Veins contain a network of values to ensure a one-way flow of blood but arteries do not. this is because veins must counteract the force of on blood flow.

They are like other investigations? say there was a murder u have to do blood test and finger prints which run through a lab. 

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).