The body has mechanisms to alter or maintain blood pressure and blood flow. There are sensors that sense blood pressure in the walls of the arteries... My WebMD Sign In, Sign Up
Perhaps while we're not watching, the denizens of Dark Shadows carry-on, caught in another inter-dimensional wing of Collinwood. If you peer out the corner of your eye, you may catch a glimmer of this shadowy activity, perhaps in Dark Shadows Collection 14, or Dark Shadows Collection 22.
If you turn on a black light bulb in a dark room, what you can see from the bulb is a purplish glow. What you cannot see is the ultraviolet light that the bulb is also producing. Our eyes can see visible light in a spectrum ranging from red through orange, yellow, green, blue and violet.