Work of breathing is the work done to expand the lungs and overcome airway resistance during breathing.
W = P × ΔV, where P is trans-pulmonary pressure and ΔV is change in lung volume; the normal lung spends about 5% of the body's total energy expenditure on this work.
Energy Source
Energy for the work of breathing is spent through muscular contraction, which generates trans-pulmonary pressure as the force that inflates the lungs.
Pressure-Volume Curve
The pressure-volume curve depicts the work required for breathing in the normal lung; the shaded area of the curve represents the work of breathing.