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Blood-Brain Barrier

Definition – It is a selective barrier between the blood & the brain tissue, called the blood-brain barrier. It is an important conduit of nutrients and cells from the blood to the brain.

Structure: Formed by –

  1. The capillary endothelium.
  2. The basement membrane.
  3. The perivascular foot plates of astrocytes on the basement membrane.

Tight junctions are present between adjacent endothelial cells; these are the structures that prevent the passage of a substance across the endothelium. It is formed by tight junctions between brain endothelial cells, which selectively prevent the diffusion of hydrophilic molecules and pathogens such as bacteria from entering the brain parenchyma.

The blood-brain barrier is absent in several parts of the brain, notably in the neurohypophysis and pineal body. It is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain.

Blood-Brain Barrier 1

Fig: Schematic of the blood-brain barrier and associated components


  • The purpose is to protect against circulating toxins or pathogens that could cause brain infections, while at the same time allowing vital nutrients to reach the brain.
  • Its help to maintain relatively constant levels of hormones, nutrients, and water in the brain – fluctuations in which could disrupt the finely tuned environment.
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