Mages call mana the lifeblood of magic. It is the fuel that powers spells. When a mage focuses his mind he can shape this mana into whatever magic he wills, but were he to run out of his reserve of mana his spell will falter and die just as a human would if drained of their own lifeblood.
There is a good reason that Magical Scholars refer to mana as the 'lifeblood' of magic, because mana is very closely attached to one's life blood. Mana flows through our own blood. Some mages argue that without mana to fuel it human life would be impossible to sustain. Recently Magical scholars have suggested an alternative claim, that mana is not the fuel of the mind, but rather that the human mind is the fuel, or perhaps the furnace, which generates mana itself! Whatever the cause or affect it's well known that the human body has always been tightly linked to mana, and the main source of it.
A mage is able to slowly refill his reserve of mana for casting spells by siphoning off the excess mana generated from his own body, the the process is slow, taking more then 8 hours of time to refill, but ensuring they usually awake full of magical power.
More relevantly though mana that mana within one's body can be spilled just as easily as the blood. When one is cut they bleed both blood and mana. Even the smallest of wounds allows some of the mana which infuses one's body to leak out. It's been well documented that upon death a huge surge of mana is also released.
This nature of mana has served humanity in the past. Mages wounded by some unfortunate danger have often tapped into the mana released by their own energy to cast spells to defend themselves. In fact many natural disasters have been mitigated by mages taping into the magical energies released by those wounded in the disaster to fuel the very spells used to save those still in danger.
Still, one can't deny some unsavory tactics have resulted from the nature of mana. The infamous blood mages willing to hurt, or even kill, to fuel greater works are a horrible thought, though strict laws and harsh penalties have ensured few go down this dark path in the civilized world. But one can not ignore the escalating nature of war magics and the death toll that has resulted. Battle mages draw on the mana released by the death's during war to fuel their lethal spells, which in turn leads to more deaths and more ambient mana for both sides to draw upon. Indeed the most fearsome magics are always worked during warfare, when mana is most plentiful. For this reason magic has always been, right or wrongly, closely tied in most men's minds with war and combat. Some sadly think of magic as only useful when used to harm another, though I and my fellow Magical Scholars would hasten to stress there are many benign and beneficial uses for magic outside of combat.
After all let us not forget about the great Merlin, who's made the ultimate sacrifice by drawing on the mana that fueled his own lifeforce, sacrificing his own life to to cast the spell of binding which saved so many lives. While I prey no mage find's themselves in the situation where they must cast a their Final spell, it does show the good that magic can do for the common man!
And this is but one example. Remember common mage spells have helped protect farmer's crops from insects and encourage higher yields. From repelling dangerous creatures to speeding new construction to warding off infection of wounds magework regularly makes the life of the common man easier!
But this important magework is limited by Mana. There are far too many demands for Workings then our mages can manage from their own ambient mana. To fuel these important daily works, to improve and even save lives of you, the common man, our mages must supplement their mana reserves with Potions. Without Potions we will fail! We can not afford to allow our Potion supplies to dwindle!
And so I plead to you good folks, help support our magework! A simple quick procedure will allow us to draw upon your ambient mana to help us make potions to support the important mageworks of today! The procedure takes only 30 minutes! The process is simple and while Yes we must release a small flow of blood it is just a small pinprick, a tiny moment of discomfort to support our noble magework! You will be compensated for your time and inconvenience, and we provide free snacks afterwards to help you refill your mana supplies.
Remember you can donate once a week! One donation can fuel up to three potions! Please do your part to support magework and help save lives!
Side effects may include temporary dizziness, lower energy for the day after, and lower Mana reserves for casting and dry mouth. Do not donate if you are cursed, treating other's curses, or may be cursed in the near future. If you continue to experience difficulty Casting after a nights rest please seek magical attention from your local witchdoctor Immediately!
Okay, I may have had a little too much fun with this idea. It's similar to the already suggested idea of blood being the source of magic, but it's a bit more general. Mana is released whenever someone is hurt or killed, but it's not directly tied to anyone's blood. In combat whenever someone is hurt, be they ally, enemy, or even yourself, some ambient mana is going to be made available, at which point anyone can draw upon it. This means conflict that involves actual harm is the time that mana is most usually available to be drawn in. Mediators manually reach out to draw the ambient mana, while naturals just sock it in.
in a totally unrelated, and certainly not at all apropo, aside everyone should consider donating blood this weekend, Your local red cross likely is open Saturdays for convenient walk in times if you just google them! ;P )