Internal Fluid Environment
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| Figure 33-2 French physiologist Claude Bernard (1813 to 1878), one of the most influential of nineteenthcentury physiologists. Bernard believed in the constancy of the milieu intérieur (“internal environment”), which is the extracellular fluid bathing the cells. He pointed out that it is through the milieu intérieur that foods and wastes and gases are exchanged and through which chemical messengers are distributed. He wrote, “The living organism does not really exist in the external environment (the outside air or water) but in the liquid milieu intérieur . . . that bathes the tissue elements.” |
Internal Fluid Environment
The body fluid of a single-celled organism is cellular cytoplasm, a liquid-gel substance in which the various membrane systems and organelles are suspended. In multicellular animals body fluids are divided into two main phases, intracellular and extracellular. The intracellular phase (also called intracellular fluid) is the collective fluid inside all the body’s cells. The extracellular phase (or fluid) is the fluid outside and surrounding the cells (Figure 33-1A). Thus the cells, sites of the body’s crucial metabolic activities, are bathed by their own aqueous environment, the extracellular fluid that buffers them from the often harsh physical and chemical changes occurring outside the body. The importance of extracellular fluid was first emphasized by the great French physiologist Claude Bernard (Figure 33-2). In animals having closed circulatory systems (vertebrates, annelids, and a few other invertebrate groups;) extracellular fluid is further subdivided into blood plasma and interstitial (intercellular) fluid (Figure 33-1A). Blood vessels contain plasma, whereas interstitial fluid, or tissue fluid as it is sometimes called, occupies spaces surrounding the cells in the body. Nutrients and gases passing between vascular plasma and cells must traverse this narrow fluid separation. Interstitial fluid is constantly formed from plasma by filtration through capillary walls.
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| Figure 33-1 Fluid compartments of the body. A, All body cells can be represented as belonging to a single large fluid compartment that is completely surrounded and protected by extracellular fluid (milieu intérieur). This fluid is further subdivided into plasma and interstitial fluid. All exchanges with the environment occur across the plasma compartment. B, Electrolyte composition of extracellular and intracellular fluids. Total equivalent concentration of each major constituent is shown. Equal amounts of anions (negatively charged ions) and cations (positively charged ions) are in each fluid compartment. Note that sodium and chloride, major plasma electrolytes, are virtually absent from intracellular fluid (actually they are present in low concentration). Note the much higher concentration of protein inside cells. |
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