system
Given the following post, respond as if you're commenting on Reddit:
human
What makes a chemical able to pass the blood brain barrier?

Why can theanine pass the BBB but vitamin C can't?  What specifically is stopping the nutrients from most things we ingest from crossing?
gpt
Unlike in normal capillary endothelium, the endothelial cells of the cerebral capillaries are connected by tight junctions that prevent the diffusion of molecules between the endothelial cells. Normal capillaries have gaps between the cells that allow free diffusion of many molecules between them, but the cerebral capillary endothelial cells are held directly in contact with each other by the zonula occludens that forms a seal between the cells, preventing any movement of molecules from the lumen of the blood vessel in the the cerebral interstitial fluid. Thus, in order for a molecule in the blood to enter the CNS, it must pass through endothelial cells. To do this, it can take one of two pathways, diffusion or transport. To travel by diffusion it must be a small, lipophilic molecule, such as O2, CO2, NO, some hormones, etc., that will diffuse directly through the luminal plasma membrane of the endothelial cell, the cytoplasm, and then the abluminal plasma membrane. Otherwise it must be transported by the endothelial cells through a membrane protein. Transport can occur through one of many types of transport proteins, but is strictly regulated.