Blood-Brain Barrier

The blood-brain barrier is a physiological barrier which prevents substances in the ECF of the body moving freely into the ECF of the brain. The functions of the BBB are:

  • Maintain a stable extracellular milieu
    Optimises neuronal function by preventing fluctuations in plasma K+, Na+, and H+ affecting cerebral cells.
  • Protection of the brain
    Isolates the brain from toxins.
  • Protection of the body
    Isolates the rest of the body from CNS neurotransmitters.


The BBB occurs in three layers:

  • Capillary endothelial cells
    Joined with tight junctions, preventing free movement of solvent and solute.
    • Substances must move through capillary endothelium to reach the brain
      Capillary endothelial cells contain high numbers of mitochondria, due to the higher energy cost of the active transport mechanisms.
  • Basement membrane
  • Astrocytes
    Glial cell which extends foot processes around the basement membrane, and reduce permeability of endothelial cells.

Due to their function, several important CNS structures must exist outside of the BBB. These are known as the circumventricular organs, and include:

  • Sensing structures
    • Chemoreceptor trigger zone (Area Postrema)
      Identifies toxins in the systemic circulation, triggering vomiting.
    • Hypothalamus
      Osmoreceptors detect systemic osmolarity.
    • Subfornical organ
      Role in CVS and fluid balance.
    • Organum vasculosum
  • Secreting structures
    • Pituitary
      Secretes hormones.
    • Pineal gland
      Secretes melatonin.
    • Choroid plexus
      Produces CSF via secretion and ultrafiltration of plasma.

Movement of Substances

Substances can move via:

  • Diffusion
    For lipid soluble molecules only; e.g:
    • CO2
    • O2
  • Facilitated diffusion
    For movement of larger/less soluble molecules down their concentration gradient, e.g:
    • Glucose
    • Water
  • Active transport
    Responsible for movement of most small ions; e.g:
    • Na+
    • Cl-
    • K+
    • Mg2+
    • Ca2+

Other substances are specifically excluded:

  • Catecholamines
    Metabolised by MAO in capillary endothelium, preventing their action as CNS neurotransmitters.
  • Amino acids
    Prevent action as neurotransmitters.
  • Ammonia
    Metabolised in astrocytes to glutamine, limiting its neurotoxic effects.


  1. Lawther BK, Kumar S, Krovvidi H. Blood–brain barrier. Continuing Education in Anaesthesia Critical Care & Pain, Volume 11, Issue 4, 1 August 2011, Pages 128–132.
  2. Brandis K. The Physiology Viva: Questions & Answers. 2003.
Last updated 2019-07-18

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