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Chapter 12. MATERIALS TRANSFER ACROSS MEMBRANES There is much traffic of materials (water, gases, solutes) across membranes. Because of the wide array of physicochemical properties of these substances, and the need to regulate what can and cannot traverse the membrane, several mechanisms exist. Membranes are best thought of as being selectively permeable to molecules: they allow certain substances to pass with ease and exclude others. The lipoprotein nature of the membrane, and its orderly arrangement into hydrophilic and hydrophobic domains, determines most of these permeability characteristics. For example, lipid-soluble molecules such as carotene, the orange pigment of many plants, pass from the interstitial fluid through the lipid domains in the cell membrane by simply dissolving in the membrane, establishing a concentration gradient from outside to inside, then flowing from regions of high concentration to regions of low concentration. A. Basic physical mechanisms The simplest form of bulk flow is filtration , a term usually applied to the movement of solutions across barriers when hydrostatic pressure is the driving force. Chapter 34 describes the role of filtration from the pressurized cardiovascular system into the extravascular tissue spaces. In the kidney, similar filtration causes enormous bulk flow of a plasma filtrate into the most proximal portion of the nephron , or renal tubule. Filtration direction is purely the result of a pressure gradient : fluid flows Figure 12-1. Filtration. This simple diagram shows a solution of macromolecules in a reservoir on a filter. The solvent passes through, but the large molecules are retained and therefore concentrate in the vicinity of the filter. The “column” of solution above the filter provides a hydrostatic driving force for the passage of solvent. from regions of high pressure to those of lower pressure in an attempt to equalize pressure on either side of the barrier. The fluid consists of solvent, usually water, and dissolved solute. The barrier can be thought of as a rather coarse filter (Figure 12-1). There is only limited opportunity to retard the passage of solute, although very large molecules (e.g., proteins and lipoproteins) may be held back; and in most cases free cells such as blood cells are unable to traverse filtration barriers. In the case of the circulatory system, the "barrier" is made up of the inner lining of endothelial cells with structures between the cells (leaky junctions) dictating the extent of filtration. 2007 version – page 89
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There is an enormous ongoing transfer of fluid and smaller polar solutes from the capillary lumen to the interstitium in most vascular beds; the capillaries in the brain are notable exceptions where the endothelial cells are connected with tight junctions creating the so-called Blood-Brain-Barrier ( BBB ). Another major mixing process that
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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