We have performed first-principle calculations of the electronic structure and exchange couplings for the nanotube compound Na2V3O7 using the local-density approximation Hubbard parameter U approach. Our results show that while the intraring exchange interactions are mainly antiferromagnetic, the inter-ring couplings are ferromagnetic. We argue that this is a consequence of the strong hybridization between filled and vacant 3d vanadium orbitals due to the low symmetry of Na2V3O7, which results into strong, and often dominant, ferromagnetic contributions to the total exchange interaction between vanadium atoms. A comparison with results of previous works is included.