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| indicating a standard low level of activity of an organism, as during total rest |
| an innate periodicity in an organism's physiological processes like sleep and wake cycles |
blood-brain barrier n.
Abbr. BBB
A physiological mechanism that alters the permeability of brain capillaries so that some substances, such as certain drugs, are prevented from entering brain tissue, while other substances are allowed to enter freely.
| blood-brain barrier
A physiological mechanism that alters the permeability of capillaries in the brain, so that some substances, such as certain drugs, are prevented from entering brain tissue, while other substances are allowed to enter freely. |
The separation of the brain, which is bathed in a clear cerebrospinal fluid, from the bloodstream. The cells near the capillary beds external to the brain selectively filter the molecules that are allowed to enter the brain, creating a more stable, nearly pathogen-free environment.
Note: Oxygen, glucose, and white blood cells are molecules that are able to pass through this barrier. Red blood cells cannot.