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.
Often when shading, the shadows don't go past dark gray. If your value range is restricted to in some cases half what it ought to be, you are limiting the modeling and depth in your drawing. Put a piece of black paper at the corner of your drawing, and don't be afraid to go dark.