1. An obstacle or impediment. 2. In psychiatry, a conflictual agent that blocks behavior that could help resolve a personal struggle. [M.E., fr. O.Fr. barriere, fr. L.L. barraria]
- blood-air b. the material intervening between alveolar air and the blood; it consists of a nonstructural film or surfactant, alveolar epithelium, basement lamina, and endothelium.
- blood-aqueous b. a selectively permeable b. between the capillary bed in the processes of the ciliary body and the aqueous humor in the anterior chamber of the eye; consists of two layers of simple cuboidal epithelium joined at their apical surfaces with junctional complexes.
- blood-brain b. (BBB) a selective mechanism opposing the passage of most ions and large–molecular weight compounds from the blood to brain tissue located in a continuous layer of endothelial cells connected by tight junctions; similar capillaries are found in the retina, iris, inner ear, and within the endoneurium of peripheral nerves.
- blood-cerebrospinal fluid b., blood-CSF b. a b. located at the tight junctions which surround and connect the cuboidal epithelial cells on the surface of the choroid plexus; capillaries and connective tissue stroma of the choroid do not represent a b. to protein tracers or dyes.
- blood-testis b. an occluding b. formed by Sertoli cells in the seminiferous tubules of the testis, which separates the more mature cells of spermatogenesis in the adlumenal compartment of the tubule from blood-derived products in the basal compartment.
- blood-thymus b. a sheath of pericytes and epithelial reticular cells around thymic capillaries that prevents the developing T lymphocytes of the thymus from being exposed to circulating antigens.
- incest b. in psychoanalysis, the learning or internalization of parental and social prohibitions against incest.
- placental b. SYN: placental membrane.

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bar·ri·er 'bar-ē-ər n
1) a material object or set of objects that separates, demarcates, or serves as a barricade see BLOOD-BRAIN BARRIER, PLACENTAL BARRIER
2) a factor that tends to restrict the free movement, mingling, or interbreeding of individuals or populations <behavioral and geographic \barriers to hybridization>

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bar·ri·er (barґe-ər) an obstruction.

Medical dictionary. 2011.