Some areas in or near the brainstem, situated outside the blood-brain barrier, are

The blood brain barrier is a semipermeable barrier, that protects brain and Spinal cord from potentially dangerous substance. Some areas are outside the BBB for apecialised functions and homeostasis :SENSORY -area postrema (AP), the subfornical organ (SFO)& vascular organ of lamina terminalis. They have the ability to sense plasma molecules and then pass that information into other regions of the brain. Through this, they provide direct information to the autonomic nervous system from the systemic circulation.SECRETORY-the subcommissural organ (SCO), the posterior pituitary, the pineal gland, the median eminence and the intermediate lobe of the pituitary gland. These organs are responsible for secreting hormones and glycoproteins into the peripheral vascular system using feedback from both the brain environment and external stimuli.(Ref: Vishram Singh textbook of clinical neuroanatomy second edition pg 183)