I'm not a doctor, either--but I've known a bunch--I used to work as an RN, from the age of 23 to 34 or so. Now I am 'retired' and am an 'artist.'

These days I nurse a soul here and there--and no doctor around for miles!
But I still know a few things about doctor stuff.
- Bleeding from a severed artery can lethally drain the body in less than 6 minutes.
- When there is severe bleeding where a major artery has been severed, pressure may be insufficient and a tourniquet may be necessary. Tourniquets are an effective way of stopping bleeding from an extremity. They do, however, stop circulation to the affected extremity and should ONLY be used when other methods, such as pressure dressings, have failed (or are likely to fail). Pressure from tourniquets must be relieved periodically to prevent damage to the tissue from lack of oxygen.
- Bright red blood that comes out in spurts (timed by each heartbeat) or a heavy, steady flow, indicates that an artery has been severed. The rapid loss of blood can quickly result in 'shock' (which is a partial shut-down of the body in order to preserve the vital organs by only giving them what oxygenated blood is available). Also, dehydration is a immediate effect of a large amount blood loss--and can also pose a life-threatening situation in the subsequent 2 to 3 days (if the victim is not re-hydrated).
- The severed artery will require surgical repair unless totally unavailable--and that unavailability drastically reduces odds of survival.
- As far as 'heavy lifting'--that's the 'short-cut' to bleeding to death--the exertion is going to nullify any positive results of the pressure applied to stop the bleeding, and the seemingly smallish cardiovascular effort required is beyond the limited survival mode that occurs during this type of trauma.
Here is a link or two I found for your reference--also, there is a lot of info on the web--just google 'severed artery repair' or 'first aid severed artery'--things like that. My own search turned up reliable information that I know to be true--so it is a good source for your writing purposes. You could ask me a question, if you wanted to--if I know the answer I will tell you--and if I don't, I'll tell you that, too.
http://www.answerbag.com/a_view.php/25492
http://www.coolnurse.com/bleeding.htm
http://www.drkoop.com/ency/93/000045trt.html
I'm not disparaging Boomerang's answer--because it is very possible to survive such a trauma; and has been documented many a time (amazing every time!)--but not as likely as not surviving--and for a severed artery, surgery must be done at some point in time, from what I understand. It can't repair itself because the constantly pumping blood doesn't allow the needed environment for the tissues to graft back together--and such a thing would cause the other arm tissue (muscle and fascia, etc) to die in the meantime--either you lose your life or your arm. And being all alone adds double jeopardy to the situation.
And there is always, without exception, the extenuating circumstances of each individual case--which, of course, you are creating yourself. I think the type of injury that caused the severed artery might sometimes render conditions to where the artery is somehow stifled by the effects of the injury, and no doubt it has a lot to do with a person's stamina--something that is purely mental and not at all physical. It is their 'John Wayne True Grit' type of toughness that I refer to as 'stamina.'