Gibbs-Donnan Effects & Traffic between Fluid Compartments: A Digest for Students

Abstract

This article is an attempt to devise a short text aimed at improving students' understanding of Gibbs-Donnan effects on fluid traffic between cellular and interstitial fluid and between interstitial fluid and plasma. The text is given to students as reading material for a discussion during seminars on microcirculation and lymphatics, usually scheduled for the following week. The students can use their textbooks (Guyton & Hall, 2000; Ganong, 2005) or other references (Schultz, 2003). Students are expected to understand the mechanisms behind the Donnan effect and, in case of any doubt, are encouraged to ask questions during seminars. They are also encouraged to try to calculate missing values from the Table (shaded fields in Table 1). Seminar Tips * Interstitial fluid is taken across the capillary wall by the Donnan effect of plasma proteins. At the same time, it is in balance with the cellular fluid due to opposed Donnan effects (of cytoplasmic proteins and of ECF sodium) (Leaf, 1959; Nguyen & Kurtz, 2006; Kurbel, 2008). Passive Gibbs-Donnan balance on the capillary wall would result in slightly higher electrolyte plasma values than reported. The lower actual data are probably results of water movement from ECF to compliant capillaries (circulatory volume is not restricted since blood vessels are not rigid). * The only way for the cell to reach the osmotic equilibrium is to alter cell volume, until concentration of nondifusible intracellular ions (mainly charges on intracellular proteins) is equal to the ECF restricted ions (mainly [Na.sup.+] ions, restricted by pumping out of cells) (Baumgarten & Feher, 2001). * The achievement of electroneutrality requires that the sum of all anions equals the concentration of positive ions in the cell (mainly [K.sup.+]). Negative charges on cytoplasmic proteins are the most stable component among ionized particles and other ions have to adapt to their concentration. Positive and negative soluble intracellular ions are all osmotically active and to achieve balance of osmotic forces on the cell membrane, the sum of their intracellular concentrations must equal the concentration of osmotically active extracellular particles. * Described forces (osmosis and electric charges) force mobile chloride ions to move to ECF, leaving only a small quantity in the cell (Table 1). Higher intracellular chloride levels are found in some marine animals with high ECF sodium concentration due to immersion in sea water. For instance, almost four times, higher chloride concentration is reported in squid axons' cellular fluid ([Na.sup.+] - 50, [K.sup.+] - 400, Cl - - 40)(Hodgkin, 1958). In cases of reduced ECF [Na.sup.+] concentration, even lower cytoplasmic chloride concentration can be expected, further reducing the ability to compensate osmotic or electrolyte changes and thus allowing the Donnan effect of charges on cytoplasmic proteins to prevail and lead to cell swelling. * Erythrocytes are a probable exemption from the described situation. Since they float in protein rich plasma, the Donnan effect of their cytoplasmic proteins is reduced by the same effect of plasma proteins, so the role of ECF [Na.sup.+] is probably less important for maintenance of their cell volume (Kurbel, 2008). This seems to be in concordance with the fact that erythrocytes from several carnivores do not have a functional [Na.sup.+][K.sup.+]-ATPase and probably use other ions to manage their volume (Sarkadi & Parker, 1991). References Baumgarten, C.M. & Feher, J.I. (2001). Osmosis and regulation of cell volume. In N. Sperelakis (Ed.), Cell Physiology Sourcebook: A Molecular Approach (p. 339). San Diego, CA: Academic. Boron, W.F. & Boulpaep, E.L. (2004). Medical Physiology. Philadelphia, PA: Saunders. Ganong, W.F. (2005). Review of Medical Physiology. Stamford, CT: Appleton & Lange. Guyton, A. …