Cause of this secret are tight junctions and mucous. As
Wikipedia explains: 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. The
corresponding junctions that occur in invertebrates are septate
junctions. They perform vital functions:They hold cells
together.Barrier function, which can be further subdivided into
protective barriers and functional barriers serving purposes
such as material transport and maintenance of osmotic
balance:They help to maintain the polarity of cells by
preventing the lateral diffusion of integral membrane proteins
between the apical and lateral/basal surfaces, allowing the
specialized functions of each surface (for example receptor-
mediated endocytosis at the apical surface and exocytosis at the
basolateral surface) to be preserved. This aims to preserve the
transcellular transport.They prevent the passage o