Find Cross Sectional area perpendicular to an axis at every point

I have a variable diameter hose and I want to find the mass flow through this hose at every point. There are no bends in the hose except to change its diameter so the hose's central axis is in a straight line. Is there a way to create a variable for the cross section of the hose so that I may use this variable to find the mass flow rate through the hose at every cross section? Say my hose's axis is x. How may I get an A(x) that I may use for the equation below:
mass flow rate = density*velocity*cross sectional area? I'm hoping there is a mathematics module or an operation that is analogous to the Wall Distance mathematics module.

Thanks,

Thomas