The researchers used plasma because blood cells are actually too large to travel through the blood-brainbarrier into the brain.
而之所以研究人员在实验中使用血浆，是因为血细胞较大，无法通过血脑壁障进入大脑。
2
Tryptophan has to fight with other amino acids to cross the blood-brainbarrier and get into the brain.
色氨酸必须打败其他的氨基酸，才能通过血管-大脑的障碍物，到达大脑。
3
This is mainly because it is a large molecule, and has difficulty passing through the blood-brainbarrier which protects the brain from potentially harmful compounds in the blood.