Systems behavior, feed-back loops, and high blood pressure research.

• In spite of the fact that much is known regarding various factors which determine blood pressure, the mechanism (s) responsible for hypertension remain elusive. The mystery applies not only to essential hypertension, but also to renal and, indeed, even to the hyper-tension associated with coarctation, adrenal tumors, and elevated intraeranial pressure. Almost characteristically, investigators who have discovered promising clues have become frustrated in their efforts to apply these clues to the "intact" hypertensive. These clues seem to become swallowed up and lost in the many interacting functions related to blood-pressure control. This discussion constitutes an attempt to consider why many clues to the etiolog3 of hypertension appear promising when considered alone but do not seem to explain the abnormality when sought in the hypertensive patient or experimental animal. Investigators and clinicians alike have tended to divide the body into traditional systems, such as gastrointestinal, cardiovas-cular, endocrine, etc. Such conventional segregation may, however, become inappropriate for investigative purposes since few physiological functions themselves follow such traditions. Furthermore, investigators have found it difficult to consider more than one function at a time. The usual biomedical experiment consists of measuring one, or at most very few, independent and dependent variable (s) with the hopeful assumption that other related variables are in a "steady state.'' It has, of course, long been recognized that such experiments are not ideal, but may be necessitated by methodological difficulties and by the nature of experimental subjects. not easy, it is nevertheless essential that he learn to deal with multiple simultaneous variables and their functional interactions. Conversely , he will be misled and nature : s mechanism will elude him if he cannot consider complex systems, since such are nature's mechanisms. It is possible to select many examples of biological systems to illustrate the fact that, in such systems, the relationships between any two variables are dependent upon the simultaneous state of other variables. Thus, if an investigator is interested in determining the relationship between two variables , he will be misled if the variables are also affected by other functions of which he is unaware or which he does not measure simultaneously. An example of this "fact of life," is the familiar and relatively simple feedback loop, as illustrated in figure l (a). Here (Y) represents some significant input into the system and (0) is the related significant output. (A) represents the properties and behavior or function …