The problem with your head being severed from your body is that;
One: Your brain no longer receives the much needed dose of oxygen and sugar so the cells start to die and then - cerebral death occurs.
Second: Once the nervous cells from your spine are severed at the level of the neck they cannot regenerate the connections with the other side of the body which means you get detached from the rest of your body permanently - tetraplegia occurs.
Third: Once a major section is cut on a body it is 100% certain that at least on major vein will be severed as it happens with a neck sever. This will mean that as long as gravity or heart normal function are present there will be a mojor blood loss that will result in death - death by blood loss.
It is also true that your body starts shutting down once your head is removed so the heart does not pump and the poisonous fluids normally generated from body functions start accumulating. Yet if the time limit is not crossed the damages can be managed and full recovery is achievable.
Curious enough these are the exact same problems one has with the sever of the torso. If the torso get's severed in two on a horizontal plane perpendicular to the spine the same problems jump in, nervous cells must eat constantly and it is, as today stands, impossible to regenerate nervous cells connections. Yet we've seen it happen and stated to be recoverable a body in such state on the narutoverse.
The slug tells tsunade that she will put her body back together hinting that said sever does not imply permanent damage. Also Senju's ability to regenerate, which Tsunade has might grant her the ability to regenerate her neurones and nervous cells at a constant rate.
In other words, theoretically a head severed would present no different problems to Tsunade than a severed torso. The slug told us that a severed torso presents no problem, so the logical conclusion can only be that a severed head will be of no problem as well, provided that the slug is around.
The only not covered issue is the bleeding part. But it would act the same way on the torso and Tsunade wasn't dead so I'm assuming the same would happen with the head.