The blood brain barrier is a structural and functional barrier which impedes and regulates the influx of most compounds from blood to brain. It is essential for normal functioning of the CNS.

The BBB:

• Regulates passage of molecules in and out of the brain to maintain environment of neurons
• Is responsible for metabolic activities such as the metabolism of L-dopa to regulate its concentration in the brain
• Is involved in certain pathological conditions like meningitis

It is made up of the cerebral capillaries, meninges and the choroid plexus (secretes CSF).

Fluid compartments and sites of barriers:

There are different fluid compartments in and around the brain as well as specific barriers which separate these compartments. The below image depicts this. The solid red line is the BBB and surrounds almost the entire nervous system (separates blood from the brain tissue).

The cerebrospinal fluid brain barrier is not a true barrier as it lacks tight junctions and allows some degree of free diffusion of substances. It separates the CSF from the brain tissue via the ependymal cells. Within the brain itself there are two fluid compartments, ICF within cells and ECF outside of the cells.

Walls of blood capillaries, meninges and the choroid plexus make up BBB

Brain capillaries versus peripheral capillaries:

• Structural differences in brain capillaries:
  • Absence of fenestrations
  • More extensive tight junctions
  • Numerous mitochondria (indicating that they require a lot of energy for things like selective transport across the endothelial cells.)
  • Prominent basal lamina and astrocyte foot processes
• Functional differences:

  • Impermeable to most substances

  • Sparse pinocytotic vesicular transport

  • Increased expression of transport and carrier proteins: receptor mediated endocytosis

  • No gap junctions, only tight junctions

  • Limited paracellular and transcellular transport

    

Pericytes:

• Cells of microvessels including capillaries, venules, and arterioles
• Provide structural support and vasodynamic capacity to microvasculature
• Role in structural stability of vessel wall
• Endothelial cells associated with pericytes are more resistant to apoptosis than isolated endothelial cells
  • Indicates role of pericytes in structural integrity and genesis of the BBB
• Phagocytic activity