A surgical system for controlled application of ultrasound energy to engaged tissue volumes for creating effective biological welds or seals in tissue. The invention provides an ultrasound transmission assembly including piezoelectric elements coupled to an elongate waveguide that is reciprocatable in an interior of an introducer. The reciprocatable waveguide assembly of the invention is adapted to have multiple functionality: (i) to couple ultrasound energy to both opposing jaws to thereby deliver energy to both sides of engaged tissues to create uniform thermal weld effects; (ii) to apply very high compressive forces to captured tissues over the length of elongate jaws by engaging substantially the entire length of the jaws, and (iii) to transect the captured tissues contemporaneous with the delivery of energy to create the thermal weld. The invention further provides for use of other energy sources to deliver thermal energy to tissue, for example microwave energy, Rf energy and laser energy, either in combination with ultrasound energy or independently.