Like Madiosi, I think that the bleeding/blood loss has a lot to do with the skills of the executioner.
When the nails are driven at the right spot through the wrist, the victim won't lose that much blood.
Most likely, when the arms are stretched, the body will be lower than the wounds.
A fact that will slow down the blood loss , too.
Still enough though, that small streams of blood were running down the arms, the side of the ribcage, hips and legs.
With the nailed feet it's, I think, a different story. Much more blood must have been lost there.
I doubt that the victims died because of the blood loss alone. Bleeding weakened the crucified, but they suffocated, died because of stress, had a sepsis, heart attacks, strokes, thrombosis, as in the regular life.
The cross though, as we know, was extremely shortening the life span of the crucified.
For a good while, the crucified were able to live and communicate on the cross.
An entertainment factor for the spectators, besides the actual spectacle of the crucifiction.
Ancient documents are even stating, that it could take days until the crucified were killed by the cross.
Of course, I'm not a doctor and don't have profound knowledge of the human physis. That's just what I think about it.
Eventually somebody can get more into detail.