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.
In this type of low blood pressure, there is a drop of 10 to 20 points of blood pressure when a person moves from a sitting or supine (flat) position to a standing position, Pacold says. It causes a sudden sense of lightheadedness and can be due to a heart that is starting to have trouble compensating..