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Homeostasis of the extracellular fluid (the blood plasma and interstitial fluid; see Key Concept 39.2) is critical for several reasons:
The solute concentration of the extracellular fluid determines the water balance of the cells of the body.
The specific ionic composition of the extracellular fluid influences many cell functions.
The health of cells requires the elimination of nitrogenous wastes.
The problems that have to be solved to maintain homeostasis of the extracellular fluid depend on the environment in which a species lives (salt water, fresh water, or terrestrial) and its lifestyle, as you saw in the case of the vampire bat in the opening story. Animals depend on excretory systems to maintain the volume, concentration, and composition of their extracellular fluids, and to excrete wastes.
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Animal excretory systems use filtration, secretion, and reabsorption to regulate osmolarity and ionic composition of the extracellular fluid.
Aquatic animals either conform to the osmolarity of the environment or expend energy to maintain a constant internal osmolarity.