Why does the brain have a semi permeable brain?

The brain does not have a semi-permeable brain structure. The blood-brain barrier (BBB) is a specialized structural component that regulates the passage of substances from the bloodstream into the brain. It acts as a protective mechanism, ensuring that potentially harmful substances are prevented from entering the brain and disrupting its delicate environment. The BBB selectively allows the entry of essential nutrients and hormones while restricting the passage of toxins and pathogens. It consists of tightly connected endothelial cells, astrocytes, and pericytes, forming a semipermeable barrier that maintains the brain's homeostasis and protects its vulnerable neural tissue.