Western Diamondbacks, like most other rattlesnakes and other venomous snakes in the pit viper subfamily, have so-called hemotoxic venom."Hemotoxic" means blood-damaging, but it turns out that the chemicals in the venom target all sorts of tissues, not just blood, so this name actually tends to upset nitpicky scientists who study snake venom.

But in any case, rattlesnake venom contains a whole cocktail of damaging chemicals, primarily proteins and enzymes. It seems that new components are being found all the time, and I could not find a comprehensive list. I also could not find a list specific to Western Diamondbacks. But it seems that a common set of components are proteins that mimic the proteins in our blood that produce blood clotting (prothrombin, thrombin, and other 'procoagulants'). Some viper venom components cause immediate blood clotting in parts of the body, often inside the kidney, which can block up the kidney and kill you. There is so much clotting that all of the fibrin proteins needed for clotting are depleted, and the body cannot clot anymore. It appears that some rattlesnakes accomplish the same task a slightly different way, by disrupting the formation of clots at all, so that no clotting is possible. After the clotting factors are either all used up or disrupted, a separate set of chemicals known as hemorrhagins takes effect. They damage the inside walls of arteries and veins. Since clotting is not possible, this leads to massive, uncontrolled bleeding into the tissues. Among other problems, this will disrupt the delivery of oxygen to tissues by the blood. Without oxygen, the tissues in the affected area will start to die.

I should mention that some rattlesnakes (specifically the Mohave rattlesnake) also have neurotoxic venom, although I don't think this is confirmed for Western Diamondbacks. This venom inactivates acetylcholine, a major neurotransmitter in the body. Acetylcholine is used to transmit nerve impulses from neurons to muscle cells. When it is inhibited, muscles stop working. Other compounds in rattlesnake venom known as crotamines can interfere with sodium channels in neurons, which also blocks nerve signals and causes muscles to stop working (think heart and diaphragm). This type of venom can kill much faster than the hemotoxic type.

So there is some pretty nasty stuff in there - try to avoid getting bitten!

sources:
Karalliedde, L (1995) Animal toxins. British Journal of Anaesthesia 74: 319-327
Mackessy, S. (2002) Journal of Toxicology - Toxin Reviews 21: vii-xi