Blood-brain barrier

The blood-brain barrier or BBB is a selective permeable barrier composed of endothelium that separates central nervous system from circulating blood.

Circumventricular organs
Some parts of the brain sit outside the blood brain barrier. Collectively, they are known as the circumventricular organs and include sensory and secretory organs important for neuroendocrine function including the pineal gland and the posterior pituitary gland. The olfactory bulb also sits outside the blood brain barrier.

Diseases
Several diseases have been associated with a weakened blood-brain barrier including epilepsy and multiple sclerosis.

Drugs
Most drugs are unable to cross the Blood-Brain-Barrier, which restricts treatment for certain conditions.