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Homeostasis
Chapter 2
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Definitions
• Ionoconformer: Animals with the same ion concentration of extracellular fluid as external environment
• Ionoregulators: Animals that to some extent regulate the ion composition of extracellular fluid
• Osmoconformers: Animals with the same osmolarity of extracellular fluid as external environment
• Osmoregulators: Animals that to some extent regulate the osmotic concentration of their body fluids
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Homeostasis & Regulation
• Regulation: The active maintenance of a physiological variable at a specific value, the setpoint.
• Homeostasis: The occurrence of a constant interior environment with respect to physiological variables (e.g., osmo- and ionoregulation, blood gasses, blood pressure, temperature)
Note: regulation is not a precondition for homeostasis and regulation does not imply perfect homeostasis
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Tolerance & Resistance
ToleranceQuantified by measuring
survival at different levels of a chemical or other stressor
LD50 (mol/kg): Lethal dose for 50% of the population
LC50 (mol/L): Lethal concentration for 50% of the population (applicable to aquatic organisms)
ResistanceQuantified by measuring
the time to death after administration of a chemical or other stressor at a level that is beyond the tolerance range.
LT50 (h): Lethal time (or tolerance) for 50% of the population
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Individuals and Populations may Alter their Ranges of Tolerance
and Resistance
• Acclimatization: Physiological readjustment to a change in the natural environment
• Acclimation: Physiological readjustment to a change in laboratory conditions (or by anthropogenically induced changes in the natural environment)
• Adaptation: Adjustment at the genetic level to specific environmental conditions. Often brought about by natural selection