This might fit in with the picture and the message...

English prefers the first person approach, phrasing this as a caption expressing the woman's indomitable will.
I think that any reference to 'nature' just detracts from the message.
The full stop at the end is optional.

A wildly free translation, but with the context, it seems appropriate.
As to spending blood, this also occurred to me:

Give generously of what nature gave you

No, this is not an accurate "translation" of the German. Instead, it’s my rather blunt American (i.e. undiplomatic) attempt to convey what could be the underlying message of the campaign: Donating blood won't cost you and arm or a leg, but it saves lives.

It always amazes me how companies and organizations can put so much time, energy and money into developing advertising campaigns and yet simply dump the job of "translating the untranslatable" into a translator's lap. Advertising agencies are paid a lot of money to come up with snappy slogans and captivating images. How much money does the translator get for the thankless task of coming up with a corresponding catchy phrase in another language? Pennies per word ... and often no guidance whatsoever for the translator as to the extensive decision-making process that went into the campaign. Absolutely mind-boggling.

Pictures of people missing limbs may be eye-catching, but if the message is muddled, the campaign is admittedly not very effective. In my opinion, this particular campaign is less about flexibility or the laws of nature and more about the life-saving measures (blood transfusions, etc.) that allowed people to survive horrific accidents and yet remain vibrant human beings (minus a limb or two, aber immerhin). At any rate, if the idea is to encourage people to donate blood, I see no reason why you can't spell it out.

Why the low confidence level, Katy? Because I doubt that you could hand in such a “translation” to the client without getting into hot water because it ignores all of the obvious “clever” plays on words with “bending” and “flexible” and instead goes in a completely different direction.

Okay, to be honest, I also just couldn't resist suggesting it.

But what do you have to lose? It won’t cost you an arm or a leg. ;-)

By the way, if the client insists that the message here is that we should admire the resiliency of the flexible and well-balanced one-legged woman in the photo, then you could always write something along the lines of the following:

First law of nature: you can’t keep a good woman down

And the connection to donating blood? Just as obscure as in the original! Then it becomes (merely) a celebration of diversity, or the "feel-good factor" (thanks, Andrew!) without directly pointing to the underlying message of “donate blood” ... which might just be the solution that the client wants.

Sorry this turned into such a novel. May the force be with you, Katy!

Suggests that perseverance is this woman’s lifeblood while reminding viewers that blood sustains life.