What is the purpose of the blood-brain barrier?

The purpose of the blood-brain barrier is to protect the brain from harmful substances while maintaining a stable environment for neuronal function. This barrier is formed by tightly packed endothelial cells in the brain's capillaries, which prevent many potentially toxic substances, pathogens, and drugs from entering the brain tissue. By selectively allowing certain substances to pass, such as glucose and amino acids, while keeping out larger molecules and harmful pathogens, the blood-brain barrier plays a crucial role in safeguarding the brain’s delicate neural environment.

Other choices do not align with the primary function of the blood-brain barrier. For example, while the barrier helps regulate the environment of the brain, it does not enhance drug absorption, transport nutrients directly—though it allows specific necessary nutrients to cross, nor does it directly regulate blood flow to the brain. Thus, the correct answer highlights the essential protective role the blood-brain barrier plays in maintaining brain health and function.