The mechanisms controlling systemic arterial blood pressure and the distribution of blood flow in different tissues in the body involve highly complex regulatory systems which are interdependent and interactive. Changes in any one system necessarily affect the others. The advantage of this “mosaic'' (Page 1966) multilevel control is the flexibility the cardiovascular system has in the maintenance of arterial blood pressure within narrow limits, the preservation of blood flow to vital organs such as the heart and brain during emergency situations, and the ability to redistribute blood flow to tissues in accordance with their metabolic needs. A disadvantage of this complexity is the increased chance that the regulatory mechanisms can misfunction and, themselves, produce disease states. Indeed, hypertension is often called a “disease of regulation.”