What are the NPC Blood Vials meant to be used for?

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When you speak to several NPC's at varying times they will give you a vial of their blood. They seem to just be additional healing items but they also appear to be too special for that to be there only purpose. So is there any point in the game where they are traded for something or affect gameplay in some way?
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What are blood vials for?
Blood Vials are a Consumable Item, and your primary source of health regeneration in Bloodborne. Vials can be found off many defeated Enemies, lootable corpses, bought from Merchants, and more.How much health do blood vials heal?
Blood Vials restore 40% of a player's total health upon use.Bloodborne 20 Blood Vials in 4 Minutes Easy Farming No-BS Guide
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