Blood is a form of life force. As it is said, that blood is the life, but our bodies make more blood every so often. Correct me if I am wrong, but it varies, and it may be about every two weeks that blood is regenerated.
It is like when summoning something. The way that ceremonial magicians get the entity to appear, it is partly by having enough energy, or else the entity won't manifest itself, because there needs to be given 'life' in order for the entity to take its form. The more blood the more energy. It is like when animals die they let off a lot of life force, which is why 'black' magick can be so powerful, because some magicians, excuse the term, will use blood of their own or sacrifice an animal. When something dies it lets off a lot of life force, a lot of energy. We expel energy all the time, but exerting it during a ritual will explain why the most successful spells will sometimes leave the spell caster tired afterwards.
I personally found that repeatedly casting will make the intended outcome stronger, or the spell stronger. So that just might be another concept to life-force. What energy we give basically accumulates after a while, if we are persistent in ritual. Kind of makes sense to be consistant in our meditations, especially something like chakra meditation, or the middle pillar ritual.
That is a rule of thumb, pardon the term, but you know the spell worked when you get dog tired and or hungry after casting. One time, I could barely drive comfortably because I was so tired, after casting a spell. One can exert a lot of energy out of his or herself, in many ways, and creative ways, and in ways by accident. Interesting what one finds, from time to time.
Blood can be addicting, I think. I don't know why it is very addicting, but I have my hunches; leave it at that. You know what, I, at times, crave blood or the taste of blood. I see that as the most exciting thing in the world, is the sight of blood. When I say exciting, I mean erotic. But I am stuck with just psi-feeding, breathing in peoples' aura, or at times, extending tentacles to subconsciously feed. It is always the weirdest thing, but true, that the partners I sleep next to always complain about not having energy the next day, or so. Well, I guess it goes into that idea that the longer an object is near one's aura, then that object is connected to that person that much stronger than a mere touch of contact. I guess sleeping next to another would leave those tentacles strongly connected, and that donor wouldn't be really regenerating their energy while sleeping, because the psi-vampire would be naturally feeding all night.
One last note. Anyone can make themselves into whatever they so choose, or do what they want, to say the least. Psi-vampires do exist. If one can learn to become empathic, then why not 'pranic.'