We have discussed the complex hemodynamic changes that occur throughout the respiratory cycle during spontaneous and mechanical ventilation. In patients, many cumulative factors account for the total hemodynamic burden of inspiration and expiration. Thus, the change in pleural pressure, ventilation rate, mode of ventilation, amount of spontaneous ventilation, underlying lung and cardiac disease, and intravascular volume status all affect cardiocirculatory equilibrium. In addition, in the patient with ARDS, various immune mediators that are released into the circulation will affect LV function. Knowledge of the effects discussed in this article will aid the physician in choosing therapeutic options for patients with ARDS.