The relaxation pressure of lungs of lungs becomes zero only at the minimal volume and negatives at lower volume

The statement refers to the elastic properties of the lungs themselves, specifically their tendency to recoil.

1. Relaxation Pressure of Lungs at Minimal Volume: The "minimal volume" for the lungs is the residual volume (RV), which is the volume of air remaining in the lungs after a maximal exhalation (represented as 0% Vital Capacity on the graph you previously saw). At this volume, the elastic fibers within the lung tissue are in their most relaxed, unstretched state, and the surface tension within the alveoli is at its lowest. Consequently, the lungs exert no inward elastic recoil force, meaning their relaxation pressure is $\boxed{\text{zero}}$ at residual volume.

2. Relaxation Pressure of Lungs at Lower Volumes: Physiologically, the lungs cannot be deflated to a volume lower than residual volume. If, hypothetically, the lung tissue were forced to a volume below RV (e.g., by extreme external suction), the lung tissue would be stretched outwards. In this non-physiological scenario, the lung tissue would still exert an inward recoil force, meaning its relaxation pressure would remain positive, as it would be trying to return to its collapsed state at RV. Therefore, the statement that the relaxation pressure of the lungs themselves becomes negative at lower volumes is generally not considered physiologically accurate for the intrinsic recoil of the lung tissue. The "LUNGS" curve on the relaxation pressure graph always shows positive pressure above residual volume and zero pressure at residual volume, never negative.