These are often referred to (and not with total accuracy) as trace elements in the human system. It might be observed that they are called this not only because that is the degree to which they are present, but also because that is the degree to which they are understood. What is understood is that they can be harmful in too great a quantity and are certain to be harmful if missing from the human system entirely. They are also highly interactive with certain vitamins, coenzymes, proteolytic enzymes and are greatly affected by the action of both.
ref: Silent Healer (pg. 51) by Bill coats R.Ph., with Robert Ahola