Balrog,
How extensive?
Not very much. You can't carry an intensive care unit on your back just in case, and you can't cover every possible scenario in the bush... so the first option is to be CAREFUL not to get injured in the first place.
I mainly worry about 5 things when out there:
1) Injury leading to the impossiblity to move: broken leg, ankle, etc. Against that, I carry a knife and a roll of duct tape (I can strap a twisted ankle, make an expedient cast, carve a set of crutches, etc.). I also carry some serious antalgics (paracetamol 1g... lethal dose is around 10g for the average adult) to help me "walk on it" if I really have no other option. I choose paracetamol because you can give it to kids (adjust the dose!) or pregnant women... unlike acetaminophen, high aspirin doses, etc. Paracetamol is also causing less allergic reactions, AFAIK. Third reason, paracetamol won't mess too much with blood coagulation (as aspirin does).
2) Hemorragies: an open fracture or a deep cut can sever big blood vessels and lead to an important hemorragy. That sucks big time, as it can quicly lead to shock, coma and death. It's very important to limit the blood loss as much as possible whenever you're seriously bleeding (elevate affected limb, apply direct pressure with hemostatic dressing if available). Against that I carry one of those huge hemostatic foam pads with one handed attachable strapping. They work well, and they can help you keep some blood so that the paramedics still have something to save when you meet help. Hemorragies can lead to serious complications in the following hours so you have to seek medical assistance ASAP, even if the bleeding has stopped.
3) Allergic reactions : mostly anaphylactic shocks (decrease of blood pressure leading to coma and death), oedemas (in the face area, they can impair vision, movement or breathing) and so long. Against that I carry anti-histamines, some serious cortico-steroids (ask your doctor) and an epinephrin seringe. The trick is that for most situations, that's not yet enough to save your ass in case of a serious allergic reaction, so still be careful. I'm not personally allergic to anything past penicillin, but you never know when you can BECOME allergic to something, so better be prepared.
4) Infection : with gradually more antibiotics resistant bugs, prevention really is the only serious option regarding infection, especially when you're out there by yourself. I've nearly lost an arm due to an open fracture that got infected, so I know how bad those bugs can be. Any open wound must be cleaned thouroughly and disinfected using a safe and clear product (over here we have this great stuff called biseptine, that is both antiseptic and efficient against fungus). I then wrap the wound with hypafix to keep it closed and clean. Keep your shots (especially tetanos) up to date.
5) Exhaustion : sometimes you just can't move anymore (depends on your outdoor activities, of course). I carry some simple sugars (dehydrated fruits, usually mixed with nuts of some sort) and some caffeine pills. I've used this mix with some serious effectiveness. Besides, the sugar can help a diabetic/hypoglycemic, and the caffeine pills can really make a difference for an asthma crisis (really, it's better than nothing as it will expand airways a little), for example. I had never thought of that caffeine pills one before I read Cody Lundin's 98.6 - the art of keeping your ass alive. As ephedrin is not legal over the counter in France, I must stick with caffeine alone, but a mix of both can be even better.
In any way, the best thing to do when injured in the backcountry is to avoid further aggravation, stabilize the victim/yourself as well as possible, and seek medical help (get to them if safely possible, or call them in). If you're already lost/in a survival scenario, well... you just have made your case much worse. Unwanted survival situations are a tough one by themselves, even when you're healthy, so don't fall down into more trouble than what you're already in. It's extremely important to be careful back there, especially if you have no immediate means to get real medical help (like an uninjured friend with communication instruments who can call in for rescue).
Cheers,
David