Do you need to do this one time on this geometry or hundreds of times on different geometries?
Please attach a .wbpz archive of this geometry to your reply if that is possible to share.
One-Time method using SpaceClaim 2021 R1
Make a copy of the solid artery and hide the original. Create a plane, move and orient the plane normal to the artery by eye where you want a point. Slice the solid to create a face. Hide one solid so you can see the newly cut face on the other solid. Select the face and start a sketch. Place a point by eye at the centroid of the face. Use the Move tool to slide the plane along the artery, orienting it to keep the plane normal aligned with the artery centerline. Repeat this process until you have a collection of points in space on all branches of the artery. Hide all the solids. Now go into Sketch mode. Enable 3D sketch. Click on the Spline tool and draw a spline through the points that were created. Create one spline per branch.
Programmatic Method
If you have to do this hundreds of times, then you will want to write a program to automatically find the centerline. I don't yet know how to do that, but I would try to do that in Grasshopper, a module in Rhino7 that has some powerful abilities to programmatically read in points, curves and surfaces in order to create new geometry, such as a centerline. Here is one example: