A thorough illustration of Polygraphs. Very informative...Polygraphs as you mentioned acquire details of different things like Pulse, HR, Blood Pressure, etc. which normally are at optimum or have certian range for most of the People, but there are certian other persons in the world whose basic vital signs are bit higher or sometimes lower; Although they have a normal life. So polygraph in these conditions can have a deceiving result. Certainly there are some individuals who can tell a pure lie without any physiological changes happening to them, they have adapted themselves to such kindda change. Moreover there are some gruesome criminals who can fool Polygraphs as well.
So maybe Polygraphs can be True in almost 70 or 80 pc of cases but there are some Cases where it fails.
P.S. Great Post broda.