The final urine that leaves the nephrons to enter the bladder and be excreted is very different from the initial glomerular filtrate, because the composition of the filtrate is modified by selective re-absorption and secretion processes. Transport of solutes and water across the renal tubular epithelium can occur between the cells (paracellular movement) or through the cells (transcellular movement). Transcellular movement occurs across two cell membranes, the apical and basal membranes of the cell. Secretion is movement from the blood, through or between the tubular cells, into the tubular fluid. Re-absorption is movement of a substance from the tubular fluid, into or between the tubular cells, and then into the blood. Neither term conveys any information about the nature of the forces causing the movement (Figure RE.5).
Conventionally, the transport of solutes has been regarded as 'active' if metabolic energy is required, and 'passive' if metabolic energy is not required. This distinction is too simplistic, since many transport processes that do not directly require metabolic energy would nevertheless not occur if metabolic energy were not available.
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