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Transport Pathways

figureThis chapter discusses the basic mechanisms of tubular transport including pathways of reabsorption and secretion. The volume and solute composition of both the urine excreted from the body and the ECF retained in the body represents the qualitative and quantitative difference between the reabsorptive and secretory processes which occur along the length of the renal tubule. A more detailed and quantitative discussion of transport mechanisms, particularly those relating to the establishment and consequences of electrochemical gradients is provided in Section I, Chapter 2. In the discussion which follows, these mechanisms are reviewed with regard to their application to renal processes and examples of selected mechanisms are given.

  1. Transcellular transport involves movement of the transported substance across both the apical and basolateral membranes and through the cytoplasm of the cell.
  2. Paracellular transport involves movement of the transported material through the tight junctions between the cells. Transport is by simple diffusion and/or solvent drag. The figure illustrates the pathways for transcellular and paracellular reabsorption of substance X.
  3. Reabsorption is the movement material, of solute and water, from the renal tubular lumen to the peritubular capillaries.
  4. Secretion is the movement of material from the peritubular capillaries to the renal tubular lumen.