Rob
Ansys Employee

As has been posted in response to other questions, the UDF manual is a good starting point, and from there you will need a DEFINE_PROFILE to set the pressure. You are also not defining a pressure gradient, Fluent will calculate that, you need to define the inlet pressure, and possibly the outlet depending on what you're modelling. Given blood is incompressible, I'd consider using a velocity boundary, again referring to the UDF manual and DEFINE_PROFILE  example.