What exactly does weed do?

I was curious to what exactly it does to your body when you smoke weed

I heard somewhere your blood pressure is raised, which explains the red eyes

I know it affects the Nervous system...

And that's basically it. If anybody who knows more I'd like to hear.....

I heard that your blood pressure is lowered when high. Honestly I don't really bother with what it does to your body. All I know is I never known anyone to die from it and I know that it has made me a better person overall and that's all that matters. It's a miracle worker, that's what it is :D.

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.

Here is some more good info. I had found this link in another thread when I was looking up the blood pressure effects.

Marijuana Health Mythology