Final answer:
The blood-brain barrier is a highly selective membrane that prevents pathogens, toxins, and certain drugs from reaching the brain and spinal cord while allowing essential molecules to cross.
Step-by-step explanation:
The blood-brain barrier is a specialized layer of protection that prevents many substances, such as pathogens, toxins, and certain drugs, from reaching the brain and spinal cord (also known as the central nervous system or CNS). It is a highly selective membrane formed of endothelial cells that separates the circulating blood from the extracellular fluid in the CNS. The barrier allows certain molecules, like water, glucose, and gases, to cross from the blood into the CNS while keeping out potentially harmful substances.