Water Balance Strategies in Animals

Module BL1004: Animal Physiology

• Module is about animal physiology
• Professor is Rob McAllen
• Email: [email protected]
• Office hours are by appointment

Osmoregulation and Excretion

• Animal excretory systems are discussed
• Chapter 44, page 1029 of Campbell is cited

Overview: A Balancing Act

• Animal physiological systems operate in a fluid environment
• Water and solute concentrations must be maintained within narrow limits
• Osmoregulation maintains solute concentrations and balances water gain and loss

Overview: A Balancing Act (cont.)

• Freshwater animals reduce water intake and conserve solutes due to adaptations
• Desert and marine animals face desiccating environments that can deplete body water quickly.
• Excretion gets rid of nitrogenous metabolites and other waste products

Osmoregulation Balances the Uptake and Loss of Water and Solutes

• Osmoregulation is based on controlling the movement of solutes between internal fluids and the external environment
• Cells require a balance between osmotic gain and loss of water
• Osmosis is the movement of water across a selective permeable membrane
• Osmolarity is the solute concentration of a solution, which determines water movement across a selective permeable membrane
• Isoosmotic solutions have equal water movement in both directions
• A net flow of water moves from the hypoosmotic solution to the hyperosmotic solution

Osmotic Challenges

• Osmoconformers, some marine animals, are isoosmotic with their surroundings and do not regulate osmolarity
• Most marine invertebrates are osmoconformers
• Stenohaline versus Euryhaline tolerances in Osmoconformers & regulators
• Relationship between body fluid osmolality and medium concentration in Tigriopus brevicornis and Artemia nauplii

Osmotic Challenges (cont.)

• Osmoregulators expend energy to control water uptake in a hypoosmotic environment and loss in a hyperosmotic environment
• Marine bony fish are hypoosmotic to seawater
• They lose water via osmosis and gain salts by diffusion and from food
• They balance water loss by drinking seawater and excreting salts

Osmotic Challenges (cont.)

• Freshwater animals constantly take in water by osmosis from their hypoosmotic environment
• They lose salts by diffusion, maintaining water balance by excreting large amounts of dilute urine
• Salts lost by diffusion are replaced in food and by uptake across the gills

Animals That Live in Temporary Waters

• Some aquatic invertebrates in temporary ponds lose almost all their body water and survive in a dormant state
• This adaptation is called anhydrobiosis

Osmotic Challenges (Land Animals)

• Land animals manage water budgets by drinking and eating moist foods, using metabolic water
• Desert animals conserve water via nocturnal lifestyles, underground existence, etc.

Water balance in two terrestrial mammals

• Water balance in a kangaroo rat versus in a human
• Ingested in food and liquid, derived from metabolism, water loss in feces and urine, balance of evaporation
• Data is provided in the form of diagrams

Conservation of water

• Kidneys in animals such as the kangaroo rat are adapted to minimize water loss by concentrating urine
• Dry feces from the kangaroo rats are compared to lab rats' feces
• Uric acid requires less water than urea to rid the same amount of waste

Storage of Water

• Animals store water in fatty deposits in their tails and other tissues.
• Most water is obtained from food

Energetics of Osmoregulation

• Osmoregulators expend energy to maintain osmotic gradients
• Animals regulate the composition of body fluid that bathes their cells
• Transport epithelia regulate solute movement
• These are vital to osmotic regulation and metabolic waste disposal
• They are arranged in complex tubular networks

How do seabirds eliminate excess salt from their bodies?

• Seabirds use salt glands to remove excess sodium chloride from their blood
• Nasal salt glands in seabirds are illustrated

An animal's nitrogenous wastes reflect its phylogeny and habitat

• The type and quantity of an animal's waste products greatly affect its water balance
• Nitrogenous breakdown products of proteins and nucleic acids are important waste products

Nitrogenous wastes

• Ammonia is toxic and requires large amounts of water. Common in aquatic species
• Urea is less toxic and requires less water for excretion. It is produced in the liver of mammals and amphibians
• Uric acid is insoluble in water and can be secreted as a paste with little water loss. This is found in insects, land snails and reptiles (including birds). It is more energetically expensive than urea.

Diverse excretory systems are variations on a tubular theme

• Excretory systems regulate solute movement between internal and external environments
• Most excretory systems refine a filtrate derived from body fluids to produce urine
• Key functions include filtration, reabsorption, secretion, and excretion

Excretory Systems

• Systems that perform basic excretory functions vary among animal groups. Typically involve complex networks of tubules
• Includes Protonephridia (flame cells/planaria), Metanephridia (earthworm/similar to nephrons), Malpighian Tubules (insects), and Nephrons (function unit of the human kidneys)

Protonephridia

• A network of dead-end tubules connected to external openings
• Smallest branches are capped by flame bulbs
• These tubules excrete dilute fluid and function in osmoregulation

Metanephridia

• Each segment of an earthworm has a pair of open-ended metanephridia
• Both excretory and osmoregulatory functions
• Metanephridia consist of tubules that collect coelomic fluid, produce dilute urine

Malpighian Tubules

• In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation
• Open into the digestive tract, producing relatively dry waste matter for terrestrial life
• Highly efficient in water conservation

Kidneys

• Kidneys, excretory organs of vertebrates, function in both excretion and osmoregulation
• Mammalian excretory systems center on paired kidneys which are primary site for water and salt regulation
• Each kidney is supplied by renal artery and drained by renal vein
• Urine leaves through ureters
• Ureters drain into urinary bladder, and urine is expelled via the urethra

Kidneys: Nephrons

• The nephron is the functional unit of the vertebrate kidney
• Consists of a single long tubule and a ball of capillaries (glomerulus)
• Bowman’s capsule surrounds the glomerulus, receiving filtrate from the glomerulus capillaries

Filtration : Glomerulus --> Bowman's Capsule

• Filtration occurs as blood pressure forces fluid from the blood in the glomerulus to the Bowman’s capsule
• Filtration of small molecules is non-selective
• The filtrate contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules

Pathway of the Filtrate

• From Bowman's capsule, the filtrate passes through three regions of the nephron: proximal tubule --> loop of Henle --> distal tubule
• Fluid from several nephrons flows into a collecting duct, then the renal pelvis, and finally the ureter

Pathway of the Filtrate (cont.)

• Vasa Recta are capillaries that serve the loop of Henle.
• The vasa recta and loop of Henle function as a countercurrent system.
• The mammalian kidney conserves water by producing urine more concentrated than body fluids

The nephron is organized...

• Reabsorption of ions, water, and nutrients takes place in the proximal tubule
• Molecules are actively or passively transported from the filtrate into interstitial fluid, then capillaries
• Toxic materials are secreted into the filtrate
• Filtrate volume decreases

The nephron is organized... (cont.)

• Reabsorption of water continues through channels formed by aquaporin proteins driven by high interstitial osmolarity compared to filtrate
• Filtrate becomes increasingly concentrated in the descending limb of the loop of Henle
• In the ascending limb, salt, but not water, diffuses from the tubule into the interstitial fluid
• Filtrate becomes increasingly dilute in the ascending limb

The nephron is organized... (cont.)

• The distal tubule regulates K+ and NaCl concentrations of body fluids
• Collecting duct carries filtrate through the medulla to the renal pelvis
• Water, salt, urea are lost
• Urine is hyperosmotic to body fluids

The nephron is organized... (cont.)

• Human kidneys process approximately 180 liters of filtrate per day
• 99% of water and nearly all sugars, amino acids, vitamins are reabsorbed

Adaptations of the Vertebrate Kidney to Diverse Environments

• The form and function of nephrons vary based on requirements for osmoregulation in an animal's habitat
• The juxtamedullary nephron contributes to water conservation in terrestrial animals
• Mammals in dry environments have long loops of Henle; freshwater mammals have shorter loops

Adaptations of the Vertebrate Kidney to Diverse Environments (cont.)

• Birds and other reptiles have shorter loops of Henle but conserve water by excreting uric acid instead of urea
• Other reptiles have only cortical nephrons but still excrete nitrogenous waste as uric acid

Acknowledgements

• Majority of text and PowerPoint slides from Campbell's Biology
• Dr Ramiro Crego, School of BEES