Final answer:
The incorrect statement about intrapleural pressure is that 'It is always negative.' Although intrapleural pressure is typically negative, it fluctuates with the breathing cycle and is not a fixed value.
Step-by-step explanation:
The statement that is NOT true regarding intrapleural pressure (Pip) is 'It is always negative.' While intrapleural pressure remains negative relative to atmospheric pressure throughout the breathing cycle, it fluctuates, becoming less negative during the inspiratory phase when the chest cavity expands and more negative during the expiratory phase as the chest cavity returns to its resting size. Intrapleural pressure remains approximately – 4 mm Hg but can vary. Furthermore, because of the elasticity of the lungs and the chest wall, the lungs do not always try to assume their largest size; rather, they attempt to recoil inward while the chest wall pulls outward.
Fluid level must indeed be kept at a minimum to maintain healthy intrapleural pressure. Excess fluid can lead to increased pressure, which can compromise lung function. The lymphatic system helps manage the fluid levels within the pleural space. Additionally, intrapleural pressure does fluctuate with breathing as described.