How does he survive? (spoilers)

His chest is full of shrapnel, he's lost a tremendous amount of blood, his ribs are shattered, and he spends hours and hours lying on the ground with the dead weight of another human directly on his chest. How could have continued respirating with that weight on him? He'd have suffocated in short order. And this doesn't even get into the operation without any transfusions, the miraculous lack of infection, etc...