Homeostasis

Chapter 2

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

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

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

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