What does the blood-brain barrier primarily do?

The blood-brain barrier primarily functions to control the passage of molecules into and out of the brain. This selective permeability is crucial for maintaining the brain's homeostasis, as it protects the central nervous system from potentially harmful substances in the bloodstream while allowing essential nutrients to pass through.

The barrier is formed by endothelial cells in the brain's blood vessels, which are tightly joined together, limiting the types of substances that can enter the brain. In this way, the blood-brain barrier plays a vital role in regulating the internal environment of the brain, ensuring that it can function optimally despite changes in the blood composition.

In contrast, the other options pertain to functions that are not directly related to the main function of the blood-brain barrier. For instance, while oxygen transportation is critical for brain health, it is not the barrier's primary role. Similarly, structural support is provided by glial cells, but this is separate from the barrier's function. Lastly, blood pressure regulation is managed through different mechanisms in the cardiovascular system and is not the primary concern of the blood-brain barrier itself.