Paracellular transport refers to the transfer of substances between cells of an epithelium

Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

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It is in contrast to "transcellular transport", where the substances travel through the cell, passing through both the apical membrane and basolateral membrane.

The distinction is in particular significance in renal physiology

Renal physiology

Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including reabsorption of glucose, amino acids, and other small molecules; regulation of sodium, potassium, and other electrolytes; regulation of fluid balance and blood pressure;...

. Transcellular transport is more likely to involve energy expenditure than paracellular transport.

Capillaries of Blood Brain Barrier have only transcellular transport, in contrast with normal capillaries which have both transcellular and paracellular transport. This is due to the presence of tight junctions in Blood Brain Barrier.

Structure of paracellular channels

Some claudin

Claudin

Claudins are a family of proteins that are the most important components of the tight junctions, where they establish the paracellular barrier that controls the flow of molecules in the intercellular space between the cells of an epithelium...

s form tight junction

Tight junction

Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. It is a type of junctional complex present only in vertebrates...

associated pores that allow paracellular ion transport.