So how does the zombie body function? How is it that only a head shot is guaranteed to kill one of them?
As mentioned in a previous post on Zombie Ecology, there have been multiple ideas of what a zombie is. The original concept of spirits animating dead flesh requires no physiological explanation--that's how it works just because it does.
But the zombie stories that feature a virus, some sort of non-supernatural phenomenon animating dead flesh such as in AMC's The Walking Dead, could use some scientific explanation. How is it that the zombies are able to move about when most of their major organs are non-functional? How is it they resist completely rotting away into skeletons?
I've got some proposed answers (and I thank Paul Clyde and James Lehye for engaging in the conversation that inspired this post):
First off, the easy stuff. Zombies don't feel pain because their brains don't function right...the pain signals arrive at what is left of their brain, but nothing there particularly cares. However, even though the brain is hardly working well, it's still working (as is the nervous system) directing the body of the zombie to move forward and attack. That's why destroying the brain by a head shot or traumatic head amputation stops a zombie every time. Without a brain, no signal arrives at the zombie muscles to drive them forward...
This is a bit of a sidetrack, but that should produce an effect that I've never seen in The Walking Dead. Carrion birds and other animal flesh eaters should be all over zombies, eating them as they shuffle along...since the zombies don't feel pain or react to it only feebly, they would only waive at the birds occasionally to stop them, as say, turkey vultures or magpies perch on their shoulders, eating away at the decaying body as it trudges along...So something about zombie physiology must stop this from happening--more on that in a bit.
So we know why head wounds do kill--why is it that chest wounds don't? Physiologically, the chest is the center of the circulatory system: via the lungs it oxygenates the blood, so destroying the lungs eliminates this capacity, while the heart pumps the blood to the body, so destroying the heart keeps the body from receiving the oxygenated blood. Plus the chest abounds in major blood vessels, many of which will cause someone to quickly bleed to death either externally or internally, should a bullet (or other weapon) pass through them.
So it seems the virus reworks the body so that its need for oxygenated blood is either zero or greatly reduced. Zombie body cells, operating on some form of metabolism that doesn't use much oxygen, can do without the blood, or without much of it. Bear in mind that even without the heart, some circulation of blood could still happen through extrinsic compressions of blood vessels caused by muscle movements--which is partly how lymph flows through the human body.
This could imply that the nature of blood itself is different in a zombie. It could be the blood becomes very thick (I mean with high viscosity), which could have the effect of making it less likely to leak out of damaged blood vessels. It could also be the blood greatly increases somehow its ability to carry oxygen, so what little flows through parts near the skin absorbs enough atmospheric oxygen to carry that to the zombie's internal organs, which don't need a lot of oxygen anyway...
In any case, any zombie is very resistant to bleeding to death and is immune to damage to the heart and lungs, though probably not through any form of rapid healing. This implies to me significant changes in the nature of circulation and blood but alternatively, it could be that interstitial fluid and lymph become the primary way to carry nutrients for a zombie's body. Damage to circulatory system doesn't matter because the zombies aren't using it anymore...
Clearly the zombies do use the digestive system, since they are always trying to eat human flesh. So a gut shot (or multiple gut shots) should in theory kill a zombie, though it would by no means be a quick death. It's noteworthy that among the reasons gut shots kill even if the body doesn't bleed to death from them is that the digestive system carries a lot of bacteria, which can multiply within wounded intestines and directly enter the blood stream and threaten the entire body. Clearly this is not an issue for zombies.
So it would seem then that the virus that creates zombies must also have some antibacterial properties. Actually, a lot of viruses are bacteriophages, so like them the zombie virus would probably need the secondary function of eating bacteria that would naturally attack zombie flesh. This explains why zombies don't rot into skeletons within a few months and how it is they can walk around (seemingly) for years on end, carrying all kinds of dead tissue without loosing it to bacterial decay.
Might this virus also effect the taste and smell of zombie flesh? So it's unappealing to carrion birds? And also to other zombies as well? Who instead of eating each other, always prefer live human flesh...
Since the virus would have to do a rather radical reworking of the circulatory system for zombies to make any sense, perhaps suggesting a radical reworking of the digestive system is in order. Normally, the human body takes what it eats and breaks it down into constituent amino acids, lipids, and carbohydrates and uses these basic building blocks not only to fuel the body in motion, but also to build replacements for body cells that are continually in the process of dying. Could it be the zombies are so radically re-worked that when they eat live human flesh their digestive systems takes whole cells from their unfortunate human victim and transports these whole cells somehow to the parts of the zombie's body that needs new tissues?
That a zombie would be able to process whole human cells to rebuild damaged tissues in itself is a pretty far-fetched suggestion. The virus would certainly have to be genetically engineered to achieve any such effect--but some heavy genetic engineering would also make its replacement of the circulatory system and antibacterial properties more plausible as well, so why not?
To be honest, whole cell replacement probably goes too far in terms of what is in any way possible for zombie physiology. But it would explain, very clearly, why it is that zombies want to eat live humans more anything else...because only the body tissues of a human alive at the time of eating would would have whole cells suitable to rebuild zombie body tissues.
So whatever else a zombie may eat, its deepest instinctual craving would always remain the desire for living human flesh...
(For a 99 cent short story I've written featuring Zombie Physiology, please follow the Zombie Doc Kindle link.)