The reality is that the head and centre mass inner organs need to be protected through a) movement of the entire body away from the potentially damaging blows, and b) the movement of the head, arms and legs so as to deflect, parry and/or block the incoming blows when the centre of mass cannot be adequately moved out the way. In the case of unarmed blows - the muscle, bone structure and inner connective tissue can be 'toughened' through physical fitness and martial arts training. The potential damage of these unarmed blows can be effectively absorbed by the outer physical and near surface structures of the body - this includes the bone structures and soft structures that comprise the entirety of skull including the mouth, teeth and tongue, etc. These areas can be 'damaged' but will heal given time. When dealing with bladed weapons - as these can penetrate the toughened outer layers and do fatal damage to the inner organs - movement of the centre mass is of a greater concern if no self-defensive weapon is held by the victim.
In this case the legs must be well trained for either minute or substantial (instantaneous) distance adjustment. A damaged leg will still have to move by necessity (hence the importance of squat kicks in all their variants). The upper limbs comprising the arms and hands will have to 'deflect' and/or catch or absorb the thrusts of any incoming bladed weapons that cannot be completely avoided by systemic (evasive) movement. This may involve the substantial damage of these body parts which must be conditioned to perform this function whilst carrying-on moving for the duration of the self-defence encounter. These types of injuries can heal (regardless of severity and any potential or long-term damage). An individual will probably survive broken or cut arms and hands - but will not survive damaged or cut inner organs, arteries or veins, etc, as the medical chances are greatly reduced.
The outer limbs can be sacrificed to protect the inner organs and blood supply vessels which lie across the centre line. There are cases of individuals surviving horrific injuries following attacks from blades weapons – even with severe mi-section laceration damaging and exposing the intestines, severe and deep cuts to the neck area and even limbs hacked-off – providing medical care can be found in time. Chances of survival are enhanced if the individual concerned has some medical knowledge and First Aid experience – as this can stem blood loss and prolong the chances of survival. Of course, traditional Chinese martial arts often involve extensive mental toughness regimen – and it is this attribute that can drive a severely injured individual to a) ‘survive’ a deadly encounter (despite being badly hurt), and b) seek-out assistance despite being isolated or far away from medical help.
Shifu Adrian Chan-Wyles (b. 1967) - Lineage (Generational) Inheritor of the Ch'an Dao Hakka Gongfu System.