![]() All excitable cells (nerve and muscle) are represented by one intracellular fluid compartment (ICF) enclosed by the cell membrane. The major fluid compartments of multicellular animals. In addition, the Na /K -ATPase opposes the Donnan equilibrium (see Glossary) of diffusible ions across cell membranes, such that intracellular and extracellular osmotic concentrations are the same (isosmotic) across cell membranes permeable to water. The concentration differences that drive the diffusion of K out of the cell and the diffusion of Na into the cell provide, respectively, the chemical basis for resting and action potentials in the excitable membranes of nerve and muscle. The Na /K -ATPase maintains high K concentrations and low Na concentrations in the ICF, and the excretory systems (Malpighian tubules, kidneys) maintain high Na concentrations and low K concentrations in the ECF. The unequal distribution of Na and K across cell membranes stems from the operation of (1) the Na /K -ATPase and (2) the excretory system ( Fig. 1A,B). Na and Cl − are the major electrolytes in the ECF, whereas K and organic anions are the major electrolytes in the ICF ( Fig. 1A,B). In the case of insects, tracheal tubes provide the ECF (hemolymph) with relatively constant concentrations of oxygen, carbon dioxide and hydrogen ions the gastrointestinal tract and accessory organs provide relatively constant concentrations of fuel and nutrients the Malpighian tubules and the gut regulate the volume and composition of the hemolymph and the circulatory system keeps the hemolymph continuously in motion, from metabolizing cells to the epithelial surfaces and back ( Fig. 1A). The constancy of the ECF is known as homeostasis. The ECF sustains the lives of cells because it buffers the ICF against unpredictable changes in the external environment, and epithelial cells at the surface of the organism adjust their activities to maintain a constant volume and composition of the ECF ( Fig. 1). the internal environment, and not in the external environment they inhabit ( Gross, 1998). More than 160 years ago, Claude Bernard recognized that multicellular animals really live in the life-sustaining compartment of the ECF, i.e. The water is distributed largely in two major fluid compartments, with 2/3 in the intracellular fluid (ICF) compartment and 1/3 in the extracellular fluid (ECF) compartment. In general, total body water in multicellular organisms makes up 70% of the body weight. Thus, ECF homeostasis in insects is highly dynamic and plastic, which may partly explain why insects remain the most successful class of animals in terms of both species number and biomass. Other insects can suspend ECF homeostasis altogether in order to survive extreme temperatures. This strategy may be related to the apparent absence of Malpighian tubules in aphids. Aphids are unique in handling solute and volume loads largely outside the ECF, in the lumen of the gut. Other insects osmoconform, allowing the ECF osmotic concentration to match the ambient osmotic concentration. Most insects osmoregulate by keeping ECF volume and osmotic concentration within a wide range of tolerance. While a correction simply reverses the challenge, compensation accommodates the challenge with changes in the other two variables. ![]() Challenges beyond those tolerances may be ‘corrected’ or ‘compensated’. ![]() ![]() Solute/volume plots of the ECF (hemolymph) reveal that insects tolerate large changes in all three of these ECF variables. The present Review uses this chemical perspective to examine how insects deal with challenges to extracellular fluid (ECF) volume, solute content and osmotic concentration (pressure). \): Cells maintain water volume by actively controlling electrolyte concentrations.In chemistry, the ratio of all dissolved solutes to the solution's volume yields the osmotic concentration. ![]()
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