What is hierarchy? Answer Wiki. A hierarchy is A body of persons or things ranked in grades, orders, or classes, one above another; specifically in Natural Science and Logic, a system or series of terms of successive rank (as classes, orders, genera, species, etc.), used in classification.  From the OED .
1 During the first 24 weeks of pregnancy, it's common for blood pressure to drop.  A decrease in blood volume can also cause blood pressure to drop. A significant loss of blood from major trauma, dehydration or severe internal bleeding reduces blood volume, leading to a severe drop in blood pressure.
1 Low blood pressure, also called hypotension, is blood pressure low enough that the flow of blood to the organs of the body is inadequate and symptoms and/or signs of low blood flow develop shock.2  Low pressure alone, without symptoms or signs, usually is not unhealthy. These symptoms are most prominent when individuals go from the lying or sitting position to the standing position (orthostatic hypotension). 2  Low blood pressure that causes an inadequate flow of blood to the body's organs can cause strokes, heart attacks, and kidney failure.