Paracellular transport

During paracellular transport via an epithelium , substances are transported through the spaces between the cells . In contrast, with transcellular transport ( transcytosis ) the substances are transported directly through the cell (e.g. by exocytosis and endocytosis or by receptor-mediated membrane transport ). The paracellular transport is passive and can be prevented or regulated by tight junctions (zonula occludentes) between the cells. Paracellular and transcellular transport are used to different degrees depending on the type of epithelium. In "permeable" epithelia such as the intestine, up to 90% of the substance transport takes place paracellularly, in dense epithelia such as the blood-brain barrier there is virtually no paracellular transport.

Solvent drag

Be the paracellular water transport solutes in the water flow "carried away," Speak of the solvent drag ( English solvents traction ). The direction of transport depends on the electrochemical gradient across the epithelium. Uncharged substances move towards the lower concentration. The direction of charged molecules (e.g. ions) also depends on the transepithelial potential difference , which is influenced by the charge distribution.

Solvent drag plays an important role in the reabsorption of dissolved substances (especially sodium , chloride and urea ) in the proximal tubular cells of the kidney and in the intestine, where larger amounts of water are absorbed.

Inflammation

If histamines are released as part of inflammatory processes , the barriers between the wall cells of the lymph and blood vessels ( vascular endothelial cells ) are opened. This process is called inflammatory increase in permeability . Histamine causes an intracellular increase in calcium levels , which changes the cytoskeleton of the cells in such a way that the tight junctions no longer adhere to one another. The endothelial barrier is thus permeable and paracellular transport can take place. The opening of the endothelial barrier can lead to dangerous edema (pulmonary or brain edema) in the event of excessive allergy reactions or poisoning .

Individual evidence

↑ Michael Gekle: Transport Processes and Signal Transduction . In: Michael Gekle et al. (Ed.): Pocket textbook Physiology . Thieme Verlag, Stuttgart 2010, ISBN 978-3-13-144981-8 , p. 31 .

literature

Bruce Alberts among others: Molecular Biology of the Cell . 4th edition. Wiley-VCH, Weinheim 2003, ISBN 3-527-30492-4 , pp. 1240 .
Jan Behrends: Dual Series: Physiology . 1st edition. Thieme, Stuttgart 2009, ISBN 978-3-13-138411-9 .

[1] Michael Gekle: Transport Processes and Signal Transduction . In: Michael Gekle et al. (Ed.): Pocket textbook Physiology . Thieme Verlag, Stuttgart 2010, ISBN 978-3-13-144981-8 , p. 31 .