blood brain barrier
"The blood-brain barrier is like the moat outside a castle, preventing harmful invaders from entering."
"The blood-brain barrier is like a very fine sieve; only very small molecules can cross this layer and actually enter the brain."
" The blood-brain barrier is like a semi-permeable gate between the brain and bloodstream."
"The blood-brain barrier is like a border where customs officers, i.e. P-glycoproteins, only allow specific substances to pass through whilst ..."
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.