Environmental Constraints and Adaptive Mechanisms

Abstract

In all higher animals extracellular fluid (plasma, interstitial, and transcellular fluids) provides an environment in which cells function separated from the external medium. This internal environment, as termed by the nineteenth-century French physiologist Claude Bernard, is the stable medium necessary for normal cell activity, possessing the correct levels of oxygen, carbon dioxide, and metabolic substrates. Maintenance of the optimum composition of this extracellular fluid compartment is ensured through the activity of several organ systems. Thus, the lungs, liver, and kidneys in mammals regulate levels of respiratory gases, metabolic substrates, and waste products, the ionic composition and volume of extracellular fluid, and, indirectly, the same variables within the cell. This part of the book examines current understanding of these latter aspects of homeostasis, principally the mechanisms controlling volume and specific ion and osmotic concentrations of body fluids— osmoregulation.