Adding textures with transparent areas onto other textures

Hi, I am looking to create a way of showing the players health by how wounded their hands are and as a first pass I am wanting to simply have a texture of cuts and grazes increase in opacity as they get more and more injured. I have created a texture for the hand which is all transparent except for the wounds and I was wondering if there is a way to add these on top of the existing textures of the hands? The idea is to have wounds showing health and the veins darkening to show mana so being able to layer both would be ideal too.

Any pointers towards tutorials would be appreciated, cheers in advance

 

You can search for texture blending shaders

For example:
http://wiki.unity3d.com/index.php?title=Blend_2_Textures