So Betterave, if someone blew my cerebral hemispheres out with a gun, you would think it the only morally acceptable course of action to resuscitate me and keep my body alive by all measures necessary?
Just to clarify, it is not Catholic theology that a human being must be kept alive by all measures necessary. Extra-ordinary measures do not have to be taken; there are definitely situations where it is appropriate to “pull the plug”. However,
ordinary measures, like food and drink, cannot be morally prevented.
I don’t think there is any definite material requirement for a human being; after all, a person’s body can be completely destroyed and yet that person will still exist as a human being - not as a earthly living human being, but as a soul in Heaven, Hell, or Purgatory. The requirements are, rather, to be human (to have previously been a living human with an earthly body, however brief that life), and to exist.
The term “human animal” is either a synonym for human being, or a contradiction in terms, for the definitions I can think of. It is a synonym when we are discussing a human being but drawing attention to the animal side of the human being’s nature. It is a contradiction when we are trying to refer to the nature of the being, as a human is immortal and an animal is mortal; a thing cannot be both at once. Perhaps what you are trying to ask by asking when a human is just a human animal is, “When does a human being become just a human body, with no being in it?” using terminology that Catholics would find clear and understandable.
As for the attachment of the soul and the being to the body, I don’t know that there is any official Church teaching on this. We tend to assume that death occurs and the soul seperates from the body when the free will no longer controls the body in any way, though - right? Isn’t that why brain death is considered significant - because without the brain, how can the will act upon the body? However, I don’t know that there is any evidence of this, including philosophical or theological evidence - perhaps the soul actually lingers within the body for some time, but simply with the free will unable to act upon the body.