Final answer:
The specialized capillaries in the brain create a highly selective barrier known as the blood-brain barrier. It allows necessary substances like water and glucose to pass while blocking harmful substances, protecting the central nervous system. This barrier poses challenges for CNS-targeted drug delivery.
Step-by-step explanation:
The highly selective barrier to diffusion that the specialized capillaries of the brain create is known as the blood-brain barrier. This barrier is critical for protecting the delicate nervous tissues of the central nervous system (CNS) from potentially harmful substances in the blood. The blood-brain barrier is comprised of endothelial cells that form tight junctions barring most substances from entering the brain. It allows selective permeability, letting through necessary substances like water, certain gases, glucose, and some amino acids, which are crucial for the CNS. At the same time, it blocks harmful chemicals and pathogens, maintaining the CNS's isolated and protected environment.
The capillary endothelial cells which make up the blood-brain barrier are unique to the CNS, having fewer fenestrations and different characteristics compared to other capillaries. The continuous capillaries in the brain, part of the blood-brain barrier, have tight junctions and no intercellular clefts, further contributing to this highly selective barrier. These capillaries also feature a thick basement membrane and astrocytic end feet, intensifying the barrier effect.
Moreover, the intricate blood supply to the brain through the internal carotid and vertebral arteries, including the circle of Willis, allows for constant perfusion to the CNS while still maintaining this selective barrier. As such, while the blood-brain barrier is crucial for protection, it also poses challenges for the delivery of pharmaceuticals to the CNS, making the treatment of diseases within the CNS more complex.