bone cells are arranged in concentric circles around longitudinal (lengthwise) tubes containing blood vessels. these longitudinal tubes in the center of each osteon are called?

hormones in blood stream

image: dtc.ucsf.edu

endocrine glands secrete hormones into the blood stream near them; the hormones then travel in our bloodstream until it reaches its destination, called a target cell, in distance parts of the body. in the target tissue, hormones lock onto the target’s plasma membrane, called the receptor site.

not sure if this is what you'r looking for eubacteria and archaebacteria