of the collecting duct to water.
  • In the presence of ADH, more water is reabsorbed, producing concentrated urine.
  • If ADH is absent, dilute urine is formed.

Kidney Structure and Function Recap

• Glomeruli: Site of ultrafiltration, removing water and most small solutes from the blood (except for large proteins and blood cells).
• Proximal Convoluted Tubule: Selective reabsorption of water and essential solutes (glucose, amino acids, ions), along with some active secretion of waste products.
• Loop of Henle: Reabsorbs water and salts, crucial for water conservation by creating a concentration gradient in the kidney medulla.
• Distal Convoluted Tubule: Reabsorption of salts, calcium ($Ca^{2+}$), and plays a role in regulating blood pH.

Comparative Vertebrate Kidney Structure

Marine Mammals

Challenges

• Blood is hypotonic relative to seawater.
• Salt uptake occurs through their diet which is either fish (Odontoceti) or marine invertebrates (Mysticeti).
• They are air breathers thus always losing air through ventilation.

Solutions

• Produce hyperosmotic urine.

Marine Reptiles

Challenges

• Blood is hypotonic relative to seawater.
• Salt uptake occurs through their diet of marine invertebrates.
• They are air breathers.
• Lack the loop of Henle meaning they can't produce urine more concentrated than seawater.

Solutions

• Salt glands produce hyperosmotic fluid.