Final answer:
Certain areas of the brain, like the pineal gland and the pituitary gland, lack a functional blood-brain barrier, allowing free exchange of substances which can expose them to toxins. The blood-brain barrier is critical for protecting the brain from harmful substances but can also hinder drug delivery to the CNS.
Step-by-step explanation:
Organs that lack a functional blood-brain barrier include certain regions of the brain such as the pineal gland, the pituitary gland, and the area postrema. These areas have fenestrated capillaries, which allow for the free exchange of substances between the blood and the brain, making them susceptible to toxins and pathogens. The blood-brain barrier is designed to protect the brain from harmful substances in the blood, controlling the transfer of materials and preventing the movement of nearly all substances, including many drugs. However, this intently selective barrier can also be problematic, especially while treating infections within the central nervous system (CNS), as it inhibits many therapeutic agents' penetration into the brain.