So as yet another blog written within a relatively short time period, I have found myself curiously investigating and taking notes on the Undead physiology in comparison to baseline human physiology out of a desire to better understand the Undead Curse for the sake of immersion in the Souls series.
Note that this is purely speculation and much of this is based on assumptions and interpretations of dialogue and behavior, as well as some facts regarding certain charcters that may or may not be relevant. Now to start...
There is little to no information or dialogue relating to the nervous system or senses, and as players we largely experience being undead through a character for whom "yes" and "no" are his/her whole vocabulary (barring roleplaying), but I have made certain guesses as to these parts of the undead anatomy.
For one, I believe that the undead and hollows seem to possess all of the senses that normal humans have, and there does not seem to be any particular differences between them in this regard. A hollow who has been shot at with an arrow will react to it whether it strikes the target or not, indicating that the sense of sight is intact, and if I am not mistaken they may react to sound as well since the sound of breaking items like furniture or footsteps may alert them (or they're just programmed to react once the player gets within a certain distance.)
The undead nervous system seems functional as well, since we can hear a hollowed player character and some other hollows/undead grunt, groan, or otherwise make noises associated with pain when injured, meaning that the undead's nerve cells work even when hollowed.
Circulatory System, Respiratory System, and Digestive System:
Starting with the circulatory system, it appears to be just as functional as a baseline human's, since the undead can be made to bleed and sweat using the Flash Sweat pyromancy.
The respiratory system of the undead also seems to work normally, since both undead and hollows can be seen breathing and one needs air to talk after all.
The digestive system's usage is somewhat more difficult to determine, as none of the undead are shown to eat or drink, and neither do hollows. Neither have any of them expressed a need to urinate or defecate as well and there doesn't seem to be much in the way of edible plants or animals in Lordran, so the Undead may not need food or water to survive. On the other hand there are the butchers in Lordran, so perhaps they do need to eat and the developers just didn't want to spend a lot of time on the matter in favor of more interesting aspects to the Souls series.
The undead appear to be resistant to most diseases and infections that would plague a standard human since they can traverse areas like the poison swamp and sewers, though in the former case and due to certain enemies/weapons and pyromancies they can still be poisoned.
They also seem to be succeptable to parasitic infestation, since the undead can be infected with some sort of worms or eggs that feed on souls.
Undead children or infants are unheard of and all the undead the player encounters appear to be at least in their young adulthood, with the general age estimated to be around mid twenties to thirties with some exceptions here and there who are older.
However, there are at least two cases wherein the player can meet an undead who is a parent. In the first game we have Siegmeyer of Catarina who is the father of Sieglinde of Catarina and in Drangleic we have Lenigrast the blacksmith and his daughter Chloanne the stone trader.
It is not known whether either of them sired their respective daughters before or after their curses manifested, though I for one believe them to have been wholly human when either child was concieved. However, this does not mean that the undead are sterile at least in human form, it just means that we have no evidence of undead children. Thus, I believe that it might be possible for an undead/human couple or undead/undead couple to have children so long as both parents were in human form during conception and the mother remained in human form and did not 'die' whilst pregnant or hollowed too much.
The Undead seem to slowly turn hollow as time goes on, this rate can be sped up due to at least two factors, death (such as being killed in battle) or on a case by case basis wherein the undead may hollow at a faster rate than others of their kind. It is possible that the undead may hollow faster due to psychological issues, as more deranged, depressed or defeatist undead appear to hollow at a faster rate than the more determined or optimistic undead, though there is no concrete evidence to verify this and the aforementioned mental instability may be a result of hollowing itself.
The only way to reverse hollowing appears to be the consumption of humanity (the sprite), which appears to rejuvinate the recipient and return them to human form.
This is the summary of my thoughts on the matter right now, though as I am not a trained physician or student of the medical arts it is likely that I have missed some details.
As always, thoughts and debate are welcome.