Upright positioning allows for improved lung volumes and alveolar recruitment through gravity assisted offloading of abdominal contents off the diaphragm. Additionally, the heart is positioned more anteriorly in the chest offloading the left lower lobe, which is compressed in supine.

 

During verticalization, there is improved ventilation-perfusion (V/Q) matching through a more homogenous pressure gradient in upright position due to repositioning of fluid. This helps to improve the arterial partial pressure of oxygen (PaO2) compared to the concentration of oxygen inspired or delivered (FiO2) or improved P/F ratio.