I'm going to try and find another link for you, but the overview in the link below is fairly good from a scientific/chemistry standpoint. It also contains some of the standard anti-cannabis propoganda, too, so beware of that. Read throught the various sections and see what you think.
Howstuffworks "How Marijuana Works"

As far as blood pressure, right after it's smoked, cannabis does raise blood pressure in lots of people. But shortly after that, it actually dilates blood vessels and can lower blood pressure in many. The initial after-smoking period is when the BP goes up, along wth the heart rate. Sativa strains tend to cause this more than Indicas, which are more relaxing and sedating (and also harder to find). It's the dilation (relaxation) of your peripheral blood vessels that actually causes the bloodshot eyes. That's why eyedrops containing vasoconstrictors (vessel tighteners) make red-eye go away--because they cause the vessels to shink and contract and appear less noticeable.

Let me see if I can find other links for you. May take me some time. I want to ask a friend about one that I once saw posted here that was very very good.