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Ionic and Osmotic Regulation and Excretion
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Marine/Terrestrial Environments, Excretion, Excretory System Structure and Function,
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Invertebrate ZoologyChris Foster
Week 4
Ionic and Osmotic Regulation and Excretion
- Body fluids = dilute saline solutions [NaCl is the predominant electrolyte]
- 1920 Macallum suggested evidence for a gradual change in the composition of the
Ocean. Recent palaeochemical studies show it hasn’t changed much.
- Living in sea water, have to regulate the composition of bodily fluids, intensifies in
the estuaries/freshwater more dilute aqueous environments. Terrestrial concern is
water retention.
- Solute ions pass from more concentrates to less via diffusion
- Solvent [liquid] passes from less to more concentrated solution by osmosis [under
osmotic [pressure]
- Pressure dependent on number of solute particles, membrane is semi-permeable to
water and solutes.
Marine Invertebrates;
- Isoosmatic body fluids, ionic composition of fluids differ from seawater [ionic
regulation]
- Jellyfish regulate sulfate ions [low concentration than sea water] Sulphate ions are
heavy and plays a part in buoyancy
- Echinoderms do not regulate appreciably [stay lives at sea]
- Arthropod; lowers Magnesium as very active creatures and magnesium is a anesthetic.
- Advantageous to maintain density; achieved via ionic regulation [Heliocranchia] large
fluid filled pericardial cavity with dense fluid, lowers sodium but increases
ammonium.
- Methods to increase buoyancy;
o Removal of ions without a replacement [osmotic costs = hypoosmotic]
o Reduction of heavy substances [no shells]
o More substances fat/oil; less dense than water
o Use of gas floats [soft walled/ hard walled/ cuttle bone]
- Osmoconformer;
o Follow osmotic condition of the environment [Stenohaline; limited tolerance
to salinity]
o Estuarine and littoral animals subjected to periodic dilution [tides//heavy rain]
crustacean exoskeleton is less permeable
- Osmoregulator;
o Resist dilution of body fluid
o Regulate throughout the life; epidermis has a low permeability and a large
amount of urine.
- Freshwater urine is hypoosmotic relative to body fluids after useful ions selectively
removed [some ions lost and must be replaced]
o Some replaced from food or direct active uptake of ions]
o Evidence for this; soak in distilled water, lower conc of ions in the body fluid.
Place organisms back in freshwater and body fluid osmolality returns to
normal [freshwater more dilute than body fluid = uptake via active transport]
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