• water enters body through mouth and gills
• must be balanced by renal excretion
• salts enter body
• kidneys, gills, and didgitiform gland helps regulate

• hyperosmotic
• tendencies and compensations:
• water moves into body, swelling, large amounts of dilute urine
• continuously lose salts:
• replaced by food, and uptake from gills by chloride cells
  

• hypoosmotic
• tendencies and compensations
• lose water to environment:
• continuously drinking water
• uptake large amounts of salt:
• concentrated urine

• slightly isotonic to hyperosmotic
• water:  mOsm
• teleost: 300 mOsm
• elasmobrnachs: 1100 mOsm

elasmobranchs

(cont)

• sharks contain 250mmol of NaCl; similar to marine teleosts
• kidney reabsorbs urea and trimethylamineoxide (TMAO); 500mOsm
• can result in a 5% increase of salts in sharks
• never drink water, passive water influx

• nitrogen waste from metabolism
• reabsorbed in kidneys by elasmobranchs
• urea is a highly toxic protein destabilizer
• TMAO can counter act the urae effects
• 2:1, TMAO:urea
• possess urea adapted proteins: hemoglobin

• major site of salt excretion
• kidneys remove the excess salts and pass them to the digitiform gland
• gland excretes a concentrated salt paste out of the cloaca
• sharks can live without it; other organs pick up excretion
• when ingected with NaCl the digitiform gland is stimulated

• bullshark
• potomatotrygonidae
• other stingrays

• urea, TMAO, NaCl internal concentrations are lower
• digitiform gland is reduced- fewer secretory tubules

• lost ability to retain urea
• 1 mM urea in freshwater
• 6 mM urea in 17ppt

• osmoregulate similarly to freshwater teleosts
• digitiform gland is nonfunctional