I know what effects haemotoxic venom has on the human body, but what is in it that makes it so deadly?

Question: I know what effects haemotoxic venom has on the human body, but what is in it that makes it so deadly?
- Keywords:
  haemotoxic,
  human body,
  venom
Asked by woooody to Cathal, Daphne, Darren, Jon, Katherine on 13 Mar 2012.
- Katherine Haxton answered on 13 Mar 2012:
  
  The venom causes massive internal bleeding which means lots of blood where there isn’t normally much blood, and not enough where there should be blood. The body can’t cope with this so the organs fail because they don’t get enough oxygen or other important chemicals.

Question: I know what effects haemotoxic venom has on the human body, but what is in it that makes it so deadly?