One of the most consistent physiological effects of marijuana is an increased pulse rate. This does not appear to be a direct drug effect on the heart. Instead, the drug appears to cause complex changes in the autonomic nerves regulating heart rate. Another consistent effect of marijuana is the suppression of the normal sinus arrhythmia usually produced by respiration. That last part means (I think) that normally, slow, deep breathing will slow the heart rate but this does not happen when you're high. So your body is acting "panicked". This is when your brain starts to play catch up by inventing "reasons" why you should be panicked. Hence the paranoia. You can investigate this effect for yourself. Every time you start to feel paranoid, stop and check whether your heart is pounding or beating fast in your chest. Ask yourself if there is anything in your immediate environment (a mountian lion, hot chick removing shirt, etc) that would cause your heart to beat faster, and if there is not, you know it's just the chemical effect of the weed.
Some strains of weed produce less paranoia, probably due to the particular combination of active ingredients (d9 THC, d8 THC, CBD, CBN, THCV, CBC, etc). Sativas, with their more "speedy" effect, tend to be associated more with paranoia.
More on the physiological effects of marijuana...
http://www.druglibrary.org/schaffer/Library/studies/nc/nc1e_3.htm
I usually find a couple of beers takes the edge off nicely. Not that I would want to encourage alcohol consumption.