What is the primary role of the blood-brain barrier?

The primary role of the blood-brain barrier is to protect the brain from harmful substances. This selective permeability barrier is formed by endothelial cells in the brain's blood vessels that are tightly joined together. This structure prevents potentially harmful chemicals, pathogens, and toxins present in the bloodstream from entering the brain, while still allowing essential nutrients and certain other substances to pass through.

The blood-brain barrier also plays a critical role in maintaining the brain's homeostasis, regulating the environment around neurons to ensure optimal functioning. It allows for the essential transport of glucose and amino acids which are crucial for brain metabolism, but restricts many larger or fat-soluble molecules that could disrupt neural activity.

In contrast, the other options do not accurately reflect the functions of the blood-brain barrier. It does not allow all substances to pass freely, contrary to the first option. The barrier's primary purpose is protective, rather than enhancing nutrient absorption as suggested in the third option. Lastly, it does not block oxygen, which is vital for brain function; instead, oxygen can easily diffuse through the barrier due to its small molecular size.