Using numerical treatment of experimentally obtained Fowler-Nordheim emission plots, radii of nanotips, formed by the build-up process in strong electric fields oil the surface of the tungsten base cathode were determined. It was found that the typical size of such structures depended upon the conditions of their forming and was in the range of 10-50 Angstrom. The field emission characteristics of these emitters for extremely high electric fields and current densities J were studied. Studies of the emission current kinetics from a single nanotip show that at J>J(blunting) similar to 3-5x10(9) A/cm(2) an irreversible change of the tip submicrogeometry-"blunting" of the nanoemitter-takes place. Its apex radius increases 3-5 times. The maximum attainable J values from nanotips exceed the values for annealed substrate emitters up to 1 to 2 orders of magnitude. These values range from 10(10) to 10(11) A/cm(2) depending on the size, structure, and geometry of tips. The results are presented of numerical simulation of cathode thermal regime. using the heat-transfer equation approach. It has been shown, that the explosive destruction of emitter and initiation of the vacuum breakdown is due not to heating of nanotips by high-density emission currents but to the energy accumulation in the base emitter bulk. The theoretical approach has been developed to the description of energy extraction accompanying the emission from ultrasmall structures (smaller than the free-path length for electron-phonon scattering). It was found that the maximum value of the total emission current was practically constant for emitting object from similar to 10 up to similar to 50 in in size.